LA Course #GCE-6-1405
Planning for Urban Trees
Edited by Len Phillips, exceptions noted
Updated in January 2021
Sections
Planning for Urban Trees
Edited by Len Phillips, exceptions noted
Updated in January 2021
Sections
Planning to Install Trees
Planning the urban forest pays off in the form of healthier trees and reduced maintenance costs. Good planning requires starting with an inventory of the existing trees and proceeds to goal setting and finally to action.
Resident participation is essential and residents of the city who are interested in the city's environment should become involved with a Street Tree Master Plan as early into the process as possible. If the city has a Tree Advisory Board or committee, they should be guiding the planning process.
The planning process should start with an overview of the Street Tree Master Plan. For a comprehensive review of The Plan see "How to Write a Street Tree Management Plan".
Street Tree Master Plan
'The Plan' is a comprehensive review that evaluates the city along with its trees, the forestry department, and the citizens. The purpose of having this plan is to insure that the community will continue to appreciate the benefits from trees through proper arboricultural management in a cost-effective manner. It is the goal of The Plan to state what is necessary for the management of the urban forest and describe the measures and services required to fulfill these responsibilities to the community. The Plan should develop criteria for deciding priorities when installing street trees. Another approach might be to create a plan on-line using the California Forestry Management Plan Toolkit.
Goals
The goals of The Plan should start with the purpose and process for making The Plan. It could indicate a realistic plan of achieving a 100% complete planting of trees in every available site in the community. The planting should be done over a long time span and should be properly diversified. The Plan should also indicate the maintenance goals so tree management can be completed in a cost-effective manner.
The Plan should be developed with the cooperation of the community's municipal arborist or landscape architect. The arborist will need assistance from volunteers, paid staff, or consultants. This assistance will be necessary to gather information, conduct evaluations, and do the actual report writing. Staff members might assist the arborist depending upon their workload and depending upon whether funds are available to pay for staff time assistance. Another option utilizes landscape architects or forestry consultants who specialize in plan preparation. The Plan should contain a description of all the items listed below.
Inventory
The inventory is the counting and description of all municipal and public shade trees growing within a community and it should be kept up-to-date. Use the inventory to find out what you have and what you need before you choose tree species for future plantings. Make sure the inventory considers overhead wires, underground utilities, and the spaces available for replanting. The inventory can also be used to look at diversification of the existing urban forest. A good rule is to have no more than 10% of the population in one species. For design purposes, some communities may choose one species or cultivar of tree per block. More progressive communities however, are choosing to develop a diversification plan where no more than four trees of the same species or cultivar will be planted in a row.
An analysis of the inventory might include looking at needs, problems, tree values, and the comparisons between species, vigor, and location. Use the analysis to develop written goals for what the community forest should be. For example, if one of the goals is to create a forest that withstands pests and climatic extremes, plan to install a diverse mix of tree species. If the intention is to replace trees–species for species, that should be stated in the goals as well. Still another goal might be to replace a dead or missing tree with a new introduction to see how well it will do in the city. The information can also be used to annually plan, prioritize, and budget tree removals, maintenance work, and the installation of trees. The inventory can determine the value of the community trees for increasing property values and storm water management, which helps emphasize a tree program's importance to the community leaders.
In some situations where the tree population is small, the inventory could be taken every five years and not constantly kept up to date. The changes in the community's forest can be seen in the number, age, and species of trees. A well maintained inventory can be used in cases of liability to demonstrate there was no negligence in the inspection or care of public trees. The inventory will also improve the chances of receiving grants and other assistance by documenting the condition of and care of the community forest. Regardless of size, successive inventories should be compared to look for long-term trends.
Tree List
The tree list is composed of natives, cultivars of native trees and introductions that are tolerant of the urban environment and are most suited to the specific community. In a municipal situation, the trees should not require high maintenance or be highly susceptible to pests and diseases. Additionally, no species with thorns or fruiting characteristics should be used in areas of pedestrian traffic. To repeat, the list should be diversified.
Tree Selection
Choose the appropriate trees for the planting site by observing the following:
- match the USDA hardiness zone and annual rainfall of the site to the tree;
- match the tree to the size of the planting site;
- match the tree to the available sunlight;
- match the tree to the soils, water, and drainage;
- consider potential soil compaction;
- consider the available root growth space;
- avoid all utilities, overhead and underground;
- avoid invasive trees;
- avoid trees that could drop messy fruit, large leaves or twigs, except at appropriate sites;
- consider the need to repeat pruning high maintenance trees and the health of and type of existing plants, ordinance restrictions and species diversity.
Tree Size
Bigger is not always better. In fact, smaller stock at planting time usually outgrows and outperforms larger stock in the long-term. Smaller sizes are easier for volunteers to plant, they are cheaper for contractors to purchase and plant, and they require less maintenance. For many areas, 6 – 8 feet (2 – 2½ m) BR stock is a great choice. On some city streets or intensively used areas, larger specimens such as 2 – 2½ inch (5 – 6 cm) diameter stock may be necessary to hold up to foot traffic, snow plowing, wind, or vandalism. Sizes have a big impact on determining the number of trees to be installed within the amount of available funds because with smaller stock more trees can be planted within the budget limitations.
Municipal Environment
The Plan should contain an evaluation of the municipal environment and tree sites. The evaluation should look at soil, climate, existing trees, land use, aesthetic character, vegetative history, etc. The evaluation should also look at the municipal residents, their attitude about trees, their willingness to pay for proper tree management, and their perception of the existing urban environment.
Aesthetics
Not only is the aesthetic value of a tree itself important, a tree should also be selected that is best suited in terms of its aesthetic value at that site.
Nurseries
Municipal street trees can be grown in municipal nurseries, purchased from commercial nurseries, or both.
Follow-Up
At least once a year, there should be a follow-up process done that will evaluate the work performance, planting progress, trees trimmed, inventory update, etc. The process will examine the work actually accomplished compared to what was budgeted and this can be reported in The Plan as well as the City's Annual Report.
IPM
Tree management should also look at disease and pest control through Integrated Pest Management (IPM) techniques.
Pruning
The Plan should require the pruning cycle for trimming street trees to be systematic and planned, as opposed to trimming on demand. This will decrease the cost per tree while increasing the safety and condition of the trees and reduce demand and storm damage requests. This effort also results in a better public image.
A plan to conduct structural pruning on newly installed trees should be the exception. In this case the pruning cycle should be modified so that the person trained in structural pruning follows a street layout that allows the person to efficiently go from one tree to the next, skipping trees not scheduled for structural pruning. Many cities require the structural pruning to be done annually on trees that were planted 3 years previously. This timing is appropriate because the transplant shock will often result in the tree compensating for the loss of roots by sending up co-dominant leaders, one of which can be removed by the structural pruning person/crew. The pruner should also look at projected branch spacing and remove those that will be too close together on the mature trunk.
Urban Forestry Department
To begin an analysis of the Urban Forestry Department, a study should be made of the following items: the number of trees to be maintained; frequency of trimming cycle; workload per year; equipment and personnel required; budget; support services and contracts; employee training and morale; and other related items unique to each community. An evaluation should be made of the municipal forestry management program and a comparison made with the industry, standards, and practices in other communities. The Plan should also identify economical work methods and procedures that will enhance existing operations. Careful planning through The Plan will improve the cost effectiveness of the operations.
Equipment
A general knowledge of the basic tools is essential for efficient street tree management. Tree pruning is the most costly operation in urban forestry department budgets. Mechanization in this area will reduce the costs for the benefit of the entire department. This requires expenditures for up-to-date vehicles and equipment, especially items like battery operated electric chain saws, electric vehicles, etc.
Contracting
Contracting versus using in-house labor versus some of both should be studied and prices compared in order to ensure that quality tree maintenance is being performed at the lowest possible cost. Some cities are writing their contract specifications to stipulate that only ISA Certified and/or TCIA accredited companies may submit bids. This reduces many low quality companies from bidding as well as unsafe and poor workmanship in the trees. (The highly trained employees of accredited companies seldom need municipal inspectors on the job with them, so overall costs to the city are often less, despite the sometimes higher costs for TCIA accredited employees.)
Regulations
The municipal regulations pertaining to tree planting, tree management, and tree removals as well as other local and state regulations pertaining to street trees should be evaluated and made a part of The Plan.
Operations and Maintenance Plan
One of the key elements to the Plan is the development of a long-term implementation strategy including annual goals that the forestry department intends to implement. The budget and funding section will also be individualized according to the local community and should provide details that will meet the needs of the community's urban forest sustainability as well as the practical limitations of municipal funding issues. The budget section should not only include cost estimates to implement each section of the plan, but it should also indicate funding sources including grants and private gifts that might be used to offset the local government costs.
Public Relations
Public support comes in the form of the public's perception regarding the Urban Forestry program. Public support is necessary in order to obtain the funding necessary to pay for street tree management. Public support can be enhanced through a wide variety of public relations programs. Some of the best programs are described below.
Arbor Day
Arbor Day is the day to celebrate trees. Everyone involved in arboriculture should take advantage of, observe, and celebrate this holiday. Most celebrations consist of a ceremony where a tree is installed correctly and according to the results of recent research such as root stabilizers instead of guy wires. The history of Arbor Day is also discussed and awards such as Tree City USA are presented.
Tree City USA
This program, sponsored by the National Arbor Day Foundation, is a very popular program that draws attention to a city's urban forestry program. The city must:
1) have a tree board or department;
2) have a tree ordinance or by-law;
3) have a comprehensive urban forestry program supported by a minimum of two dollars per capita;
4) make an Arbor Day proclamation and hold a commemorative tree planting at a public ceremony.
Volunteers
All public support must be channeled through a street tree advisory board. This is a group of volunteers who establish the policies and recommendations that an Urban Forestry Department must follow.
Volunteers can also be used to assist in other aspects of the urban forestry program, especially for tree planting projects. The volunteers should receive training in proper tree planting and tree care. Many cities also have citizen pruning organizations that provide training in proper pruning and tools to prune small and recently planted trees. Any city that has a program for volunteers must also be sure to have a “Volunteer Appreciation Day”.
Street Tree Inventory
Management of any resource begins with an inventory of that resource. Street trees are no exception. A street tree inventory should be undertaken with the following objectives:
1. To count and provide a tree identification number for all public shade trees growing within the community boundaries by the common and botanical names and to determine their size, location and installation date (if known).
2. To record the observation of needs such as pruning, fertilizing, cable/bracing, removal etc. and problems such
as insects, disease, aesthetic value, condition of the trees, shade canopy, insect damage, conflicts with
overhead and underground utilities and other evaluative data.
3. To use the information gathered in programming tree care activities and to point out needs for additional
plantings or alterations to the street tree canopy.
When a city is planning to change its image through a major tree planting effort, or set up a new forestry program, the city will usually consider undertaking an inventory so proper planning is done. Once this data is obtained, it can be organized to serve as a useful and available source of information for the ongoing tree care program. The inventory can be used to justify the municipal tree budget for popular planting programs as well as for maintenance programs. The inventory information, when properly used, can decrease a community's liability for tree damage and damage done by trees. The inventory also lends itself to monthly and yearly reports of completed work on trees in the inventory. The inventory is also desirable when a new tree planting or tree maintenance program is being established. The inventory also allows the municipal arborist the opportunity to make intelligent responses to calls from the public. This public service is appealing to administrators. The information in the inventory is a valuable asset to the municipal arborist to provide a diverse tree planting program and establish certain maintenance needs. It can also be used to project heavy periods of removals, required pruning, and planting needs. The inventory is also valuable in preparing the monetary value of the community's trees using a program such as i-Tree from the USDA Forest Service. This is available at no cost from the i-Tree website. i-Tree can also analyze maintenance costs and other costs requested by city officials.
Mapping
Once the guidelines are established, the street tree location should be mapped at a scale convenient for the arborist, landscape architect and the community to depict the overall tree canopy of the community. Each shade tree is shown on the map by using various symbols. The symbols represent the actual size of the tree as well as if the tree is coniferous or deciduous. A brief review of these maps enables one to quickly determine if an area is well foliated and if the foliage provides shade cover over the street. Also apparent are the size, number, mixture and relative locations of the trees. Computer programs are available to perform this task with ease.
Information Collected
The information collected on each tree varies according to the needs of the Arborist but they can include: tree location, street name, house number, tree number, width of the tree lawn, presence and condition of sidewalk, map number, management unit if applicable, distance from curb, and house set back. The species can be identified by name in scientific, common, both, or coded names.
The physical conditions of the tree should also be noted. This includes the tree's condition, trunk diameter, monetary value, tree crown, spread, biomass and approximate height. Environmental conditions are also important and physical conditions should also include underground and overhead utilities, irrigation needs, soils, land use, pollution problems, growing space, size and type.
Finally maintenance needs a tree report that provides condition codes, pruning needs, structural soundness, maintenance and miscellaneous remarks can be added to the inventory record for each tree. The miscellaneous remarks might include: location such as spacing, shaded, or poor location; form and shape such as good, poor, deformity, multiple trunks, etc.; exposed or girdled roots; insect damage; disease damage; mechanical damage; pruning needs; suggested treatments such as insect control, pruning, removal, fertilize, cabling, aerate or water. Tree maintenance records should be kept in the inventory to include the date, work done, and labor report.
All of the information being gathered will be useful in establishing a meaningful tree management program. For example, the utility information will indicate that low trees can be planted under the wires or shallow rooted trees can not be planted beside the sidewalks. Maintenance needs can be recorded and projected if the inventory information is sufficient. Before an inventory is undertaken however, the municipal arborist or landscape architect should evaluate what reports, graphs, charts, maps, and other information are required for maximum utilization of the inventory.
The Inventory Computerization
The most important purpose for computerizing the street tree inventory is making the inventory information more readily available and enabling the contents to be analyzed. This provides the municipal tree department an easy opportunity for answering complaints and questions from the public about any tree in the city. The computerization of the inventory will also benefit the maintenance operations by improving the scheduling of routine maintenance, spraying operations, leaf pickup programs, etc. and overall improvements in the community tree care program.
Administrative duties such as scheduling for more efficient use of resources, budget justifications and preparations of annual reports will be reduced and simplified by the use of a computerized tree inventory system. Species and tree age, determined by the diameter information in the tree inventory, enables the arborist or landscape architect to project years of extra removal costs followed by high planting costs; whereas, a young tree population indicates the maintenance costs will be needed in trimming. Other municipal departments such as the Planning department may also want access to the information for their long-term planning and construction projects.
Another reason for having an inventory is the statistical analysis that is available. The statistics include: the number of trees, lists of trees by species; plus reporting history and work demands. Finally, the computerization will ease any planning operations which might require, for diversification purposes, the selection of species for tree planting programs and landscape planting plans. Planning capability can also be useful for projecting maintenance costs such as pruning needs. Using this information in conjunction with programs such as i-Tree will allow the arborists and landscape architects to calculate the value and the monetary benefits that the urban forest provides to the community.
The computer program lists the trees according to street address and tree number for reference purposes, the location of the tree and distance from the curb, house set back, tree species, caliper, tree condition and monetary value. The program will also store the data and keep a history of the tree by incorporating completed work such as trimming, spraying, removals or annual tree growth. This allows for periodic summaries to be produced which indicate numbers of removals, miles of street tree pruning, etc. A maintenance distribution report enables the arborist to justify the budget before the City Council.
At one time, communities would develop their own programs to store and retrieve inventory information. Today however, several computerized systems for automating the storage, retrieval and analysis of street trees have been developed by consultants for municipalities. There are also several companies and firms which offer computerized inventory services to municipalities. One of the most popular programs is i-Tree developed by the USDA Forest Service.
The inventory costs can sometimes be combined with a major municipal tree planting initiative. The Plan will justify the need for this major initiative.
Data Collection
Once a decision has been made regarding whether or not to proceed with an inventory; what information is to be collected; and whether or not the information is to be computerized; the next major decision deals with how the information is to be collected. If volunteers or municipal laborers are to be used, they need training. If a consultant is to be hired to do this, are sufficient funds available and is the entire inventory process to be done as a single contract or in small components?
The next decision deals with collecting the information. What types of hand held computers should be used to store the field data? If portable data collection devices are used, the information can be easily and quickly transferred to the computer program. The device will also act somewhat as an accuracy check if a non-existent species is incorrectly entered. Operating the data collection device may also require training. A popular approach many cities have used is the Trimble Nomad hand held computer running ESRI ArcPad. The information collected is then placed in Excel, Access or similar software.
Other Inventory Options
Success of the entire inventory data collection process is the result of excellent training, supervision, quality control and input from the municipal forestry personnel. Success is also ensured if all work is entered into the inventory system as the work is completed. Future re-inventorying can be done periodically; and is also an option which allows the inventory updates to be a budget item. This practice however, results in information which quickly becomes out of date so it is generally not recommended. Windshield inventories, which may be quickly done and useful for statistical purposes, are not accurate enough for compiling a complete inventory and for maintenance records. Windshield inventories are useful for a general overview of trees and quick estimates of tree numbers, planting vacancies or inspecting special problems like topped trees or insect and disease attacks.
A partial inventory may be useful as a trial run to provide the estimated number of trees and maintenance requirements and from which, a budget can be developed to prepare a complete inventory. A statistically accurate, quick inventory to establish a data base can be done. However, to consider anything less than a 100% inventory as complete, would be faulty reasoning. This information cannot be used from one year to the next nor used as a base for a working document. This is because trees are maintained individually and work performed on one tree is not guaranteed to be necessary on the next one.
Key to success is to determine what information is needed and how often it will be used. Each community is different and requires different street tree management techniques. Therefore, street tree inventory programs should be adaptable to meet the individual needs of the individual community.
Management of any resource begins with an inventory of that resource. Street trees are no exception. A street tree inventory should be undertaken with the following objectives:
1. To count and provide a tree identification number for all public shade trees growing within the community boundaries by the common and botanical names and to determine their size, location and installation date (if known).
2. To record the observation of needs such as pruning, fertilizing, cable/bracing, removal etc. and problems such
as insects, disease, aesthetic value, condition of the trees, shade canopy, insect damage, conflicts with
overhead and underground utilities and other evaluative data.
3. To use the information gathered in programming tree care activities and to point out needs for additional
plantings or alterations to the street tree canopy.
When a city is planning to change its image through a major tree planting effort, or set up a new forestry program, the city will usually consider undertaking an inventory so proper planning is done. Once this data is obtained, it can be organized to serve as a useful and available source of information for the ongoing tree care program. The inventory can be used to justify the municipal tree budget for popular planting programs as well as for maintenance programs. The inventory information, when properly used, can decrease a community's liability for tree damage and damage done by trees. The inventory also lends itself to monthly and yearly reports of completed work on trees in the inventory. The inventory is also desirable when a new tree planting or tree maintenance program is being established. The inventory also allows the municipal arborist the opportunity to make intelligent responses to calls from the public. This public service is appealing to administrators. The information in the inventory is a valuable asset to the municipal arborist to provide a diverse tree planting program and establish certain maintenance needs. It can also be used to project heavy periods of removals, required pruning, and planting needs. The inventory is also valuable in preparing the monetary value of the community's trees using a program such as i-Tree from the USDA Forest Service. This is available at no cost from the i-Tree website. i-Tree can also analyze maintenance costs and other costs requested by city officials.
Mapping
Once the guidelines are established, the street tree location should be mapped at a scale convenient for the arborist, landscape architect and the community to depict the overall tree canopy of the community. Each shade tree is shown on the map by using various symbols. The symbols represent the actual size of the tree as well as if the tree is coniferous or deciduous. A brief review of these maps enables one to quickly determine if an area is well foliated and if the foliage provides shade cover over the street. Also apparent are the size, number, mixture and relative locations of the trees. Computer programs are available to perform this task with ease.
Information Collected
The information collected on each tree varies according to the needs of the Arborist but they can include: tree location, street name, house number, tree number, width of the tree lawn, presence and condition of sidewalk, map number, management unit if applicable, distance from curb, and house set back. The species can be identified by name in scientific, common, both, or coded names.
The physical conditions of the tree should also be noted. This includes the tree's condition, trunk diameter, monetary value, tree crown, spread, biomass and approximate height. Environmental conditions are also important and physical conditions should also include underground and overhead utilities, irrigation needs, soils, land use, pollution problems, growing space, size and type.
Finally maintenance needs a tree report that provides condition codes, pruning needs, structural soundness, maintenance and miscellaneous remarks can be added to the inventory record for each tree. The miscellaneous remarks might include: location such as spacing, shaded, or poor location; form and shape such as good, poor, deformity, multiple trunks, etc.; exposed or girdled roots; insect damage; disease damage; mechanical damage; pruning needs; suggested treatments such as insect control, pruning, removal, fertilize, cabling, aerate or water. Tree maintenance records should be kept in the inventory to include the date, work done, and labor report.
All of the information being gathered will be useful in establishing a meaningful tree management program. For example, the utility information will indicate that low trees can be planted under the wires or shallow rooted trees can not be planted beside the sidewalks. Maintenance needs can be recorded and projected if the inventory information is sufficient. Before an inventory is undertaken however, the municipal arborist or landscape architect should evaluate what reports, graphs, charts, maps, and other information are required for maximum utilization of the inventory.
The Inventory Computerization
The most important purpose for computerizing the street tree inventory is making the inventory information more readily available and enabling the contents to be analyzed. This provides the municipal tree department an easy opportunity for answering complaints and questions from the public about any tree in the city. The computerization of the inventory will also benefit the maintenance operations by improving the scheduling of routine maintenance, spraying operations, leaf pickup programs, etc. and overall improvements in the community tree care program.
Administrative duties such as scheduling for more efficient use of resources, budget justifications and preparations of annual reports will be reduced and simplified by the use of a computerized tree inventory system. Species and tree age, determined by the diameter information in the tree inventory, enables the arborist or landscape architect to project years of extra removal costs followed by high planting costs; whereas, a young tree population indicates the maintenance costs will be needed in trimming. Other municipal departments such as the Planning department may also want access to the information for their long-term planning and construction projects.
Another reason for having an inventory is the statistical analysis that is available. The statistics include: the number of trees, lists of trees by species; plus reporting history and work demands. Finally, the computerization will ease any planning operations which might require, for diversification purposes, the selection of species for tree planting programs and landscape planting plans. Planning capability can also be useful for projecting maintenance costs such as pruning needs. Using this information in conjunction with programs such as i-Tree will allow the arborists and landscape architects to calculate the value and the monetary benefits that the urban forest provides to the community.
The computer program lists the trees according to street address and tree number for reference purposes, the location of the tree and distance from the curb, house set back, tree species, caliper, tree condition and monetary value. The program will also store the data and keep a history of the tree by incorporating completed work such as trimming, spraying, removals or annual tree growth. This allows for periodic summaries to be produced which indicate numbers of removals, miles of street tree pruning, etc. A maintenance distribution report enables the arborist to justify the budget before the City Council.
At one time, communities would develop their own programs to store and retrieve inventory information. Today however, several computerized systems for automating the storage, retrieval and analysis of street trees have been developed by consultants for municipalities. There are also several companies and firms which offer computerized inventory services to municipalities. One of the most popular programs is i-Tree developed by the USDA Forest Service.
The inventory costs can sometimes be combined with a major municipal tree planting initiative. The Plan will justify the need for this major initiative.
Data Collection
Once a decision has been made regarding whether or not to proceed with an inventory; what information is to be collected; and whether or not the information is to be computerized; the next major decision deals with how the information is to be collected. If volunteers or municipal laborers are to be used, they need training. If a consultant is to be hired to do this, are sufficient funds available and is the entire inventory process to be done as a single contract or in small components?
The next decision deals with collecting the information. What types of hand held computers should be used to store the field data? If portable data collection devices are used, the information can be easily and quickly transferred to the computer program. The device will also act somewhat as an accuracy check if a non-existent species is incorrectly entered. Operating the data collection device may also require training. A popular approach many cities have used is the Trimble Nomad hand held computer running ESRI ArcPad. The information collected is then placed in Excel, Access or similar software.
Other Inventory Options
Success of the entire inventory data collection process is the result of excellent training, supervision, quality control and input from the municipal forestry personnel. Success is also ensured if all work is entered into the inventory system as the work is completed. Future re-inventorying can be done periodically; and is also an option which allows the inventory updates to be a budget item. This practice however, results in information which quickly becomes out of date so it is generally not recommended. Windshield inventories, which may be quickly done and useful for statistical purposes, are not accurate enough for compiling a complete inventory and for maintenance records. Windshield inventories are useful for a general overview of trees and quick estimates of tree numbers, planting vacancies or inspecting special problems like topped trees or insect and disease attacks.
A partial inventory may be useful as a trial run to provide the estimated number of trees and maintenance requirements and from which, a budget can be developed to prepare a complete inventory. A statistically accurate, quick inventory to establish a data base can be done. However, to consider anything less than a 100% inventory as complete, would be faulty reasoning. This information cannot be used from one year to the next nor used as a base for a working document. This is because trees are maintained individually and work performed on one tree is not guaranteed to be necessary on the next one.
Key to success is to determine what information is needed and how often it will be used. Each community is different and requires different street tree management techniques. Therefore, street tree inventory programs should be adaptable to meet the individual needs of the individual community.
Grow the Right Tree in the Right Place for the Right Purpose
By Gordon Mann
Planting sites can be a major challenge for the arborist and landscape architect working in an urban environment. We have all heard the answer is simply planting the right tree in the right location. But what does this really mean? Before we can select the right tree, we have to understand the purpose for planting the tree. The real mantra should be grow the right tree in the right place for the right purpose. This requires working backwards to define the purpose, understanding the available planting space, and then picking the appropriate tree for the site.
The Right Purpose
Trees are infrastructure elements that provide valuable ecosystem and social services to the property and community where they grow. The services produce benefits that can be quantified. Although it is difficult to quantify exactly the value each individual tree provides, average costs for individual tree species have been calculated by the USDA Forest Service researchers. Benefits from trees to communities include: enhanced air quality, stormwater interception, energy conservation, shade, reduced crime, improved student learning, wildlife habitat, traffic calming, increased property values, and increased retail revenues. A popular method of calculating benefits is the i-Tree program available at no cost.
Most of the services and benefits of trees come from the canopy. The foliar crown is the measure from where the benefits are received, and size does matter with trees. The bigger the canopy, the more benefits the tree provides us. Planting a tree gives us a good feeling but it takes growing a tree to provide the benefits and services.
Selecting the Right Place
Once we know what the desired services and benefits are, we need to look at the available space. If we grow our trees only to have to remove them prematurely due to conflicts at the growing site, we lose the investment in the benefits we sought to obtain.
When selecting the right place, the primary focus should be: Is the site able to provide the soil, the space, and the needs for the tree to reach the size desired? It is very difficult to try to squeeze a large shade tree into a three foot (1 m) wide ribbon of green between the sidewalk and the street, or a cut-out in an expanse of concrete. If the root space is restricted, the tree's development and probably the tree's health will be impacted. Or worse, the adjacent infrastructure is damaged and the repair costs may be significant. Communities growing large trees (trees that grow to 24-inch [60 cm] dbh or larger) in less than six-foot wide planting spaces have a 90% chance of experiencing adjacent infrastructure damage over the life of the tree. Damage can be caused by surface roots or the root crown and flare growing larger.
There are several methods to expand the available root space. Some of the best choices are increasing the width and depth of the planting pit. Root channels and soil space can be deeper and wider, improving the soil deep under the sidewalk leading to soil areas behind the sidewalk. Structural soils or structural cells can be used under the street and sidewalk to provide the necessary construction compaction while also providing space for roots, oxygen, and water.
If adequate root space for the tree to reach its potential is not available, select a smaller stature tree. If there are not any opportunities to enlarge the substandard planting strip, call it a green ribbon, a utility strip, or garden bed and plant attractive flowers or lawns. Using this approach, new trees can be installed behind the sidewalk, on private property, with the landowner’s permission, or in planting easements, where there is adequate room for the roots to grow and crown to spread. The trees can grow faster and achieve a nice canopy over the street in a shorter period of time behind the sidewalk compared to the same tree installed next to the street in a limited growing space, subjected to road salt, exhaust fumes, damage from vehicles, soil compaction, etc.
Selecting the Right Tree
Trees are wonderful organisms with an incredible array of characteristics including: deciduous leaf/evergreen needle size and shape; four seasons of interest and color; height; spread; crown shape; branch structure; flowers; fruit; scent; attraction to wildlife; shade density; ability to withstand heat or cold; pest susceptibility and resistance; and trunk color and texture.
To select the right tree, start with a tree list. Select trees that match the form and aesthetic value suitable for the site. Select trees that possess the attributes you want for the site. Check if its liabilities require more care than the tree is expected to receive. Are there sufficient maintenance funds available to care for the tree?
The soil conditions at the site should be suitable to support the long-term growth of the tree. Consider the soil's nutrient, organic matter, and pH levels. Check the soil type and porosity to be sure it is suitable for the tree being selected.
The mature tree size is one of the most important factors in selecting the right tree. One obvious reason is to not plant a large central leader tree under the utility wires or in too small a space. While this should be obvious, there are occasions when working with residents who may have a certain tree in mind, or while discussing where to install their tree, that something may be overlooked or unknown. The list includes underground utilities, overhead utilities, shade and root competition from other trees, lawn sprinklers, property lines, tree qualities desired by the resident, walkways, driveways, curbs, etc.
Installation sites should be large enough to accommodate the tree's roots at maturity. Published research reports indicate that trees need four (4) square feet of soil surface area for every one (1") inch of trunk diameter the tree is expected to attain, or two (2) cubic feet of soil for every square foot of the future crown projection (which is the area under the drip line). This number is the same as the tree's spread in the nursery catalog description of the tree. The soil should be a minimum three feet (90 cm) deep for normal growth and vigor.
The last important consideration is species diversity. We have learned the hard way, the risk of catastrophic loss from monoculture plantings. Dutch Elm Disease, Emerald Ash Borer, and Chestnut Blight are diseases and pests focused on one genus or species. There is not one perfect tree, and there is not one tree that should be installed solely across an entire community.
Preparing a Tree List
To develop a palate of trees, first prepare a list of trees that will do well with the urban atmosphere; will tolerate urban soils; and will grow despite all the challenges one finds for trees in our cities. A second list could be trees that will do well in suburban soil locations and urban sites with improved soil. The right trees should be resistant to known serious insect and disease problems. The trees on your list should be sorted according to their height, spread, and trunk diameter at maturity, and the minimum amount of surface area in square feet needed for a planting site three feet (90 cm) deep. The hardiness zone is another factor. List trees that are exceptionally tough. The list might also include the typical size of each tree at its maximum growth. The mature sizes are provided by nurseries where the trees may be growing in near perfect soil and space conditions. These numbers may need to be adjusted for your local climate. Irrigation, growing season, and local rainfall may also alter the size and root area requirements. Do not forget to consider heat tolerance due to climate change.
Your list should also include the qualities of each tree. What are the colors the tree has in each of the seasons; what soil pH will it tolerate; what are the leaf characteristics and branching structure; what is the tree's disease and pest resistance; are fruits and drooping branches an issue; how fast will it grow; and which means of transplanting works best. One excellent resource for more information about tree characteristics is the University of Florida website, developed in cooperation with the USDA Forest Service. This site develops a list of trees according to soil, desired tree size, plant attributes, site requirements, etc. Click on various items and a list appears for you. This is a great resource to save on your computer's favorites/bookmark list.
Summary
Selecting a tree is a process. Learn the details about the trees on your plant list, so you can grow the right tree in the right place for the right purpose. First, consider the function and purpose of each tree in the landscape. Is it there to provide shade, screen or frame a view, provide a street canopy, etc.? On the other hand, will the tree block the view of a store, sign, or neighbor? Will the dropping leaves or fruit be a problem to typical urban uses? Next consider the available space. Finally, select the tree with the desired characteristics to suit the purpose and fit the available space.
Gordon Mann is an ASCA Registered Consulting Arborist, an ISA Certified Arborist and Municipal Specialist, PNW-ISA Certified Tree Risk Assessor, and urban forestry consultant in Auburn, CA. He is the past SMA ANSI A300 Committee representative and president of ASCA.
By Gordon Mann
Planting sites can be a major challenge for the arborist and landscape architect working in an urban environment. We have all heard the answer is simply planting the right tree in the right location. But what does this really mean? Before we can select the right tree, we have to understand the purpose for planting the tree. The real mantra should be grow the right tree in the right place for the right purpose. This requires working backwards to define the purpose, understanding the available planting space, and then picking the appropriate tree for the site.
The Right Purpose
Trees are infrastructure elements that provide valuable ecosystem and social services to the property and community where they grow. The services produce benefits that can be quantified. Although it is difficult to quantify exactly the value each individual tree provides, average costs for individual tree species have been calculated by the USDA Forest Service researchers. Benefits from trees to communities include: enhanced air quality, stormwater interception, energy conservation, shade, reduced crime, improved student learning, wildlife habitat, traffic calming, increased property values, and increased retail revenues. A popular method of calculating benefits is the i-Tree program available at no cost.
Most of the services and benefits of trees come from the canopy. The foliar crown is the measure from where the benefits are received, and size does matter with trees. The bigger the canopy, the more benefits the tree provides us. Planting a tree gives us a good feeling but it takes growing a tree to provide the benefits and services.
Selecting the Right Place
Once we know what the desired services and benefits are, we need to look at the available space. If we grow our trees only to have to remove them prematurely due to conflicts at the growing site, we lose the investment in the benefits we sought to obtain.
When selecting the right place, the primary focus should be: Is the site able to provide the soil, the space, and the needs for the tree to reach the size desired? It is very difficult to try to squeeze a large shade tree into a three foot (1 m) wide ribbon of green between the sidewalk and the street, or a cut-out in an expanse of concrete. If the root space is restricted, the tree's development and probably the tree's health will be impacted. Or worse, the adjacent infrastructure is damaged and the repair costs may be significant. Communities growing large trees (trees that grow to 24-inch [60 cm] dbh or larger) in less than six-foot wide planting spaces have a 90% chance of experiencing adjacent infrastructure damage over the life of the tree. Damage can be caused by surface roots or the root crown and flare growing larger.
There are several methods to expand the available root space. Some of the best choices are increasing the width and depth of the planting pit. Root channels and soil space can be deeper and wider, improving the soil deep under the sidewalk leading to soil areas behind the sidewalk. Structural soils or structural cells can be used under the street and sidewalk to provide the necessary construction compaction while also providing space for roots, oxygen, and water.
If adequate root space for the tree to reach its potential is not available, select a smaller stature tree. If there are not any opportunities to enlarge the substandard planting strip, call it a green ribbon, a utility strip, or garden bed and plant attractive flowers or lawns. Using this approach, new trees can be installed behind the sidewalk, on private property, with the landowner’s permission, or in planting easements, where there is adequate room for the roots to grow and crown to spread. The trees can grow faster and achieve a nice canopy over the street in a shorter period of time behind the sidewalk compared to the same tree installed next to the street in a limited growing space, subjected to road salt, exhaust fumes, damage from vehicles, soil compaction, etc.
Selecting the Right Tree
Trees are wonderful organisms with an incredible array of characteristics including: deciduous leaf/evergreen needle size and shape; four seasons of interest and color; height; spread; crown shape; branch structure; flowers; fruit; scent; attraction to wildlife; shade density; ability to withstand heat or cold; pest susceptibility and resistance; and trunk color and texture.
To select the right tree, start with a tree list. Select trees that match the form and aesthetic value suitable for the site. Select trees that possess the attributes you want for the site. Check if its liabilities require more care than the tree is expected to receive. Are there sufficient maintenance funds available to care for the tree?
The soil conditions at the site should be suitable to support the long-term growth of the tree. Consider the soil's nutrient, organic matter, and pH levels. Check the soil type and porosity to be sure it is suitable for the tree being selected.
The mature tree size is one of the most important factors in selecting the right tree. One obvious reason is to not plant a large central leader tree under the utility wires or in too small a space. While this should be obvious, there are occasions when working with residents who may have a certain tree in mind, or while discussing where to install their tree, that something may be overlooked or unknown. The list includes underground utilities, overhead utilities, shade and root competition from other trees, lawn sprinklers, property lines, tree qualities desired by the resident, walkways, driveways, curbs, etc.
Installation sites should be large enough to accommodate the tree's roots at maturity. Published research reports indicate that trees need four (4) square feet of soil surface area for every one (1") inch of trunk diameter the tree is expected to attain, or two (2) cubic feet of soil for every square foot of the future crown projection (which is the area under the drip line). This number is the same as the tree's spread in the nursery catalog description of the tree. The soil should be a minimum three feet (90 cm) deep for normal growth and vigor.
The last important consideration is species diversity. We have learned the hard way, the risk of catastrophic loss from monoculture plantings. Dutch Elm Disease, Emerald Ash Borer, and Chestnut Blight are diseases and pests focused on one genus or species. There is not one perfect tree, and there is not one tree that should be installed solely across an entire community.
Preparing a Tree List
To develop a palate of trees, first prepare a list of trees that will do well with the urban atmosphere; will tolerate urban soils; and will grow despite all the challenges one finds for trees in our cities. A second list could be trees that will do well in suburban soil locations and urban sites with improved soil. The right trees should be resistant to known serious insect and disease problems. The trees on your list should be sorted according to their height, spread, and trunk diameter at maturity, and the minimum amount of surface area in square feet needed for a planting site three feet (90 cm) deep. The hardiness zone is another factor. List trees that are exceptionally tough. The list might also include the typical size of each tree at its maximum growth. The mature sizes are provided by nurseries where the trees may be growing in near perfect soil and space conditions. These numbers may need to be adjusted for your local climate. Irrigation, growing season, and local rainfall may also alter the size and root area requirements. Do not forget to consider heat tolerance due to climate change.
Your list should also include the qualities of each tree. What are the colors the tree has in each of the seasons; what soil pH will it tolerate; what are the leaf characteristics and branching structure; what is the tree's disease and pest resistance; are fruits and drooping branches an issue; how fast will it grow; and which means of transplanting works best. One excellent resource for more information about tree characteristics is the University of Florida website, developed in cooperation with the USDA Forest Service. This site develops a list of trees according to soil, desired tree size, plant attributes, site requirements, etc. Click on various items and a list appears for you. This is a great resource to save on your computer's favorites/bookmark list.
Summary
Selecting a tree is a process. Learn the details about the trees on your plant list, so you can grow the right tree in the right place for the right purpose. First, consider the function and purpose of each tree in the landscape. Is it there to provide shade, screen or frame a view, provide a street canopy, etc.? On the other hand, will the tree block the view of a store, sign, or neighbor? Will the dropping leaves or fruit be a problem to typical urban uses? Next consider the available space. Finally, select the tree with the desired characteristics to suit the purpose and fit the available space.
Gordon Mann is an ASCA Registered Consulting Arborist, an ISA Certified Arborist and Municipal Specialist, PNW-ISA Certified Tree Risk Assessor, and urban forestry consultant in Auburn, CA. He is the past SMA ANSI A300 Committee representative and president of ASCA.
Selection Criteria of Plant Material
By Francisco Ferrini and Alessio Fini
Very often tree selection for urban planting does not take into consideration all of the issues related to survivability and growth performance. Successful planting depends on many factors. Not only is it necessary to use the right type of tree of the highest quality, but it is essential that the site be suitable for the tree. Keep in mind that attractive trees can be selected, but aesthetics alone does not provide a sufficient reason to select trees for a high survival rate and vigorous growth.
Selection Criteria
In Europe, our street tree planning process is slightly different from the models used in the US. Our primary focus is on the selection of trees and to analyze the sensitivity of different species toward global change; and does this change cause significant negative impacts on tree growth. If a species appears to be sensitive to climate change which might harm the tree, select an alternative species. If a species appears to be tolerant to climate change, move to the next step which is site assessment.
Site assessment should consider the following features of trees:
Planting Programs
Not too long ago, many cities planted trees of the same species on street after street. It was nice to see a wide avenue made of identical trees, but unfortunately this practice became outweighed by disadvantages. Rapid development of disease and pest attacks on these weakened city trees made the need to diversify extremely necessary.
Some of the most notable examples of mono-culture problems include the American Elm (Dutch elm
disease), American Chestnut (chestnut blight), Honey Locust (honey locust plant bug), Norway Maple (giant tar spot and verticillium wilt), London Plane tree (canker, anthracnose) and crabapple (scab, fireblight, cedar apple rust and powdery mildew). Currently, the emerald ash borer is killing Ash trees in many states and several Provinces in Canada and the Beech leaf disease is quickly spreading and killing Beech trees growing in Ohio, throughout the tri-state areas, Pennsylvania, Maine and Virginia.
To avoid similar problems in the future, it is clear that uniform monoculture plantings must be avoided. Fortunately, it is possible to gain diversity without giving up the aesthetic advantages of uniformity. Through the careful selection and grouping of trees into similar sizes and textures, communities of trees can be created. This provides a genetic diversity while maintaining a visual uniformity. As new pests and diseases inhabit our woody landscapes, species diversity will be a critical key to minimizing their impact. (Editor's note: an example might be using Betula populifolia ‘Whitespire Sr.’, Fagus sylvatica 'Fastigiata', and Carpinus betulus 'Frans Fontaine' if you are looking for a row of narrow columnar trees with gray, smooth bark. A description of these trees can be seen on the J. Frank Schmidt & Son Co. Reference Guide (see link below in sources).
Carbon Dioxide and Urban Trees
As for carbon dioxide (CO2), the evidence so far suggests that increased CO2 from global warming and air pollution, may result in increased tree growth if other factors (water, temperature, nutrients, etc.) are not limiting. This effect may be temporary for some species according to several researchers. It is also well-known that trees sequester carbon in their tissues at different rates and the amounts are based according to their size at maturity, life span, and growth rate, as well as state of health. At the same time tree care equipment is releasing carbon back into the atmosphere from fossil-fuel emissions. It is therefore not only necessary to perfectly match the species to the site, but also to choose those species that require minimal maintenance.
Planting strategies could be adapted to select species that are not only very efficient in sequestering CO2 but also have a high water use efficiency. Comparative studies are important for helping to understand how future climate changes will affect the carbon and the energy budget of different ecosystems. So it becomes of paramount importance to select species for the urban forest with a low transpiration rate for the amount of carbohydrates produced. The efficiency of water use can also be improved by increasing in the density of tree cover and mulches so that less heat reaches the soil surface and evaporation is kept to a minimum. Under these conditions, the largest possible amount of energy can be used in photosynthesis for the most photosynthates produced per unit of water evaporated.
Maintenance Cost
The selection of trees can cause the annual maintenance cost to vary from 20% to 50% of the purchase and planting price. Pruning requirements have a significant impact on whether the tree will fit your program and budget. Some trees require annual pruning to correct bad habits or excess suckering, e.g. Acer saccharinum, A. negundo, A. rubrum, Crataegus spp., Gleditsia triacanthos inermis, Malus spp., Robinia pseudoacacia, and Tilia x europaea. These trees are, in general, aggressive growers and frequent pruning for several years after planting is necessary to prevent problems in the future.
Compartmentalization or wound response is also important. Acer saccharinum, A. rubrum, Aesculus spp., Betula spp., Fagus spp., Fraxinus spp. and Ulmus spp. do not have a good wound response and resistance to heartwood rot. Even when considering actively growing trees, many show heartwood decay. Trees that require little or no pruning after the initial two to three prunings during the establishment period include Acer saccharum, A. platanoides, Fraxinus spp., Ginkgo biloba, Koelreuteria paniculata, Nyssa sylvatica, Quercus alba, Q. coccinea, and Q. palustris to mention a few.
Reduced Conflicts with Human Activities and Health
Trees do not always bring only benefits: in some cases their presence makes the sharing of space and coexistence difficult, in addition to the expenditure of considerable economic resources for their management. Rarely does a tree throughout its lifetime satisfy or even maintain the objectives for which it was planted. Trees become larger over time, often outgrowing their original growing space both above and below ground. Some trees also produce fruits or seeds that may be troublesome for the municipality and homeowners.
Pollen – The most frequently encountered problems are those linked to the normal cycle of growth and seasonal phrenology of the plants. For example:
Litter – Debris from urban trees is another widespread problem, although it varies in magnitude among trees of many species.
BVOC
It is also well-known and documented that plants emit a substantial amount of biogenic volatile organic compounds (BVOCs) such as isoprene and monoterpenes into the atmosphere. BVOCs react with nitrogen oxides to form ozone. Although their contribution may be small compared with other sources, BVOC emissions from plants could exacerbate a smog problem.
Trees that are well adapted to and thrive in certain environments should not be replaced just because they may be high BVOC emitters. The amount of emissions spent on maintaining a tree that may emit low amounts of BVOCs, but is not well suited to an area, could be considerable and outweigh any possible benefits of low BVOC emission rates. Trees should not be labeled as polluters because their total benefits on air quality and emissions reduction far outweigh the possible consequences of BVOC emissions on ozone concentrations.
The emission of BVOCs increases exponentially with temperature. This means, higher emissions will occur at higher temperatures. In desert climates, locally native trees adapted to drought conditions emit significantly less BVOCs than plants native to wet regions. As discussed above, the formation of ozone is also temperature dependent. Thus, the best way to slow the production of ozone and emission of BVOCs is to reduce urban temperatures and the effect of the urban heat island.
The most effective way to lower temperatures is with an increased canopy cover. The effect on ozone production has only recently been discovered by the scientific community, so extensive and conclusive research has not yet been conducted. Ongoing research demonstrates that BVOCs could protect plants against high temperatures. BVOC emissions are probably increasing with warming and with other factors associated with global change, including changes in land cover. These increases in BVOC emissions could contribute in a significant way (via negative and positive feedback) to the complex processes associated with global warming.
The trees with the highest isoprene emission rates should be planted with caution. For instance, most species from genera Eucalyptus, Casuarina, Liquidambar, Robinia, Liriodendron, Populus, Quercus, Nyssa, Platanus, Salix, and all conifers are important emitters of volatile isoprenoids while others such as Acer and Tilia have low emitting potential.
Without a doubt, whenever possible, careful selection of species and proper placing of trees will avoid, or at least minimize these problems.
By Francisco Ferrini and Alessio Fini
Very often tree selection for urban planting does not take into consideration all of the issues related to survivability and growth performance. Successful planting depends on many factors. Not only is it necessary to use the right type of tree of the highest quality, but it is essential that the site be suitable for the tree. Keep in mind that attractive trees can be selected, but aesthetics alone does not provide a sufficient reason to select trees for a high survival rate and vigorous growth.
Selection Criteria
In Europe, our street tree planning process is slightly different from the models used in the US. Our primary focus is on the selection of trees and to analyze the sensitivity of different species toward global change; and does this change cause significant negative impacts on tree growth. If a species appears to be sensitive to climate change which might harm the tree, select an alternative species. If a species appears to be tolerant to climate change, move to the next step which is site assessment.
Site assessment should consider the following features of trees:
- Bio-ecological – tolerance to soils, compaction, drought, shade, pollution, and disease; low risk to become an invasive species or being a food source for local fauna.
- Functional – reduced conflicts with human activities, less than a healthy and vigorous growth rate, a short life span, reduced pollution absorption, potential conflicts with sidewalks or pavements, toleration to frequent pruning, transplanting, and maintenance.
- Pest Resistance – The selection of pest and disease resistant trees is considered to be the most efficient and long-lasting control against the insects and diseases that plague trees growing in urban areas.
- Climate Factors – Climate changes can proceed to a point where certain species can no longer survive in an ecosystem. We are becoming aware of air pollutants that may have profound effects on pests. There is an endless number of diseases that can be conceived for future urban tree problems. Research is needed around the world to continuing to develop trees that are tolerant to these stresses.
Planting Programs
Not too long ago, many cities planted trees of the same species on street after street. It was nice to see a wide avenue made of identical trees, but unfortunately this practice became outweighed by disadvantages. Rapid development of disease and pest attacks on these weakened city trees made the need to diversify extremely necessary.
Some of the most notable examples of mono-culture problems include the American Elm (Dutch elm
disease), American Chestnut (chestnut blight), Honey Locust (honey locust plant bug), Norway Maple (giant tar spot and verticillium wilt), London Plane tree (canker, anthracnose) and crabapple (scab, fireblight, cedar apple rust and powdery mildew). Currently, the emerald ash borer is killing Ash trees in many states and several Provinces in Canada and the Beech leaf disease is quickly spreading and killing Beech trees growing in Ohio, throughout the tri-state areas, Pennsylvania, Maine and Virginia.
To avoid similar problems in the future, it is clear that uniform monoculture plantings must be avoided. Fortunately, it is possible to gain diversity without giving up the aesthetic advantages of uniformity. Through the careful selection and grouping of trees into similar sizes and textures, communities of trees can be created. This provides a genetic diversity while maintaining a visual uniformity. As new pests and diseases inhabit our woody landscapes, species diversity will be a critical key to minimizing their impact. (Editor's note: an example might be using Betula populifolia ‘Whitespire Sr.’, Fagus sylvatica 'Fastigiata', and Carpinus betulus 'Frans Fontaine' if you are looking for a row of narrow columnar trees with gray, smooth bark. A description of these trees can be seen on the J. Frank Schmidt & Son Co. Reference Guide (see link below in sources).
Carbon Dioxide and Urban Trees
As for carbon dioxide (CO2), the evidence so far suggests that increased CO2 from global warming and air pollution, may result in increased tree growth if other factors (water, temperature, nutrients, etc.) are not limiting. This effect may be temporary for some species according to several researchers. It is also well-known that trees sequester carbon in their tissues at different rates and the amounts are based according to their size at maturity, life span, and growth rate, as well as state of health. At the same time tree care equipment is releasing carbon back into the atmosphere from fossil-fuel emissions. It is therefore not only necessary to perfectly match the species to the site, but also to choose those species that require minimal maintenance.
Planting strategies could be adapted to select species that are not only very efficient in sequestering CO2 but also have a high water use efficiency. Comparative studies are important for helping to understand how future climate changes will affect the carbon and the energy budget of different ecosystems. So it becomes of paramount importance to select species for the urban forest with a low transpiration rate for the amount of carbohydrates produced. The efficiency of water use can also be improved by increasing in the density of tree cover and mulches so that less heat reaches the soil surface and evaporation is kept to a minimum. Under these conditions, the largest possible amount of energy can be used in photosynthesis for the most photosynthates produced per unit of water evaporated.
Maintenance Cost
The selection of trees can cause the annual maintenance cost to vary from 20% to 50% of the purchase and planting price. Pruning requirements have a significant impact on whether the tree will fit your program and budget. Some trees require annual pruning to correct bad habits or excess suckering, e.g. Acer saccharinum, A. negundo, A. rubrum, Crataegus spp., Gleditsia triacanthos inermis, Malus spp., Robinia pseudoacacia, and Tilia x europaea. These trees are, in general, aggressive growers and frequent pruning for several years after planting is necessary to prevent problems in the future.
Compartmentalization or wound response is also important. Acer saccharinum, A. rubrum, Aesculus spp., Betula spp., Fagus spp., Fraxinus spp. and Ulmus spp. do not have a good wound response and resistance to heartwood rot. Even when considering actively growing trees, many show heartwood decay. Trees that require little or no pruning after the initial two to three prunings during the establishment period include Acer saccharum, A. platanoides, Fraxinus spp., Ginkgo biloba, Koelreuteria paniculata, Nyssa sylvatica, Quercus alba, Q. coccinea, and Q. palustris to mention a few.
Reduced Conflicts with Human Activities and Health
Trees do not always bring only benefits: in some cases their presence makes the sharing of space and coexistence difficult, in addition to the expenditure of considerable economic resources for their management. Rarely does a tree throughout its lifetime satisfy or even maintain the objectives for which it was planted. Trees become larger over time, often outgrowing their original growing space both above and below ground. Some trees also produce fruits or seeds that may be troublesome for the municipality and homeowners.
Pollen – The most frequently encountered problems are those linked to the normal cycle of growth and seasonal phrenology of the plants. For example:
- There can be problems connected with flowers (pollen production) which, in addition to the release of allergens, include the attraction of insects, production of fleshy fruits and sometimes problems related to the structural stability of the tree. The pollen responsible for the principal allergic reaction comes from, anemophilous (wind pollinated) species which, in general, produce large quantities of pollen and depend on wind for spreading the pollen to nearby trees. Entomophilous species are characterized by pollen which is frequently heavy and spread by insects, and because it is poorly dispersed through the air, it is rarely present at concentrations sufficient to trigger an allergic reaction. However, there are exceptions such as Tilia, an entomophilous (insect pollinated) genus that often causes an allergic response.
- The spread of pollen grains in the environment depends not only on the quantity produced but also on climatic events during flowering (e.g. wind, rainfall, atmospheric humidity) and the presence of barriers to their diffusion (e.g. vegetation, buildings, etc.). For example, conifers rank first for individual quantity of pollen produced, but they are toward the bottom of the ranking of species responsible for allergic reactions (with the exception of Cupressus sempervirens). Conversely, Graminaceae are at the top of the list in terms of allergenity but they are individually modest producers of pollen yet are largely spontaneous and wide-spread in nature and often highly concentrated in large biophytic associations.
- Climate change will probably trigger some changes in the species distribution and might enhance pollen production which could, in turn, increase the risk of aggravating allergies. Meteorological factors strongly influence the timing and duration of the pollen season as well as the total pollen count, thus the seasonality of pollen-related disorders such as hay fever may be affected by climate change.
Litter – Debris from urban trees is another widespread problem, although it varies in magnitude among trees of many species.
- Falling fruits can dirty an environment or give off unpleasant odors (i.e. Ginkgo biloba) or cause damage from large or particularly hard fruits (e.g. the cones of Pinus pinea). Even the simple falling of leaves can cause troubles when the pavement becomes slippery. The trees that produce the most litter in the urban environment might include: Liquidambar styraciflua and the ball-like fruits of plane trees Platanus x acerifolia which are sometimes extensively planted along urban streets, yet their fruits are a vexing litter problem. Crataegus x lavallei and Malus spp. are frequently recommended for street planting (especially in narrow streets and under overhead utility lines), and for their stress tolerance, but the fruits can increase the risk of pedestrian falling. Full-grown Prunus pissardi, are among the most appreciated ornamental trees, but their fruits are an intolerable nuisance, because they litter roads and sidewalks. In general, fleshy fruits are usually messy, but other types of fruit can also be annoying. Examples include the pods of Ceratonia siliqua, Gleditsia triacanthos, Robinia pseudoacacia, and Styphnolobium japonicum. Nuts, like those of Quercus rubra, produce an abundance of fruits and are also a major problem to clean up. In countries with Mediterranean-like climate the use of olive trees as ornamental plants can be suggested for the favorable properties of this species; but in an urban setting olive trees can be messy.
- The best solution to the fruit problem is to use non-fruiting species and cultivars. In dioecious species (i.e. Ginkgo biloba and Gymnocladus dioicus) propagation of male individuals is the easiest way to obtain
non-fruiting trees. Male cultivars of maples and ash are also very popular in areas where emerald ash borer is not a concern. Some selections of fruitless sweetgum and olive trees have been patented and their use is suggested over patios, walks, lawns, driveways, and streets and anywhere drought tolerant species are needed.
- Male trees are not always a good choice if the tree happens to be a wind blown pollen producer such as maple, oak, and pine installed in an area where people are subject to tree pollen allergies.
- Sometimes it is not the plant itself that can cause a negative interaction with human life but the pests that attack it. For example, both pine and oak processionary moth have spread throughout Europe. These two species create population explosions in limited areas at time intervals of approximately 20 years, commonly lasting for two or three years. Small stings from these caterpillars contain thaumetopoein, a nettle-poison. Now, with climate change they start to migrate a month earlier in the spring.
- Attacks of aphids on street trees can produce a strong production of honey dew that makes cars, sidewalks and pavements sticky and dirty.
BVOC
It is also well-known and documented that plants emit a substantial amount of biogenic volatile organic compounds (BVOCs) such as isoprene and monoterpenes into the atmosphere. BVOCs react with nitrogen oxides to form ozone. Although their contribution may be small compared with other sources, BVOC emissions from plants could exacerbate a smog problem.
Trees that are well adapted to and thrive in certain environments should not be replaced just because they may be high BVOC emitters. The amount of emissions spent on maintaining a tree that may emit low amounts of BVOCs, but is not well suited to an area, could be considerable and outweigh any possible benefits of low BVOC emission rates. Trees should not be labeled as polluters because their total benefits on air quality and emissions reduction far outweigh the possible consequences of BVOC emissions on ozone concentrations.
The emission of BVOCs increases exponentially with temperature. This means, higher emissions will occur at higher temperatures. In desert climates, locally native trees adapted to drought conditions emit significantly less BVOCs than plants native to wet regions. As discussed above, the formation of ozone is also temperature dependent. Thus, the best way to slow the production of ozone and emission of BVOCs is to reduce urban temperatures and the effect of the urban heat island.
The most effective way to lower temperatures is with an increased canopy cover. The effect on ozone production has only recently been discovered by the scientific community, so extensive and conclusive research has not yet been conducted. Ongoing research demonstrates that BVOCs could protect plants against high temperatures. BVOC emissions are probably increasing with warming and with other factors associated with global change, including changes in land cover. These increases in BVOC emissions could contribute in a significant way (via negative and positive feedback) to the complex processes associated with global warming.
The trees with the highest isoprene emission rates should be planted with caution. For instance, most species from genera Eucalyptus, Casuarina, Liquidambar, Robinia, Liriodendron, Populus, Quercus, Nyssa, Platanus, Salix, and all conifers are important emitters of volatile isoprenoids while others such as Acer and Tilia have low emitting potential.
Without a doubt, whenever possible, careful selection of species and proper placing of trees will avoid, or at least minimize these problems.
Principles of Tree Selection
People who regard the use of trees in the urban environment as an aesthetic nicety do not see the whole picture. Arborists and landscape architects must increasingly enumerate and quantify the many functions that trees play in order to justify the need for continued funding of the green urban environment in times of restricted budgets.
What functions do trees play in the urban environment?
What about aesthetic appeal?
An appreciation of the vast aesthetic possibilities that trees offer can be overwhelming; yet, by embracing this diversity of seasonal interest, form, color, and texture we can create wonderfully inventive landscapes. The realization of a design vision for a site and its ultimate success as a landscape requires more than creative elements of space, line, and form. It requires a thorough understanding of how people will use the site and how the site may or may not support the long-term biological needs of trees growing there.
How well is the tree adapted to the site?
All trees have a genetic potential to grow to a certain size and shape at a given rate under optimal conditions, but site conditions rarely enable a tree to grow to its full genetic potential. Knowing the tree's potential and matching its needs to the site's ability to meet those needs is the key to achieving the realization of a design vision. If enough of a tree's needs are met, such as appropriate levels of light, water, nutrients, temperatures, oxygen and carbon dioxide, we can have confidence that the proposed planting will develop into what was originally envisioned.
With all the potential choices in plant materials, the selection process of tree adaptability to site conditions can seem overwhelming. Moreover, it is important to know under what conditions trees will grow satisfactorily, if not optimally. There are many references that can help arborists and landscape architects learn about appropriate planting conditions.
How do we begin matching the tree to the site?
Space – The first consideration for successful tree growth is how that tree can fit into its envisioned space. Given the vast number of tree choices, coming up with the desired size, growth rate, soil, and form for tree growth on the site requires careful selection.
In the urban environment, clear sight lines for visual access can be important for pedestrian and vehicular safety. It is important that a tree be chosen that can be pruned to provide these conditions. Or will the natural form of the tree be so altered to fit into a particular space that its aesthetic appeal is lost?
While space for a tree above ground is important, space for root development is even more important. Trees will only grow if there is sufficient space and good soil for the roots to grow. A tree forced to grow in a small cube of soil in a sidewalk will not thrive and will be much smaller than the same tree growing on a lawn area on either side of the sidewalk.
Hardiness – Tree adaptability to extremes of heat and cold would be the next consideration when choosing trees. Knowing the USDA Hardiness Zone of your site is essential. With global warming getting worse, it is also important to understand the Heat Zone Map and select trees that will tolerate heat from two zones warmer than the site. Any micro-climatic factors such as re-radiated heat, wind, rain, shadows, and frost pockets. There are often sheltered areas in the urban environment that enable a wider range of trees to be grown than would be possible in surrounding rural areas. This is because of temperature, light, and wind-altering effects of buildings and built surfaces. Raised planters can experience lower soil temperatures than the ground. In addition, references sometimes don't agree on the hardiness parameters of trees, and cultivars may have different hardiness ratings compared to the species. It would be wise to consult several local references and experienced professionals. When there is a discrepancy, choose conservatively. Here is a list of heat tolerant trees.
Sun/Shade – Most trees require full sun (at least 4-6 hours daily) to grow to their envisioned size and form. Occasionally a few smaller trees may tolerate partial shade. This can be a significant issue in many landscapes. When it comes to shrubs or perennials however, there are many more choices for plants that prefer partial sun or moderate shade.
Soil Moisture – In the natural environment, trees are often grouped by their ability to tolerate similar soil conditions such as soil moisture. These are important parameters that should be used to choose trees. Some trees have the ability to tolerate a wide range of soil moisture conditions, from flooding on one extreme to drought on the other.
Where soil volumes are limited, it is common for trees to experience alternating periods of too wet and too dry soil conditions. When it rains, water does not drain away and the roots experience oxygen deprivation. When the soil eventually dries out, the roots are in such a small area that there is too little water to support tree growth.
Where soil volumes are large and drainage is good, it is possible to grow a large number of trees including those that cannot adapt to alternating soil-moisture extremes.
Knowing the conditions that determine moisture and oxygen availability in the soil are the keys to good design and successful tree establishment. Soil texture, depth, volume, density, drainage, and the presence of irrigation will determine how water and air are available in the soil.
Soils that are seasonably wet, but otherwise well-drained will accommodate a larger number of trees than those that are continually wet during the growing season or those that alternate between very wet and very dry. Sites that are dry most of the growing season will also pose challenges in tree selection. It might be necessary to modify soil conditions to provide a better balance between too wet and too dry conditions in order to grow trees successfully on the site. If the soil cannot be easily modified to overcome extreme moisture conditions, the number of tree choices for the site will be severely limited.
Soil pH – Testing soil pH is a simple, inexpensive technique that provides vital information about nutrient availability. Although most trees grow best at a slightly acid pH, there are many trees that will grow well in soil with a pH as high as 8.2. Fewer trees will grow well in extremely acid soils of pH 3.5 - 4.5.
It is important that trees be chosen that will do well in the existing soil pH. It is often very difficult to change soil pH, especially when trying to make a permanent change. It is better to choose trees that tolerate the existing soil pH. Research shows that many trees will have acceptable growth in a wide pH range.
Salts – When developing a planting plan, consider trees that tolerate deicing salts and seashores. There are places where salts may be wind-blown to a site, such as along the seashore or next to a highway during winter in northern climates. Salt may also accumulate in soil related conditions as in arid areas, and salt-water intrusions into soils from seawater. All woody trees will be killed by some level of salts, but some trees are quite salt-tolerant. Most salts are quite water-soluble and will wash through the root zone after a good rain if soils are well-drained, but poor soil drainage can exacerbate salt contamination. This benefit of good drainage is clearly illustrated in areas where spring rains wash away the road-salt runoff in northern climates.
What are the management issues associated with the tree?
Beyond environmental factors, there are other management concerns that are important in tree selection, including the trees' susceptibility to pests and pathogens, ability to be transplanted, availability in the trade, maintenance requirements, etc.
Pests and Diseases – Susceptibility of trees to insect and disease attack is a significant factor in tree selection. Trees that will not need pesticides to grow well should always be a first choice. Check with local growers to find out about locally troublesome pests for a particular tree. Most trees have some pest problems, so avoid trees that have severe pest problems. There is excellent information available in books, the Internet, and local universities and nurseries about species and cultivars that are resistant to particular pests and diseases. J. Frank Schmidt & Son website is an excellent source for specific information about trees including disease resistance (see their link to the Crabapple Information Chart in the Sources list below). Also, by installing trees in sites well suited for a particular tree, the species will generally suffer fewer pest problems or will be able to recover from an attack more rapidly.
Woody trees that suffer from a lack of water are more susceptible to potentially damaging boring insects. Many times there may be reasonable substitutions for trees that may be troubled by a certain disease or insect.
Dwindling landscape maintenance budgets and a growing desire to reduce or eliminate the use of chemicals for disease and insect control are driving the demand for hardy, durable, disease-resistant, sustainable trees that require minimal pruning, irrigation, maintenance, and ongoing care. If budgets allow, Arborists and landscape architects should try a few new introductions from nurseries every year to see if they are pest and disease resistant in your city. Also, according to recent nursery reports, the demand for native trees and cultivars of native trees is stronger than ever.
Cost and Availability – Many desirable trees are not available in appropriate sizes or are too costly to be specified for a landscape planting. Therefore the specifier should substitute trees that are available to meet close to what is desired. Consideration should be given to costs and availability of bare root versus balled and burlapped trees. Also consider that less expensive smaller trees will recover from transplant shock much faster than a large diameter tree of the same species. For example, a six-inch diameter tree may be desired for a particular site, but it will require about 6 years to recover from transplanting. Meanwhile, a two-inch diameter tree will recover in a couple of years and will resume normal growth so that in six years it most likely surpassed the larger tree in size.
Transplanting – Several trees and shrubs are notably difficult to transplant. There are practices that help insure greater success in transplanting, yet some remain difficult even when appropriate transplanting measures are taken. It would be reasonable not to choose too many of the troublesome trees in any one particular installation.
Timing is always important when transplanting. The arborist/landscape architect/designer may want to consider availability of trees and inform the contractor to be sure that species and sizes specified can be located. Alternatively, horticultural brokers may be hired to find trees that otherwise would be difficult to obtain. Having gone through the process of site assessment and making the best selections, it is worth trying to find the best trees before contractor substitutions need to be accepted.
Maintenance Issues – Some trees produce an unacceptable amount of fruit or leaf litter under certain circumstances or require regular pruning for an appropriate appearance. There may be small-fruited or fruitless alternatives that have less of a litter problem. If regular maintenance cannot be provided, shrubs and trees that need less maintenance should be selected.
Thorns on trees may be an issue in settings where people are likely to be in direct contact with the trees, such as a playground or a narrow street or alley.
“Weak-wooded” is a term that refers to the propensity of trees to break up or drop branches in high winds or during snow and ice storms. Often, fast-growing trees are the most frequent offenders. Weak-wooded trees should not be planted in areas with lots of pedestrian traffic or where property may be damaged. Trees that may be perfectly acceptable in a natural area may not be suited to certain urban conditions.
Native vs. non-native trees – The most important factor for the success of a tree in the landscape is its adaptability to the site and not its original geographic origin. Most urban sites have been so greatly altered that the original native soil conditions, prior to centuries of development, are irrelevant for the selection of trees today. If a tree was native to parts of New York City in the 16th century, it may not be the right tree for anywhere in New York City today, except perhaps in Central Park and even that was altered substantially a hundred years ago. Cultivars of the native tree on the other hand, may be the right tree since they are often developed for their urban tolerance. Invasive trees should always be avoided unless it is for a site where nothing else will grow.
Diversity vs. Uniformity – The selection and placement of trees in the urban environment is a complex task requiring the consideration of many factors. Issues such as visual access, spatial constraints, and disease and insect resistance can sometimes conflict with design objectives. Perhaps the most troubling conflict arises between the preference for visual uniformity and the practical need for biological and species diversity. Until recently a typical street tree planting consisted of uniform rows of a single species, generally selected for its attractive appearance and high tolerance to urban stresses. However, as over planting has brought about the decline of a number of favorite species such as the American elm, it is clear that design objectives must be balanced against the benefits of species diversity in street tree installations. This is where the arborist and landscape architect should be trying new introductions to see how well they perform in their city.
People who regard the use of trees in the urban environment as an aesthetic nicety do not see the whole picture. Arborists and landscape architects must increasingly enumerate and quantify the many functions that trees play in order to justify the need for continued funding of the green urban environment in times of restricted budgets.
What functions do trees play in the urban environment?
- Where summer temperatures are very high, trees provide shade and reduced air temperatures.
- Tree planting to reduce wind speeds has long been practiced around the world. Research shows that semi-porous windscreens that may include trees and shrubs can have a profound effect on reducing wind. A barrier or hedgerow of approximately 35% transparent material can create a long calm zone that can improve human comfort levels.
- A few trees alone do a poor job of reducing noise. However, a dense planting, especially when combined with solid barriers or landforms, can reduce noise significantly.
- Trees may play a role in reducing air pollution, both particulate and gaseous pollutants. More research is needed on the effectiveness of certain species in the reduction of air-borne pollutants in our urban areas.
- Trees play a crucial role in reducing soil erosion as well as trapping and slowing the storm-water runoff.
- Trees can significantly reduce our reliance on storm-water abatement systems, improve natural water infiltration, and reduce the velocity of water moving over a landscape.
- Urban green spaces provide animal habitat, opportunities for recreation, and are places where most people connect with the natural world.
- Tree-lined streets are safer and happier places to have a home.
- Cities with well-cared-for parks are always listed as more desirable places to live.
- Property values are significantly higher near well-cared-for green spaces, just as they are for properties that are considered well landscaped.
- Trees can change the sense of scale to a more human dimension.
- Trees may be used to design physical barriers, direct foot traffic, or screen unsightly views.
- Notable features and architectural lines can be enhanced with appropriate planting.
- Historic neighborhoods often have appropriate landscapes including large trees that are intrinsic to the historical experience and sense of place.
What about aesthetic appeal?
An appreciation of the vast aesthetic possibilities that trees offer can be overwhelming; yet, by embracing this diversity of seasonal interest, form, color, and texture we can create wonderfully inventive landscapes. The realization of a design vision for a site and its ultimate success as a landscape requires more than creative elements of space, line, and form. It requires a thorough understanding of how people will use the site and how the site may or may not support the long-term biological needs of trees growing there.
How well is the tree adapted to the site?
All trees have a genetic potential to grow to a certain size and shape at a given rate under optimal conditions, but site conditions rarely enable a tree to grow to its full genetic potential. Knowing the tree's potential and matching its needs to the site's ability to meet those needs is the key to achieving the realization of a design vision. If enough of a tree's needs are met, such as appropriate levels of light, water, nutrients, temperatures, oxygen and carbon dioxide, we can have confidence that the proposed planting will develop into what was originally envisioned.
With all the potential choices in plant materials, the selection process of tree adaptability to site conditions can seem overwhelming. Moreover, it is important to know under what conditions trees will grow satisfactorily, if not optimally. There are many references that can help arborists and landscape architects learn about appropriate planting conditions.
How do we begin matching the tree to the site?
Space – The first consideration for successful tree growth is how that tree can fit into its envisioned space. Given the vast number of tree choices, coming up with the desired size, growth rate, soil, and form for tree growth on the site requires careful selection.
In the urban environment, clear sight lines for visual access can be important for pedestrian and vehicular safety. It is important that a tree be chosen that can be pruned to provide these conditions. Or will the natural form of the tree be so altered to fit into a particular space that its aesthetic appeal is lost?
While space for a tree above ground is important, space for root development is even more important. Trees will only grow if there is sufficient space and good soil for the roots to grow. A tree forced to grow in a small cube of soil in a sidewalk will not thrive and will be much smaller than the same tree growing on a lawn area on either side of the sidewalk.
Hardiness – Tree adaptability to extremes of heat and cold would be the next consideration when choosing trees. Knowing the USDA Hardiness Zone of your site is essential. With global warming getting worse, it is also important to understand the Heat Zone Map and select trees that will tolerate heat from two zones warmer than the site. Any micro-climatic factors such as re-radiated heat, wind, rain, shadows, and frost pockets. There are often sheltered areas in the urban environment that enable a wider range of trees to be grown than would be possible in surrounding rural areas. This is because of temperature, light, and wind-altering effects of buildings and built surfaces. Raised planters can experience lower soil temperatures than the ground. In addition, references sometimes don't agree on the hardiness parameters of trees, and cultivars may have different hardiness ratings compared to the species. It would be wise to consult several local references and experienced professionals. When there is a discrepancy, choose conservatively. Here is a list of heat tolerant trees.
Sun/Shade – Most trees require full sun (at least 4-6 hours daily) to grow to their envisioned size and form. Occasionally a few smaller trees may tolerate partial shade. This can be a significant issue in many landscapes. When it comes to shrubs or perennials however, there are many more choices for plants that prefer partial sun or moderate shade.
Soil Moisture – In the natural environment, trees are often grouped by their ability to tolerate similar soil conditions such as soil moisture. These are important parameters that should be used to choose trees. Some trees have the ability to tolerate a wide range of soil moisture conditions, from flooding on one extreme to drought on the other.
Where soil volumes are limited, it is common for trees to experience alternating periods of too wet and too dry soil conditions. When it rains, water does not drain away and the roots experience oxygen deprivation. When the soil eventually dries out, the roots are in such a small area that there is too little water to support tree growth.
Where soil volumes are large and drainage is good, it is possible to grow a large number of trees including those that cannot adapt to alternating soil-moisture extremes.
Knowing the conditions that determine moisture and oxygen availability in the soil are the keys to good design and successful tree establishment. Soil texture, depth, volume, density, drainage, and the presence of irrigation will determine how water and air are available in the soil.
Soils that are seasonably wet, but otherwise well-drained will accommodate a larger number of trees than those that are continually wet during the growing season or those that alternate between very wet and very dry. Sites that are dry most of the growing season will also pose challenges in tree selection. It might be necessary to modify soil conditions to provide a better balance between too wet and too dry conditions in order to grow trees successfully on the site. If the soil cannot be easily modified to overcome extreme moisture conditions, the number of tree choices for the site will be severely limited.
Soil pH – Testing soil pH is a simple, inexpensive technique that provides vital information about nutrient availability. Although most trees grow best at a slightly acid pH, there are many trees that will grow well in soil with a pH as high as 8.2. Fewer trees will grow well in extremely acid soils of pH 3.5 - 4.5.
It is important that trees be chosen that will do well in the existing soil pH. It is often very difficult to change soil pH, especially when trying to make a permanent change. It is better to choose trees that tolerate the existing soil pH. Research shows that many trees will have acceptable growth in a wide pH range.
Salts – When developing a planting plan, consider trees that tolerate deicing salts and seashores. There are places where salts may be wind-blown to a site, such as along the seashore or next to a highway during winter in northern climates. Salt may also accumulate in soil related conditions as in arid areas, and salt-water intrusions into soils from seawater. All woody trees will be killed by some level of salts, but some trees are quite salt-tolerant. Most salts are quite water-soluble and will wash through the root zone after a good rain if soils are well-drained, but poor soil drainage can exacerbate salt contamination. This benefit of good drainage is clearly illustrated in areas where spring rains wash away the road-salt runoff in northern climates.
What are the management issues associated with the tree?
Beyond environmental factors, there are other management concerns that are important in tree selection, including the trees' susceptibility to pests and pathogens, ability to be transplanted, availability in the trade, maintenance requirements, etc.
Pests and Diseases – Susceptibility of trees to insect and disease attack is a significant factor in tree selection. Trees that will not need pesticides to grow well should always be a first choice. Check with local growers to find out about locally troublesome pests for a particular tree. Most trees have some pest problems, so avoid trees that have severe pest problems. There is excellent information available in books, the Internet, and local universities and nurseries about species and cultivars that are resistant to particular pests and diseases. J. Frank Schmidt & Son website is an excellent source for specific information about trees including disease resistance (see their link to the Crabapple Information Chart in the Sources list below). Also, by installing trees in sites well suited for a particular tree, the species will generally suffer fewer pest problems or will be able to recover from an attack more rapidly.
Woody trees that suffer from a lack of water are more susceptible to potentially damaging boring insects. Many times there may be reasonable substitutions for trees that may be troubled by a certain disease or insect.
Dwindling landscape maintenance budgets and a growing desire to reduce or eliminate the use of chemicals for disease and insect control are driving the demand for hardy, durable, disease-resistant, sustainable trees that require minimal pruning, irrigation, maintenance, and ongoing care. If budgets allow, Arborists and landscape architects should try a few new introductions from nurseries every year to see if they are pest and disease resistant in your city. Also, according to recent nursery reports, the demand for native trees and cultivars of native trees is stronger than ever.
Cost and Availability – Many desirable trees are not available in appropriate sizes or are too costly to be specified for a landscape planting. Therefore the specifier should substitute trees that are available to meet close to what is desired. Consideration should be given to costs and availability of bare root versus balled and burlapped trees. Also consider that less expensive smaller trees will recover from transplant shock much faster than a large diameter tree of the same species. For example, a six-inch diameter tree may be desired for a particular site, but it will require about 6 years to recover from transplanting. Meanwhile, a two-inch diameter tree will recover in a couple of years and will resume normal growth so that in six years it most likely surpassed the larger tree in size.
Transplanting – Several trees and shrubs are notably difficult to transplant. There are practices that help insure greater success in transplanting, yet some remain difficult even when appropriate transplanting measures are taken. It would be reasonable not to choose too many of the troublesome trees in any one particular installation.
Timing is always important when transplanting. The arborist/landscape architect/designer may want to consider availability of trees and inform the contractor to be sure that species and sizes specified can be located. Alternatively, horticultural brokers may be hired to find trees that otherwise would be difficult to obtain. Having gone through the process of site assessment and making the best selections, it is worth trying to find the best trees before contractor substitutions need to be accepted.
Maintenance Issues – Some trees produce an unacceptable amount of fruit or leaf litter under certain circumstances or require regular pruning for an appropriate appearance. There may be small-fruited or fruitless alternatives that have less of a litter problem. If regular maintenance cannot be provided, shrubs and trees that need less maintenance should be selected.
Thorns on trees may be an issue in settings where people are likely to be in direct contact with the trees, such as a playground or a narrow street or alley.
“Weak-wooded” is a term that refers to the propensity of trees to break up or drop branches in high winds or during snow and ice storms. Often, fast-growing trees are the most frequent offenders. Weak-wooded trees should not be planted in areas with lots of pedestrian traffic or where property may be damaged. Trees that may be perfectly acceptable in a natural area may not be suited to certain urban conditions.
Native vs. non-native trees – The most important factor for the success of a tree in the landscape is its adaptability to the site and not its original geographic origin. Most urban sites have been so greatly altered that the original native soil conditions, prior to centuries of development, are irrelevant for the selection of trees today. If a tree was native to parts of New York City in the 16th century, it may not be the right tree for anywhere in New York City today, except perhaps in Central Park and even that was altered substantially a hundred years ago. Cultivars of the native tree on the other hand, may be the right tree since they are often developed for their urban tolerance. Invasive trees should always be avoided unless it is for a site where nothing else will grow.
Diversity vs. Uniformity – The selection and placement of trees in the urban environment is a complex task requiring the consideration of many factors. Issues such as visual access, spatial constraints, and disease and insect resistance can sometimes conflict with design objectives. Perhaps the most troubling conflict arises between the preference for visual uniformity and the practical need for biological and species diversity. Until recently a typical street tree planting consisted of uniform rows of a single species, generally selected for its attractive appearance and high tolerance to urban stresses. However, as over planting has brought about the decline of a number of favorite species such as the American elm, it is clear that design objectives must be balanced against the benefits of species diversity in street tree installations. This is where the arborist and landscape architect should be trying new introductions to see how well they perform in their city.
Tree Diversification
A broad diversity of trees is needed in our urban landscape to guard against the possibility of large-scale devastation by natural disasters as well as native and introduced insect pests and diseases.
History
The widespread planting of American Elms in the towns and cities of eastern United States by our forefathers was a nice idea. Our early horticulturists were taking advantage of the beauty and adaptability of the American Elm, a native tree that Thomas Jefferson called “nature’s noblest vegetable”. The accidental introduction of Dutch elm disease, and the consequential destruction of millions of elms, served not only to focus attention on the loss of urban forests but also to the qualities of the American elm. So arborists began widespread planting of ash trees and that did not go very well either.
Role of Diversity
We now realize that we need a diversity of trees in our urban forests, not only to guard against disasters like Dutch elm disease, but also to “grow the right tree in the right place” as the evolution of our cities and suburbs creates challenges and new settings for tree planting.
Diversity plays an important role in the long-term stability of an ecosystem. When an area has a high diversity of tree species, it is less likely to suffer catastrophic loss from diseases or pests. For example, consider the impact the Emerald Ash Borer (EAB) has had on ash trees. The Emerald Ash Borer (Agrilus planipennis), an exotic wood-boring beetle discovered in southeast Michigan in the summer of 2002, has killed at least tens of millions of ash trees so far and threatens to kill most of the 8.7 billion ash trees throughout North America. Installing a diversity of hardy tree species throughout the urban forest can reduce the vulnerability of the trees to species specific pests.
Age Diversity
Consideration should be given to diversity by age within the urban forest. Planting the entire forest at one time will result in a future collapse of the entire forest at one time. A rule may also apply to age if the age of the existing forest and the number of existing trees are known. By using these two numbers one can prepare an age diversification factor for the tree replacement plan. For example: if the oldest tree in the urban forest died at 100 years of age and the tree inventory indicates a maximum capacity of 30,000 trees in the urban forest, then simple math tells us that 300 trees should be planted every year to create a diversity by age.
Diversity by Shapes
If given enough space to grow, trees have characteristic shapes. Some shapes fit better in a space than others. Because of their vase shape, when hackberries and American elms are planted across the street from each other, their crowns form a cathedral-like arch over the street. A tree with a rounded crown will shade the yard. Pyramidal-shaped trees with dense persistent leaves provide dense shade, and are better at breaking the wind speed nearer the ground. A pyramidal-shaped tree also takes up more space near the ground. Trees with a weeping stature provide focal points in a landscape as do columnar trees. Columnar trees are also suitable in locations where space is tight or a tall hedge is desired.
Diversity of Food Value
A diversity of trees with high food value is the single best way to increase wildlife populations. For example:
Keep in mind that every tree must be carefully selected for each location to be sure it is appropriate. For example do not install ash if EAB is on its way to your city and sweetgums with thorny fruit balls should not be installed where pedestrians are walking. In a sidewalk installation, Honeylocust might be the only tree suitable for survival in that location. Therefore Honeylocust should not be installed in other locations if it is close to meeting the diversification limit within the city or region.
A Diversity of Wildlife Habitats
Birds and small animals need concealed spaces for nesting and hiding to protect them from the eyes of predators. Planting conifers in groups, growing hedges with low branches, or using prickly or thorny plants in a few areas are all ways to provide wildlife cover.
The Ten-Percent Rule
The “10% rule” is a reaction to the likelihood that some major insect or disease pest could, at some point in time, wipe out most of the trees in a city. However, based on the latest research and study of the impact that EAB has made on all species of ash, the 10% rule has now become the 5% rule. Urban foresters, landscape architects, municipal arborists and/or anyone responsible for the selection of trees, should use the following guidelines for tree diversity within their areas of jurisdiction:
1. plant no more than 5% of any species;
2. no more than 20% of any genus;
3. no more than 30% of any family.
Individual trees as well as strips or blocks of uniformly planted species, cultivars, or clones with proven adaptability should be scattered throughout the city to achieve spatial as well as biological diversity.
Originally the rule was considered a safeguard against a “new pest” that might be introduced from a foreign country. The American experience with Dutch elm disease and chestnut blight is sufficient to explain our concern about such epidemics. However, the spread of dogwood anthracnose disease on our native, Cornus florida, has caused great alarm because it did not come from an introduced pathogen. There are also many “native” insect problems that we are aware of and that must considered as potential threats to the urban forest.
Enacting Species Diversity
While the new “5% rule” may serve as a target or goal to soothe the consciences of city councils and municipal arborists, it will not solve all potential pest problems nor guarantee the long-term stability and aesthetics of the urban forest. We need to install more of survivor trees developed through genetic research and tree selections from difficult sites. We need to utilize the practical experience from practitioners of urban forestry. We have to plan the installation of city trees and understand the problems and potentials of our actions.
Enacting a species diversity plan implies three simple actions:
1. Minimize planting overused species;
2. Increase the planting of underused species;
3. Introduce new species and cultivars into the landscape.
To achieve each of these elements, a plan containing eight tasks is necessary. The first two tasks focus on evaluating the current and potential tree resources. The remaining six tasks outline a systematic process for establishing thresholds for each species and circulating species on and off of the annual planting list.
Task 1 – Compile and assess data on the current tree population and program. An inventory is the easiest tool for collecting and summarizing this data. The necessary information includes:
Task 2 – Develop a list of all potential species to plant in the area. The list should include everything from small to large and abundant to obscure. This list can be derived from a number of sources such as:
Task 3 – Establish a target for each species on your current list. To minimize an over emphasis of any one species on the list, a cap for each species should be established. This cap should be a percentage of the total possible number of trees that can be present in the community (existing number of trees plus the number of vacant planting sites).
Task 4 – Develop a short list of species to plant each year. This list may have anywhere from five to twenty species. This is the list of species that are going to be emphasized in the current year's planting program. The list should incorporate a variety of species in various sizes.
Task 5 – Create a threshold for each species on your annual planting list. To minimize an over emphasis of any one species on the annual list, a cap for each species should be established. As that threshold is reached, that particular species is taken off of the list for the remainder of the year. A careful balance of all of suggested species for the year will guarantee an even and equitable distribution of the trees. Keep in mind that throughout most of the US, arborists landscape architects have relied on the tried and true reliable Maples, Ash, Honeylocusts and Oaks. However, because of their over-abundance everywhere, they may be the next genus to suffer a problem like EAB or DED that has not yet appeared or flared up.
Task 6 – Establish a planting cycle for each species. Each species stays on the annual list for a few years. The duration is a function of how much emphasis will be placed on that particular species (the current versus the target). For example, a city might have 16 Kentucky Coffeetrees currently and a target of 266, so 250 coffeetrees can be planted in the next few years, before this tree is removed from the tree list. Other cities have chosen to select the trees that the inventory indicates have the highest number, and place a ban on using these trees until the diversification formula has been met.
Task 7 – Rotate species on and off the annual list. Once a species has reached the end of its planting cycle, it is taken off the list for a few years. Other species are then placed on the annual list to encourage their emphasis. This down time is temporary and should last anywhere from three to five years for any one species. Researchers have also discovered that six genus are monotypic species and have only one species commonly known to exist at this time within most of the US and this genus seems to be pest resistant. These monotypic trees may be used more than the target number of trees, at the appropriate location. They are: Ginkgo, Cercidiphyllum, Davidia, Eucommia, Maclura, Metasequoia, Sequoia sempervirens, and Sequoiadendron giganteum. According to researchers, these trees had several species before the ice ages which wiped out their pests.
Task 8 – Evaluate the program. No program is complete without the ability to gauge whether or not the program is successful or not. The best way to assess the success or failure of a species diversity program is to compare the change over time to the individual species count and compare that count against the targets you have established for each species. Continue to assess the suitability of species and incorporate changes due to insect/disease, cultural, or other realizations.
Benefits of Diversification
Sources:
A broad diversity of trees is needed in our urban landscape to guard against the possibility of large-scale devastation by natural disasters as well as native and introduced insect pests and diseases.
History
The widespread planting of American Elms in the towns and cities of eastern United States by our forefathers was a nice idea. Our early horticulturists were taking advantage of the beauty and adaptability of the American Elm, a native tree that Thomas Jefferson called “nature’s noblest vegetable”. The accidental introduction of Dutch elm disease, and the consequential destruction of millions of elms, served not only to focus attention on the loss of urban forests but also to the qualities of the American elm. So arborists began widespread planting of ash trees and that did not go very well either.
Role of Diversity
We now realize that we need a diversity of trees in our urban forests, not only to guard against disasters like Dutch elm disease, but also to “grow the right tree in the right place” as the evolution of our cities and suburbs creates challenges and new settings for tree planting.
Diversity plays an important role in the long-term stability of an ecosystem. When an area has a high diversity of tree species, it is less likely to suffer catastrophic loss from diseases or pests. For example, consider the impact the Emerald Ash Borer (EAB) has had on ash trees. The Emerald Ash Borer (Agrilus planipennis), an exotic wood-boring beetle discovered in southeast Michigan in the summer of 2002, has killed at least tens of millions of ash trees so far and threatens to kill most of the 8.7 billion ash trees throughout North America. Installing a diversity of hardy tree species throughout the urban forest can reduce the vulnerability of the trees to species specific pests.
Age Diversity
Consideration should be given to diversity by age within the urban forest. Planting the entire forest at one time will result in a future collapse of the entire forest at one time. A rule may also apply to age if the age of the existing forest and the number of existing trees are known. By using these two numbers one can prepare an age diversification factor for the tree replacement plan. For example: if the oldest tree in the urban forest died at 100 years of age and the tree inventory indicates a maximum capacity of 30,000 trees in the urban forest, then simple math tells us that 300 trees should be planted every year to create a diversity by age.
Diversity by Shapes
If given enough space to grow, trees have characteristic shapes. Some shapes fit better in a space than others. Because of their vase shape, when hackberries and American elms are planted across the street from each other, their crowns form a cathedral-like arch over the street. A tree with a rounded crown will shade the yard. Pyramidal-shaped trees with dense persistent leaves provide dense shade, and are better at breaking the wind speed nearer the ground. A pyramidal-shaped tree also takes up more space near the ground. Trees with a weeping stature provide focal points in a landscape as do columnar trees. Columnar trees are also suitable in locations where space is tight or a tall hedge is desired.
Diversity of Food Value
A diversity of trees with high food value is the single best way to increase wildlife populations. For example:
- Summer Fruit – cherries, dogwoods, plums, apricots;
- Autumn and Winter Fruit – apples, crabapples, dogwoods, hackberries, hawthorns, mountain ashes;
- Seeds – ashes, birches, firs, hemlocks, maples, spruces, sweetgums;
- Nuts and Acorns – butternuts, walnuts, chestnuts, hazelnuts, hickories, oaks, pecans.
Keep in mind that every tree must be carefully selected for each location to be sure it is appropriate. For example do not install ash if EAB is on its way to your city and sweetgums with thorny fruit balls should not be installed where pedestrians are walking. In a sidewalk installation, Honeylocust might be the only tree suitable for survival in that location. Therefore Honeylocust should not be installed in other locations if it is close to meeting the diversification limit within the city or region.
A Diversity of Wildlife Habitats
Birds and small animals need concealed spaces for nesting and hiding to protect them from the eyes of predators. Planting conifers in groups, growing hedges with low branches, or using prickly or thorny plants in a few areas are all ways to provide wildlife cover.
The Ten-Percent Rule
The “10% rule” is a reaction to the likelihood that some major insect or disease pest could, at some point in time, wipe out most of the trees in a city. However, based on the latest research and study of the impact that EAB has made on all species of ash, the 10% rule has now become the 5% rule. Urban foresters, landscape architects, municipal arborists and/or anyone responsible for the selection of trees, should use the following guidelines for tree diversity within their areas of jurisdiction:
1. plant no more than 5% of any species;
2. no more than 20% of any genus;
3. no more than 30% of any family.
Individual trees as well as strips or blocks of uniformly planted species, cultivars, or clones with proven adaptability should be scattered throughout the city to achieve spatial as well as biological diversity.
Originally the rule was considered a safeguard against a “new pest” that might be introduced from a foreign country. The American experience with Dutch elm disease and chestnut blight is sufficient to explain our concern about such epidemics. However, the spread of dogwood anthracnose disease on our native, Cornus florida, has caused great alarm because it did not come from an introduced pathogen. There are also many “native” insect problems that we are aware of and that must considered as potential threats to the urban forest.
Enacting Species Diversity
While the new “5% rule” may serve as a target or goal to soothe the consciences of city councils and municipal arborists, it will not solve all potential pest problems nor guarantee the long-term stability and aesthetics of the urban forest. We need to install more of survivor trees developed through genetic research and tree selections from difficult sites. We need to utilize the practical experience from practitioners of urban forestry. We have to plan the installation of city trees and understand the problems and potentials of our actions.
Enacting a species diversity plan implies three simple actions:
1. Minimize planting overused species;
2. Increase the planting of underused species;
3. Introduce new species and cultivars into the landscape.
To achieve each of these elements, a plan containing eight tasks is necessary. The first two tasks focus on evaluating the current and potential tree resources. The remaining six tasks outline a systematic process for establishing thresholds for each species and circulating species on and off of the annual planting list.
Task 1 – Compile and assess data on the current tree population and program. An inventory is the easiest tool for collecting and summarizing this data. The necessary information includes:
- Species count and percent representation in the population;
- Species count according to planting records from previous years in the city;
- Number of vacant planting sites in the community;
- Number of trees planted each year within the normal budget;
- Number of trees removed each year;
- Contents of the annual planting list.
Task 2 – Develop a list of all potential species to plant in the area. The list should include everything from small to large and abundant to obscure. This list can be derived from a number of sources such as:
- the current tree population highlighting all the species that currently appear to be doing well;
- planting lists from neighboring communities;
- local arboreta or botanical garden lists;
- tree, shrub, and landscape books;
- nursery catalogs that match the hardiness and heat zone restrictions for your area.
Task 3 – Establish a target for each species on your current list. To minimize an over emphasis of any one species on the list, a cap for each species should be established. This cap should be a percentage of the total possible number of trees that can be present in the community (existing number of trees plus the number of vacant planting sites).
Task 4 – Develop a short list of species to plant each year. This list may have anywhere from five to twenty species. This is the list of species that are going to be emphasized in the current year's planting program. The list should incorporate a variety of species in various sizes.
Task 5 – Create a threshold for each species on your annual planting list. To minimize an over emphasis of any one species on the annual list, a cap for each species should be established. As that threshold is reached, that particular species is taken off of the list for the remainder of the year. A careful balance of all of suggested species for the year will guarantee an even and equitable distribution of the trees. Keep in mind that throughout most of the US, arborists landscape architects have relied on the tried and true reliable Maples, Ash, Honeylocusts and Oaks. However, because of their over-abundance everywhere, they may be the next genus to suffer a problem like EAB or DED that has not yet appeared or flared up.
Task 6 – Establish a planting cycle for each species. Each species stays on the annual list for a few years. The duration is a function of how much emphasis will be placed on that particular species (the current versus the target). For example, a city might have 16 Kentucky Coffeetrees currently and a target of 266, so 250 coffeetrees can be planted in the next few years, before this tree is removed from the tree list. Other cities have chosen to select the trees that the inventory indicates have the highest number, and place a ban on using these trees until the diversification formula has been met.
Task 7 – Rotate species on and off the annual list. Once a species has reached the end of its planting cycle, it is taken off the list for a few years. Other species are then placed on the annual list to encourage their emphasis. This down time is temporary and should last anywhere from three to five years for any one species. Researchers have also discovered that six genus are monotypic species and have only one species commonly known to exist at this time within most of the US and this genus seems to be pest resistant. These monotypic trees may be used more than the target number of trees, at the appropriate location. They are: Ginkgo, Cercidiphyllum, Davidia, Eucommia, Maclura, Metasequoia, Sequoia sempervirens, and Sequoiadendron giganteum. According to researchers, these trees had several species before the ice ages which wiped out their pests.
Task 8 – Evaluate the program. No program is complete without the ability to gauge whether or not the program is successful or not. The best way to assess the success or failure of a species diversity program is to compare the change over time to the individual species count and compare that count against the targets you have established for each species. Continue to assess the suitability of species and incorporate changes due to insect/disease, cultural, or other realizations.
Benefits of Diversification
- Diversification will mean making better use of the tree species commonly planted in your city, while ensuring that they are well suited to the site in the context of predicted climate change.
- Take opportunities to use tree species not historically widespread in your city, but which will become more suitable under current climate change predictions.
- Be sure that you have given appropriate consideration to a choice of silvicultural systems when making decisions on species and mixtures.
- Make sure that any group mixtures are of compatible species, and they will meet the long-term management objectives.
- Use existing or expected natural regeneration to diversify stands where the species you expect will be able to meet the set objectives and will remain well suited to the site, even where you are not practicing diversification.
- When thinning or re-spacing, take the opportunity to retain minor species where they will contribute to the long-term objectives of the urban forest.
Sources:
- “A guide for increasing tree species diversity in Wales” Forestry Commission on Wales, November 15, 2010
- Barker P.A., “Fruit litter from urban trees”. Journal of Arboriculture, 12 (12):293-298.1986.
- Bassuk, Nina, "Principles of Plant Selection and Some Favorite Trees", Online Seminar Archives #1, 2005.
- Bassuk N.L., “Street tree diversity making better choices for the urban landscape”. METRIA Proceedings: 71-78.1990.
- Bassuk N.L., P. Trowbridge, C. Grohs, “Visual similarity and biological diversity: street tree selection and design”. Paper presented at the European Conference of the International Society of Arboriculture, OSLO. 2002.
- Benedikz T., F. Ferrini, H.L. Garcia Valdecantos, M.L. Tello, “Urban Forest and Trees” Plant Quality, 2005.
- Buley, Nancy "Tree Selection and Planting", Arbor Age, March/April 2009.
- Chapman D.J., "Tree species selection with an eye toward maintenance”. Journal of Arboriculture, 7(12): 313-316. 1981.
- Clatterbuck W.K. and D.C. Fare, “Trees to reconsider after planting”. Trees for Tennessee Landscapes. Retrieved 4th April 2008.
- Dodge, Linda, "Current Research", Pacific Northwest Trees, Spring 2001
- Duntemann, Mark, “Urban Forestry Management Series: Implementing a Species Diversity Program”, Natural Path Urban Forestry Consultants, July, 2004.
- Gilman, Edward F., "Planting trees in landscapes", Environmental Horticulture Department, IFAS, University of Florida, 2004
- Hagen, Bruce, "The Ecology of Urban Trees", City Trees, The Journal of The Society of Municipal Arborists Vol. 37, Number 5, September/October 2001
- J. Frank Schmidt & Son Co., Crabapple Information Chart.
- J. Frank Schmidt & Son Co., Reference Guide.
- Johnston M., “Impacts and Adaptation for Climate Change in Urban Forests”. 6th Canadian Urban Forest Conference, Kelowna, B.C. 2004.
- "Knoxville Street Tree Master Plan", Knoxville-Knox County Metropolitan Planning Commission, 2002
- National Arbor Day Foundation, “Tree Diversity Activity”, 2009.
- Ninemets V., and J. Peñuelas, “Gardening and urban landscaping: significant players in global change”. Trends in Plant Science, Vol. 13, N°2; 60-65. 2008.
- Nowak D.J., J.C. Stevens, S.M. Sisinni, C.L. Luley, “Effects of urban tree management and species selection on atmospheric carbon dioxide”. Journal of Arboriculture 28(3):113-122. 2002.
- Phillips, Leonard, "Municipal Street Tree Master Plan", City Trees, The Journal of The
Society of Municipal Arborists Vol. 30, Number 3, May/June 1994 - Phillips, Leonard, Urban Trees, McGraw-Hill, 1993, pg. 90.
- Santamour F.S., “The selection and breeding of pest-resistant landscape trees”. Journal of Arboriculture, 3(8):146-152. 1977.
- Santamour, Frank S., Jr., “Uniformity and Common Sense”, METRIA 7 Proceedings, 1990.
- Shurtleff, M.C., “The search for disease-resistant trees”. Journal of Arboriculture, 6(9): 238-244. 1980.
- Sogni S., Arredo urbano ed “allergie: le barriere fisiologiche al fruimento del verde pubblico”. Acer, 2:42-47, 2000.
- Trowbridge, Peter J. and Nina L. Bassuk, "Trees in the Urban Landscape", John Wiley & Sons, Inc. 2004.
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