LA Course #GCE-1-1405
Tree Preservation, Diversification and Sustainability
Edited by Len Phillips and Richard W Gibney RLA/ISA
Sections Go directly to the section by clicking on the title below
Tree Preservation, Diversification and Sustainability
Edited by Len Phillips and Richard W Gibney RLA/ISA
Sections Go directly to the section by clicking on the title below
Protecting Trees During Construction
Shade trees have an extensive root system that is required to maintain the health of the tree and trees add to the value of property. Therefore, during construction, efforts should be made to protect and preserve established, healthy shade trees and their root systems. Any construction work in the area near trees, should be done with extreme care. The majority of roots are located within the top 12 inches (30 cm) of soil and extends past the dripline of the tree. The dripline makes a circle around the tree and must be considered a protected root zone.
Saving a Tree
The following items must be considered to decide whether it is better to save the tree and work around it, or remove the tree and replace it with a smaller, younger tree, after the construction is complete.
Preventing Injury
Preventing damage is less costly than correcting it. Few developers and builders are aware of the way tree roots grow and what is needed to protect them. Before construction begins, the arborist must meet with the planner to decide which trees can be saved and what design modifications must be taken to help preserve the trees that are to remain. Next, install highly-visible barricades and signs around the trees and root zone areas to be protected. The optimal size of barricaded areas varies according to tree species, size, and construction project. For example:
Types of tree injury caused by mechanized equipment include:
Avoid Soil Compaction
After a tree is established, any activity that changes the soil condition is extremely detrimental to its health. Construction traffic compacts soil most severely near the surface, where the majority of tree roots are growing. Soil compaction decreases soil permeability, reduces the amount of pore space, and interferes with essential gas exchange as well as surface and subsurface drainage. When root growth is restricted by compacted soils, less nutrients and water are available for plant growth which results in reduced tree vigor and possible death. Decline and dieback may gradually appear over a period of years because of soil compaction.
It is much easier to avoid soil compaction than to correct it. Keep construction traffic and material storage away from tree root areas and out of the barricaded areas. The 4 to 6 inch (10 – 15 cm) layer of wood chips around all protected trees as mentioned above will help reduce compaction from vehicles that inadvertently cross the barricades.
Digging Trenches
Trenches should only be dug on one side of the tree. To find the minimum distance from the tree trunk that the trench can be dug, determine the diameter of trunk at breast height (dbh) by measuring the diameter or width of the tree trunk. Multiply the dbh in inches by one foot (30 cm) to get minimum distance from tree for there to be a trench. Keep in mind this is the minimum distance and it is always best for the health of the tree to maximize this distance to beyond the dripline.
Some recent research has indicated that if the trench is going to remove more than one quarter of the tree's roots, the best place to locate a trench is directly under the tree trunk. This will minimize root loss provided
the excavating equipment starts one foot on either side of the trunk and the tunnel at the bottom of the trench, under the tree trunk is hand dug to allow the pipe to run directly under the tree.
Augering
When augering for fence posts, signs, etc., use the same distance calculation as was used for digging trenches. This should be a minimum distance. If possible, extend this outward a few feet or better still, outside the dripline.
General Rules for Cutting Roots
Grade Cuts
If more than 12 inches (30 cm) of soil is to be removed from an area containing tree roots, build a retaining wall to reduce the amount of grade change within the protected root zone. Also, if needed, create steps of grade changes.
After building the retaining wall, back fill with soil, and mulch the soil surface around the tree. Also, do not remove soil within 3 feet (1 m) of the trunk. If these rules will not work, remove tree and plant a new one once
the grade change has been made.
Filling Grade
Before raising the grade within the protected root zone, consider soil compaction. No more than 6-12" inches of soil should be added within the protected root zone. When adding soil to increase the grade within the
protected root zone, never allow the excess soil to touch the trunk or base of the tree. This will cause rot, which can kill the tree no matter how healthy it is.
To keep soil away from the base of the tree, build a retaining wall around and 3 feet (1 m) away from the trunk. When adding soil, it is advisable to lay drainage pipes in a radial pattern to provide air flow from the wall and the lower half of new fill soil should be a mix of coarse sand and shredded bark.
If the fill is going to be deeper than suggested above, build the retaining wall on or outside the dripline. Back-filling should be done as mentioned above, with the lower half the depth being a mix of coarse sand and shredded bark. Aeration is necessary when adding large amounts of fill.
Adding or Removing Soil on One Side of the Tree
If it is necessary to cut or fill on only one side of the tree, use the same methods mentioned above. Still do not work closer than 3 feet (1 m) from the trunk of the tree.
Aeration
Aerate within the protected root zone before and after grade changes. Starting 3 feet (1 m) from the base of the tree, drill 2-inch (5 cm) diameter holes into the soil about 1 to 1-1/2 feet (30 – 50 cm) deep. Drill the holes in a circle 3 feet (1 m) away from the trunk, spacing them two feet (60 cm) apart, and then go out 5 feet (1.5 m) and drill again in a circle. Continue every 2 feet (60 cm) within the protected root zone. Use the porous soil mentioned above to fill the holes.
Under the Canopy
Tips for Tree Care Before and After
The best time to prepare a tree for construction activities, within the root zone, is a year before the work begins. Trees store water and nutrients for times of need. These times can be the spring growth surge, during droughts, after a hailstorm, etc. So extra stores of water and nutrients will create a healthier, stronger tree and will increase survival following the stress of damaging work. Trees should be watered a few days before work is to begin and again as soon as possible after the work is done. Watering should be done thoroughly and deeply. Finally, remember to contact the local utility companies to find out where it is safe to dig before digging.
Sources
Shade trees have an extensive root system that is required to maintain the health of the tree and trees add to the value of property. Therefore, during construction, efforts should be made to protect and preserve established, healthy shade trees and their root systems. Any construction work in the area near trees, should be done with extreme care. The majority of roots are located within the top 12 inches (30 cm) of soil and extends past the dripline of the tree. The dripline makes a circle around the tree and must be considered a protected root zone.
Saving a Tree
The following items must be considered to decide whether it is better to save the tree and work around it, or remove the tree and replace it with a smaller, younger tree, after the construction is complete.
- Age: Younger trees will survive soil and grade changes better than older, more mature trees.
- Disturbance Sensitivity: Trees vary in sensitivity when the soil and their roots are cut or disturbed.
- Soil Compaction: Trees vary in sensitivity, but generally become stressed due to the soil around their roots being compacted.
- Severity of Change: The amount of grade change or root loss is a major factor to consider.
- Tree Health: If trees are already stressed the likelihood of surviving a major change in root loss or grade change is small.
Preventing Injury
Preventing damage is less costly than correcting it. Few developers and builders are aware of the way tree roots grow and what is needed to protect them. Before construction begins, the arborist must meet with the planner to decide which trees can be saved and what design modifications must be taken to help preserve the trees that are to remain. Next, install highly-visible barricades and signs around the trees and root zone areas to be protected. The optimal size of barricaded areas varies according to tree species, size, and construction project. For example:
- For recently planted trees (one to four years), the area to be protected should be within the dripline.
- For minimal protection of trees older than four years, barricades should extend beyond the dripline.
- For each inch of the tree’s trunk's diameter, the protection area should extend one foot of radius.
- For additional protection, a temporary layer of wood chips, 4 to 6 inches (10 – 15 cm) deep, should be placed around each tree prior to placement of the barricades.
- Examine trees and barricades at least once a week during the entire construction time frame; more often during the active construction period.
Types of tree injury caused by mechanized equipment include:
- Bark removal or "skinning" of the trunk can be caused by any type of equipment.
This can easily kill the tree if the damage is extensive and it is not treated
or repaired within a very few minutes of the injury. - Breakage of branches may make the tree unsightly and removing too many leaves will cause stress and future decline of the tree.
- Surface grading removes surface vegetation and the topsoil that contains a tree's roots.
- Trenching for utilities can also cause substantial root damage and should be done far away from existing trees. In new developments, this can be done easily. Where the trench must pass under or near a tree, substantial injury can be avoided by using a power auger to bore a tunnel under the roots. If trenching is unavoidable, place the trench as far from the trunk as possible (minimum 8 feet) (2 m), while cutting as few roots as possible. Cleanly prune cut roots and refill trenches as soon as possible to prevent excessive moisture loss. Wounds make the tree highly susceptible to root pathogens and decay fungi. Decline and death can result if more than 40% of the roots are damaged or killed. Stressed trees are also more susceptible to insect attacks such as from bark beetles and borers.
Avoid Soil Compaction
After a tree is established, any activity that changes the soil condition is extremely detrimental to its health. Construction traffic compacts soil most severely near the surface, where the majority of tree roots are growing. Soil compaction decreases soil permeability, reduces the amount of pore space, and interferes with essential gas exchange as well as surface and subsurface drainage. When root growth is restricted by compacted soils, less nutrients and water are available for plant growth which results in reduced tree vigor and possible death. Decline and dieback may gradually appear over a period of years because of soil compaction.
It is much easier to avoid soil compaction than to correct it. Keep construction traffic and material storage away from tree root areas and out of the barricaded areas. The 4 to 6 inch (10 – 15 cm) layer of wood chips around all protected trees as mentioned above will help reduce compaction from vehicles that inadvertently cross the barricades.
Digging Trenches
Trenches should only be dug on one side of the tree. To find the minimum distance from the tree trunk that the trench can be dug, determine the diameter of trunk at breast height (dbh) by measuring the diameter or width of the tree trunk. Multiply the dbh in inches by one foot (30 cm) to get minimum distance from tree for there to be a trench. Keep in mind this is the minimum distance and it is always best for the health of the tree to maximize this distance to beyond the dripline.
Some recent research has indicated that if the trench is going to remove more than one quarter of the tree's roots, the best place to locate a trench is directly under the tree trunk. This will minimize root loss provided
the excavating equipment starts one foot on either side of the trunk and the tunnel at the bottom of the trench, under the tree trunk is hand dug to allow the pipe to run directly under the tree.
Augering
When augering for fence posts, signs, etc., use the same distance calculation as was used for digging trenches. This should be a minimum distance. If possible, extend this outward a few feet or better still, outside the dripline.
General Rules for Cutting Roots
- Water tree a few days before construction activities to soak the soil and hydrate the tree roots, but not within two days before construction begins.
- If soil is wet, any weight will compact the soil pores which will cause more stress on the tree's roots.
- Make sure cuts to the roots are done with hand tools that will make clean, quick cuts. Do not use large equipment like backhoes and bulldozers to tear the roots off the tree.
- Do not disturb roots any more than necessary. Twenty percent of the tree's roots can be cut before any signs of stress will appear. Keep in mind that tree roots do extend outside of the protected root zone and beyond the dripline.
- Make sure cut roots are covered with soil and wood chips as soon as possible after the pipe is laid. If roots are going to be exposed for more than an hour, cover the roots with a damp cloth. Be sure to keep cloth moist until soil and mulch can be used for permanent cover.
- Water the tree thoroughly when job is done. This means wetting the top 12 inches (30 cm) of soil within the entire tree dripline.
- Put a permanent 3 foot (1m) (minimum) diameter ring of mulch around base of tree. Mulch 3 – 5 inches (8 – 12 cm) deep but keep the mulch 6 inches (15 cm) away from trunk. Wood chips, shredded bark or leaves, and ground corn cobs are examples of good products that can be used for mulch. Most types of mulch break down in time, so it is necessary to replenish the mulch from year to year.
Grade Cuts
If more than 12 inches (30 cm) of soil is to be removed from an area containing tree roots, build a retaining wall to reduce the amount of grade change within the protected root zone. Also, if needed, create steps of grade changes.
After building the retaining wall, back fill with soil, and mulch the soil surface around the tree. Also, do not remove soil within 3 feet (1 m) of the trunk. If these rules will not work, remove tree and plant a new one once
the grade change has been made.
Filling Grade
Before raising the grade within the protected root zone, consider soil compaction. No more than 6-12" inches of soil should be added within the protected root zone. When adding soil to increase the grade within the
protected root zone, never allow the excess soil to touch the trunk or base of the tree. This will cause rot, which can kill the tree no matter how healthy it is.
To keep soil away from the base of the tree, build a retaining wall around and 3 feet (1 m) away from the trunk. When adding soil, it is advisable to lay drainage pipes in a radial pattern to provide air flow from the wall and the lower half of new fill soil should be a mix of coarse sand and shredded bark.
If the fill is going to be deeper than suggested above, build the retaining wall on or outside the dripline. Back-filling should be done as mentioned above, with the lower half the depth being a mix of coarse sand and shredded bark. Aeration is necessary when adding large amounts of fill.
Adding or Removing Soil on One Side of the Tree
If it is necessary to cut or fill on only one side of the tree, use the same methods mentioned above. Still do not work closer than 3 feet (1 m) from the trunk of the tree.
Aeration
Aerate within the protected root zone before and after grade changes. Starting 3 feet (1 m) from the base of the tree, drill 2-inch (5 cm) diameter holes into the soil about 1 to 1-1/2 feet (30 – 50 cm) deep. Drill the holes in a circle 3 feet (1 m) away from the trunk, spacing them two feet (60 cm) apart, and then go out 5 feet (1.5 m) and drill again in a circle. Continue every 2 feet (60 cm) within the protected root zone. Use the porous soil mentioned above to fill the holes.
Under the Canopy
- Do not add more than 6" (15 cm) total of soil and/or mulch.
- Keep mulch 6” (15 cm) away from base of tree.
- Never till soil under canopy.
- Do not overwork the soil under the tree.
- Work soil with hands and hand tools only, no large equipment.
- Preserve ground covers and/or mulch around the tree that protect its roots and the soil from compaction and maintain soil moisture.
- Do not plant anything within 3 feet (1 m) of the tree trunk that will compete with the tree for moisture and nutrients, such as another tree or large shrubs. Ground covers, perennials, and small shrubs can be beneficial to the tree. They act as a barrier to soil compaction and improving moisture content by shielding the soil from direct sun exposure.
- Avoid planting anything that will be more than one-third the mature height of the tree.
- Avoid cutting roots from one inch (2 cm) diameter and up. If roots are encountered, move planting hole over until you can dig without encountering large roots.
Tips for Tree Care Before and After
The best time to prepare a tree for construction activities, within the root zone, is a year before the work begins. Trees store water and nutrients for times of need. These times can be the spring growth surge, during droughts, after a hailstorm, etc. So extra stores of water and nutrients will create a healthier, stronger tree and will increase survival following the stress of damaging work. Trees should be watered a few days before work is to begin and again as soon as possible after the work is done. Watering should be done thoroughly and deeply. Finally, remember to contact the local utility companies to find out where it is safe to dig before digging.
Sources
- Bracewell, Sara K. “Protecting Existing Trees During Construction”, University of Minnesota, Department of Horticultural Science, 2006.
- Coder, Dr. Kim D., “Tree Conservation during Site Development”, Warnell School of Forestry & Natural Resources, University of Georgia, 2010.
- Dennis, C., and W.R. Jacobi, “Protecting Trees During Construction”, Colorado State University Extension, April 2013.
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 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.
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 “put 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 more than 10 million trees in southeast Michigan. Planting a diversity of hearty 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 300 trees should be planted every year.
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 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 less shade, but 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 plant ash if the Emerald Ash Borer is on its way to your city, and sweetgums should not be planted where pedestrians are walking.
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. Urban foresters and municipal arborists should use the following guidelines for tree diversity within their areas of jurisdiction:
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 “10% 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 plant more of the superior trees developed through genetic research. We need to utilize the practical experience from practitioners of urban forestry. We have to plan the planting of city trees, and understand the problems and potentials of our actions.
Enacting a species diversity plan implies three simple actions:
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. Necessary information includes:
Task 2 – Develop a list of all potential species to potentially 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.
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.
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 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.
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 “put 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 more than 10 million trees in southeast Michigan. Planting a diversity of hearty 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 300 trees should be planted every year.
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 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 less shade, but 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
- Fall 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, pecan
Keep in mind that every tree must be carefully selected for each location to be sure it is appropriate. For example do not plant ash if the Emerald Ash Borer is on its way to your city, and sweetgums should not be planted where pedestrians are walking.
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. Urban foresters and municipal arborists should use the following guidelines for tree diversity within their areas of jurisdiction:
- plant no more than 10% of any species,
- no more than 20% of any genus,
- no more than 30% of any family.
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 “10% 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 plant more of the superior trees developed through genetic research. We need to utilize the practical experience from practitioners of urban forestry. We have to plan the planting of city trees, and understand the problems and potentials of our actions.
Enacting a species diversity plan implies three simple actions:
- Minimize planting overused species.
- Increase the planting of underused species.
- Introduce new species 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. Necessary information includes:
- Species count and percent representation in the population.
- Number of vacant planting sites in the community.
- Number of trees planted each year.
- Number of trees removed each year.
- Contents of the annual planting list.
Task 2 – Develop a list of all potential species to potentially 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 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.
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.
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
- Duntemann, Mark, “Urban Forestry Management Series: Implementing a Species Diversity Program”, Natural Path Urban Forestry Consultants, July, 2004.
- Santamour, Frank S., Jr., “Uniformity and Common Sense”, METRIA 7 Proceedings, 1990.
- National Arbor Day Foundation, “Tree Diversity Activity”, 2009.
Sustainable Trees
Definition
Careful tree selection is the first step in developing a balanced and self-perpetuating landscape. However, tree survival with minimal maintenance is not the only issue in sustainability. Landscape trees that are more sustainable are those which are not invasive and require reduced levels of pesticides, water, and maintenance. Arborists must all become aware of the limits of our natural resources and the precarious balance of nature in which we play a pivotal role.
Sustainable shade trees are those that have been selected to survive and thrive in harsh urban conditions because they are drought tolerant, disease and insect resistant, and make efficient use of nutrients and water. They are proven to be predictable and dependable large shade trees that outlive, outlast, and outperform other varieties of trees, expanding their benefits for future generations.
Another way to look at sustainability from the viewpoint of the arborist, naturalist, and gardener, means that we do not use a resource to the point that it has a negative consequence on future generations’ abilities to use the same resource. When applied to trees, sustainability means tree removals are a last resort. Sustainability also means that proper selection, placement, and care are required to ensure trees will live to their full potential, providing the maximum benefits for future generations.
In a sustainable tree-planting project, trees not only survive, but also grow stronger limbs and live to the normal life expectancy for the species. Research a particular species and its growth habit before you determine its location. A large-growing oak does not belong over a swimming pool because it drops acorns, leaves, and flowers into the pool. Also, consider the future growth of near-by trees before selecting a location. Trees without adequate sunlight for the species do not develop to their full potential.
When selecting the site, be sure a tree will have adequate root space and healthy soil. Trees surrounded by concrete survive around 20 years, while trees surrounded by organic matter can live to their full life expectancy.
Select quality trees with a proper-sized root ball, a strong branching habit, and healthy foliage or healthy buds in the dormant season. A tree, growing in a nursery container that is too small is not sustainable. A 6’ or 8’ (2-2.5 m) tall tree growing in a 5-gallon (20 l.) container, likely has circling roots inside the container and may also require staking to become established. If circling roots are not properly pruned or straightened during the planting process, they continue to grow in a circular pattern, leaving the tree without adequate support and a shorter life span. Staking should not be necessary to prevent the tree from blowing over. As a rule, a root ball or container should be at least 10”-12” (26-30 cm) wide for each inch in trunk caliper.
Sustainability also requires providing sufficient maintenance during the establishment period. Proper maintenance is critical for a minimum of 5 years for newly planted trees up to 1.5” (4 cm) caliper, and 3 years for trees between 2? and 4” (5-10 cm) caliper unless there is a long period of drought anytime during the establishment period.
The use of potential invasives cannot be seen as sustainable except in very controlled situations. There is great difficulty with invasive exotic trees that have escaped from managed landscapes, displaced native trees, and have disrupted natural ecosystems. The callery pear is a prime example of an escaped exotic tree that can now be found along roadways, city parks, and other undesirable locations throughout the southeastern United States.
Tree Lists
Lists of sustainable trees are invaluable resources for arborists, garden enthusiasts, designers, nursery trades people, and landscape architects. However, a list of sustainable trees is not the entire answer. Proper siting, planting and maintenance are necessary for a tree to prosper in the landscape. Therefore, climatic conditions, exposure to sun and wind, subsurface soil and moisture conditions, etc. must be considered when selecting trees for a particular location. Sustainable or not, if one ignores the site and a tree's cultural requirements, that tree will suffer.
Lists of sustainable trees can be obtained from colleges or extension services in your region.
Sources
* Houser, Steve, “Sustainable Tree Planting”, InSight, January 2011
* “Sustainable Trees and Shrubs”, Rhode Island Cooperative Extension , 2003
The test that follows contains 30 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
LA CES will award 1.0 PDH (HSW) credit for a passing grade. North Carolina Board of LA and New Jersey Board of Architects have approved this course for 1.0 credit.
The cost for taking this test is $20 per credit. If you purchase an annual subscription for 12 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report your passing test score to LA CES. If you are also ISA* certified we will report your passing score to ISA for no additional cost. Please be sure to add your ISA Certification number when you sign in. Tests with passing scores may be submitted only once to each organization.
*ISA has approved this course for .5 CEUs per section for a total of 1.5 CEUs applied toward Certified Arborist credit or .5 CEUs may be applied toward BCMA Practice credit, .5 Science credit and .5 Management credit.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to gibneyCE.com’. That will take you to the test sign in page followed by the test. If you are an ISA and/or CLARB member, please be sure to include your certification/member number(s) along with your LA license and ASLA numbers.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks after that.
LA CES maintains a record of earned PDH credits on their website http://laces.asla.org/
ISA maintains a record of earned CEU credits on their website http://www.isa-arbor.com/
Definition
Careful tree selection is the first step in developing a balanced and self-perpetuating landscape. However, tree survival with minimal maintenance is not the only issue in sustainability. Landscape trees that are more sustainable are those which are not invasive and require reduced levels of pesticides, water, and maintenance. Arborists must all become aware of the limits of our natural resources and the precarious balance of nature in which we play a pivotal role.
Sustainable shade trees are those that have been selected to survive and thrive in harsh urban conditions because they are drought tolerant, disease and insect resistant, and make efficient use of nutrients and water. They are proven to be predictable and dependable large shade trees that outlive, outlast, and outperform other varieties of trees, expanding their benefits for future generations.
Another way to look at sustainability from the viewpoint of the arborist, naturalist, and gardener, means that we do not use a resource to the point that it has a negative consequence on future generations’ abilities to use the same resource. When applied to trees, sustainability means tree removals are a last resort. Sustainability also means that proper selection, placement, and care are required to ensure trees will live to their full potential, providing the maximum benefits for future generations.
In a sustainable tree-planting project, trees not only survive, but also grow stronger limbs and live to the normal life expectancy for the species. Research a particular species and its growth habit before you determine its location. A large-growing oak does not belong over a swimming pool because it drops acorns, leaves, and flowers into the pool. Also, consider the future growth of near-by trees before selecting a location. Trees without adequate sunlight for the species do not develop to their full potential.
When selecting the site, be sure a tree will have adequate root space and healthy soil. Trees surrounded by concrete survive around 20 years, while trees surrounded by organic matter can live to their full life expectancy.
Select quality trees with a proper-sized root ball, a strong branching habit, and healthy foliage or healthy buds in the dormant season. A tree, growing in a nursery container that is too small is not sustainable. A 6’ or 8’ (2-2.5 m) tall tree growing in a 5-gallon (20 l.) container, likely has circling roots inside the container and may also require staking to become established. If circling roots are not properly pruned or straightened during the planting process, they continue to grow in a circular pattern, leaving the tree without adequate support and a shorter life span. Staking should not be necessary to prevent the tree from blowing over. As a rule, a root ball or container should be at least 10”-12” (26-30 cm) wide for each inch in trunk caliper.
Sustainability also requires providing sufficient maintenance during the establishment period. Proper maintenance is critical for a minimum of 5 years for newly planted trees up to 1.5” (4 cm) caliper, and 3 years for trees between 2? and 4” (5-10 cm) caliper unless there is a long period of drought anytime during the establishment period.
The use of potential invasives cannot be seen as sustainable except in very controlled situations. There is great difficulty with invasive exotic trees that have escaped from managed landscapes, displaced native trees, and have disrupted natural ecosystems. The callery pear is a prime example of an escaped exotic tree that can now be found along roadways, city parks, and other undesirable locations throughout the southeastern United States.
Tree Lists
Lists of sustainable trees are invaluable resources for arborists, garden enthusiasts, designers, nursery trades people, and landscape architects. However, a list of sustainable trees is not the entire answer. Proper siting, planting and maintenance are necessary for a tree to prosper in the landscape. Therefore, climatic conditions, exposure to sun and wind, subsurface soil and moisture conditions, etc. must be considered when selecting trees for a particular location. Sustainable or not, if one ignores the site and a tree's cultural requirements, that tree will suffer.
Lists of sustainable trees can be obtained from colleges or extension services in your region.
Sources
* Houser, Steve, “Sustainable Tree Planting”, InSight, January 2011
* “Sustainable Trees and Shrubs”, Rhode Island Cooperative Extension , 2003
The test that follows contains 30 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
LA CES will award 1.0 PDH (HSW) credit for a passing grade. North Carolina Board of LA and New Jersey Board of Architects have approved this course for 1.0 credit.
The cost for taking this test is $20 per credit. If you purchase an annual subscription for 12 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report your passing test score to LA CES. If you are also ISA* certified we will report your passing score to ISA for no additional cost. Please be sure to add your ISA Certification number when you sign in. Tests with passing scores may be submitted only once to each organization.
*ISA has approved this course for .5 CEUs per section for a total of 1.5 CEUs applied toward Certified Arborist credit or .5 CEUs may be applied toward BCMA Practice credit, .5 Science credit and .5 Management credit.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to gibneyCE.com’. That will take you to the test sign in page followed by the test. If you are an ISA and/or CLARB member, please be sure to include your certification/member number(s) along with your LA license and ASLA numbers.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks after that.
LA CES maintains a record of earned PDH credits on their website http://laces.asla.org/
ISA maintains a record of earned CEU credits on their website http://www.isa-arbor.com/