Classics
The following article has been selected because it is deemed very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Natives versus Cultivars
Edited by Len Phillips
How do you define which species are native to an urban area where the built environment has erased the native soil, understory plants, and wildlife that characterize a native ecosystem? Just about every arborist I know has an opinion about growing cultivars instead of native trees. Cultivars are cultivated varieties that have been selected because they have different characteristics from a species. Cultivars, according to definition are stem grafted onto species rootstock, while clones would be selections growing on their own roots. This article will not be including clones in the comparisons below. To be perfectly clear, Red Maple, Acer rubrum is a tree native to the eastern half of the United States, while Red Sunset® Maple, Acer rubrum 'Franksred' is a cultivar that can be found growing from coast to coast in the US. I would like to offer the following comments with the hope that your opinion on this subject will be based on some facts.
Genetic features
Native – Uniform genetics in all native trees of a species being similar but having different minor characteristics.
Cultivar – Called genetically defective from the species by some experts. Cultivars are genetically identical and many arborists feel they are more desirable than the native because you can rely on the desirable characteristics of form, flowers, pest resistance, etc. being identical in all the trees of a particular cultivar. In a formal landscape, all the trees would end up with an identical shape, without pruning.
Suitable for diversification
Native – Despite the uniform genetics of a native tree species, it is not enough to be considered diversified when dealing with pests and diseases. One reason to use natives is that the pest would be known and tolerated by the species.
Cultivar – Cultivars that have been found resistant to particular pests and diseases that attack the native trees can be used for replacing the entire urban forest and maintain proper diversification.
Long-term survival
Native – These trees were growing fine without any human interference and will continue long after humans have left the planet. They breed successfully in their home range without assistance.
Cultivar – They might become extinct without human intervention, support, and management. This would be okay because they were developed to survive in a human environment. Without humans, there is no need for these trees and the forests can revert to native species.
Growth patterns
Native – These trees have a natural form that is somewhat predictable, with minor variations in size and shape.
Cultivar – A tree's special form is one reason the tree is cross bred or selected to become a cultivar. For example, Endowment Maple, Acer saccharum 'Morton' was selected from a field of sugar maples Acer saccharum for its uniformly columnar shape.
Invasive species
Native – Invasive trees tend to be trees that have aggressive growth and reproduction functions. They are invasive because of excessive seed deployment or a high number of root suckers. In many locations, black locust, Robinia pseudoacacia is an invasive tree because of its root suckers that can destroy a hardscape or choke out understory plants.
Cultivar – Most cultivars, especially the newer introductions, do not produce viable seed and are not invasive because they must be grafted on to rootstocks, which limit their ability to spread by root suckers. Bessoniana Locust, Robinia pseudoacacia 'Bessoniana' is a non-invasive cultivar.
Planting locations
Native – Native trees grow best in native soil. Native trees can adapt to growing in poor sites because among the seedlings there will often be one seedling whose specific genotype will tolerate and grow in the poor site.
Cultivar – Cultivars cannot adapt to poor sites unless they were selected because they were found growing in a poor site. Cultivars are often selected because they tolerate urban soils better than the native parent. Since the soils in our cities seldom resemble native soil, cultivars selected for their urban soil tolerance are clearly the best choice for planting in the city. Always grow the right tree in the right place. For example many cultivars of red maple such as, Autumn Flame Maple, Acer rubrum 'Autumn Flame' have been selected from trees growing in swampy areas. These trees are very tolerant of compacted, wet soil and soil low in oxygen; hence they make very desirable street trees (along with being one of the hardiest of the red maples and being the first to turn a brilliant red in the fall).
Unique features
Native – Native trees are seldom known for having unique features but they do provide a stable appearance to the landscape and a stable ecosystem for the flora and fauna that they support.
Cultivar – Cultivars are often selected because they have unique features dealing with forms, special flowers, large fruits, unusual leaf color, etc. Crabapple, Malus spp. is a good example of trees selected for their bloom, fruit color, wildlife food, and disease resistance in certain cultivars.
Flower production
Native – Most native trees have normal single flowers that provide pollen and fruit for native insects and wildlife.
Cultivar – Cultivars are often selected because of outstanding flower production. However, sometimes a certain floral characteristic has bred the nectaries out of the plant. English tea roses and double flowers are a good example. This can have deleterious effect on our native pollinators and wildlife.
Uses in the urban landscape
Native – Use natives in an area of suitable, excellent soil, such as a garden or forest, to provide a uniform appearance, texture, or leaf color. Big Tooth Maple, Acer grandidentatum is a good example of a small round tree suitable for most native landscapes.
Cultivar – Used as a street tree as well as a garden or landscape specimen, Rocky Mountain Glow® Maple, Acer grandidentatum 'Schmidt' is a good example of a cultivar that has intense autumn color and thrives in an urban landscape.
Disease resistance
Native – Native trees have good resistance to common diseases through a long association with native diseases and through the genetic diversity in any long standing breeding population of native trees, but they may not be able to withstand an exotic or a recently introduced diseases like Chalara fraxinea in Europe and more recently in England.
Cultivar – Many cultivars have been selected because they resist a particular disease. A good example is the Stellar Dogwood series of trees that are resistant to the Dogwood anthracnose which is killing the native eastern U.S. Dogwoods. Stellar Pink® dogwood, Cornus x 'Rutgan', is free of the disease and a very popular replacement.
Pest Resistance
Native – Native trees with good resistance to certain pests are acceptable, but many exotic or introduced pests can be fatal. Some naturalists consider this a desirable feature because the pest contributes to the natural food chain. For example, the woodpecker has developed a preference for the bronze birch borer found in many native white barked birches.
Cultivar – Many cultivars have been selected because they are resistant to a devastating pest. A good example might be the Heritage® Birch, Betula nigra 'Cully' which was selected for its resistance to the bronze birch borer. What impact this may have on the woodpecker population is unknown at this time but as an apex predator it may just move to its next favorite food item.
Global Warming Survival
Native – Native trees are not likely to tolerate global warming stress. The warming subjects the trees to increased disease and pest attacks. The genetic diversity of breeding native populations may somewhat buffer that species against the worst effects of warming by favoring those in its population that can adapt to changing environmental conditions. The Amur Maple, Acer ginnala might be a good example of a tree that tolerates the cold in USDA Hardiness Zone 2 and will have to migrate even further north to survive.
Cultivar – Cultivars that have been selected from warmer climates will be better survivors. An example here might be the Red November™ Maple, Acer ginnala 'JFS-UGA' which is a Zone 4 tree that is heat tolerant and evolved from the heat of Georgia. However, cultivars will likely be from a narrow genetic base and as such, will not have the diverse genetic ability to adapt.
Water needs
Naturalists, such as the National Audubon Society, claim that cultivars will require much more water use than native trees. However, a sugar maple tree and a cultivar of the same size sugar maple tree will both require the same amount of water assuming all other aspects and the physiology of the two trees is the same.
Whether you prefer natives or cultivars, keep in mind that natives define the region where you live and provide the ultimate in sustainability because they breed and survive without intervention. Cultivars provide the diversity of amenity and interest to the landscape. Another thought to consider deals with municipal budgets and that the cost to plant and maintain a cultivar is almost always less than the cost to plant and maintain a native tree because the cultivar was bred to survive without intervention. A cultivar can be selected as suitable for a site, instead of needing frequent maintenance and pruning that might be necessary to maintain a group of native trees in the urban environment.
Sources
* Special thanks to Jay Banks and David Lloyd-Jones for their comments on this subject.
* Audubon at Home, “Plant Native Species”, National Audubon Society, Inc., 2013.
* Lloyd -Jones, David, “Tree Morphogenesis Book 1 Reduction Via Thinning” Kindle Edition, Chapter 8, 2013.
* Ramstad, Kristin and Cynthia Orlando, “Tree Choices: Native? Non-native? Invasive? The terms can be relative”, Oregon Department of Forestry Urban and Community Forestry Assistance Program, 2009.
Editor's Note: If you know of other reasons to use natives or cultivars, please click here to send an email with your comments. lenphillips@on-line-seminars.com
The following article has been selected because it is deemed very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Natives versus Cultivars
Edited by Len Phillips
How do you define which species are native to an urban area where the built environment has erased the native soil, understory plants, and wildlife that characterize a native ecosystem? Just about every arborist I know has an opinion about growing cultivars instead of native trees. Cultivars are cultivated varieties that have been selected because they have different characteristics from a species. Cultivars, according to definition are stem grafted onto species rootstock, while clones would be selections growing on their own roots. This article will not be including clones in the comparisons below. To be perfectly clear, Red Maple, Acer rubrum is a tree native to the eastern half of the United States, while Red Sunset® Maple, Acer rubrum 'Franksred' is a cultivar that can be found growing from coast to coast in the US. I would like to offer the following comments with the hope that your opinion on this subject will be based on some facts.
Genetic features
Native – Uniform genetics in all native trees of a species being similar but having different minor characteristics.
Cultivar – Called genetically defective from the species by some experts. Cultivars are genetically identical and many arborists feel they are more desirable than the native because you can rely on the desirable characteristics of form, flowers, pest resistance, etc. being identical in all the trees of a particular cultivar. In a formal landscape, all the trees would end up with an identical shape, without pruning.
Suitable for diversification
Native – Despite the uniform genetics of a native tree species, it is not enough to be considered diversified when dealing with pests and diseases. One reason to use natives is that the pest would be known and tolerated by the species.
Cultivar – Cultivars that have been found resistant to particular pests and diseases that attack the native trees can be used for replacing the entire urban forest and maintain proper diversification.
Long-term survival
Native – These trees were growing fine without any human interference and will continue long after humans have left the planet. They breed successfully in their home range without assistance.
Cultivar – They might become extinct without human intervention, support, and management. This would be okay because they were developed to survive in a human environment. Without humans, there is no need for these trees and the forests can revert to native species.
Growth patterns
Native – These trees have a natural form that is somewhat predictable, with minor variations in size and shape.
Cultivar – A tree's special form is one reason the tree is cross bred or selected to become a cultivar. For example, Endowment Maple, Acer saccharum 'Morton' was selected from a field of sugar maples Acer saccharum for its uniformly columnar shape.
Invasive species
Native – Invasive trees tend to be trees that have aggressive growth and reproduction functions. They are invasive because of excessive seed deployment or a high number of root suckers. In many locations, black locust, Robinia pseudoacacia is an invasive tree because of its root suckers that can destroy a hardscape or choke out understory plants.
Cultivar – Most cultivars, especially the newer introductions, do not produce viable seed and are not invasive because they must be grafted on to rootstocks, which limit their ability to spread by root suckers. Bessoniana Locust, Robinia pseudoacacia 'Bessoniana' is a non-invasive cultivar.
Planting locations
Native – Native trees grow best in native soil. Native trees can adapt to growing in poor sites because among the seedlings there will often be one seedling whose specific genotype will tolerate and grow in the poor site.
Cultivar – Cultivars cannot adapt to poor sites unless they were selected because they were found growing in a poor site. Cultivars are often selected because they tolerate urban soils better than the native parent. Since the soils in our cities seldom resemble native soil, cultivars selected for their urban soil tolerance are clearly the best choice for planting in the city. Always grow the right tree in the right place. For example many cultivars of red maple such as, Autumn Flame Maple, Acer rubrum 'Autumn Flame' have been selected from trees growing in swampy areas. These trees are very tolerant of compacted, wet soil and soil low in oxygen; hence they make very desirable street trees (along with being one of the hardiest of the red maples and being the first to turn a brilliant red in the fall).
Unique features
Native – Native trees are seldom known for having unique features but they do provide a stable appearance to the landscape and a stable ecosystem for the flora and fauna that they support.
Cultivar – Cultivars are often selected because they have unique features dealing with forms, special flowers, large fruits, unusual leaf color, etc. Crabapple, Malus spp. is a good example of trees selected for their bloom, fruit color, wildlife food, and disease resistance in certain cultivars.
Flower production
Native – Most native trees have normal single flowers that provide pollen and fruit for native insects and wildlife.
Cultivar – Cultivars are often selected because of outstanding flower production. However, sometimes a certain floral characteristic has bred the nectaries out of the plant. English tea roses and double flowers are a good example. This can have deleterious effect on our native pollinators and wildlife.
Uses in the urban landscape
Native – Use natives in an area of suitable, excellent soil, such as a garden or forest, to provide a uniform appearance, texture, or leaf color. Big Tooth Maple, Acer grandidentatum is a good example of a small round tree suitable for most native landscapes.
Cultivar – Used as a street tree as well as a garden or landscape specimen, Rocky Mountain Glow® Maple, Acer grandidentatum 'Schmidt' is a good example of a cultivar that has intense autumn color and thrives in an urban landscape.
Disease resistance
Native – Native trees have good resistance to common diseases through a long association with native diseases and through the genetic diversity in any long standing breeding population of native trees, but they may not be able to withstand an exotic or a recently introduced diseases like Chalara fraxinea in Europe and more recently in England.
Cultivar – Many cultivars have been selected because they resist a particular disease. A good example is the Stellar Dogwood series of trees that are resistant to the Dogwood anthracnose which is killing the native eastern U.S. Dogwoods. Stellar Pink® dogwood, Cornus x 'Rutgan', is free of the disease and a very popular replacement.
Pest Resistance
Native – Native trees with good resistance to certain pests are acceptable, but many exotic or introduced pests can be fatal. Some naturalists consider this a desirable feature because the pest contributes to the natural food chain. For example, the woodpecker has developed a preference for the bronze birch borer found in many native white barked birches.
Cultivar – Many cultivars have been selected because they are resistant to a devastating pest. A good example might be the Heritage® Birch, Betula nigra 'Cully' which was selected for its resistance to the bronze birch borer. What impact this may have on the woodpecker population is unknown at this time but as an apex predator it may just move to its next favorite food item.
Global Warming Survival
Native – Native trees are not likely to tolerate global warming stress. The warming subjects the trees to increased disease and pest attacks. The genetic diversity of breeding native populations may somewhat buffer that species against the worst effects of warming by favoring those in its population that can adapt to changing environmental conditions. The Amur Maple, Acer ginnala might be a good example of a tree that tolerates the cold in USDA Hardiness Zone 2 and will have to migrate even further north to survive.
Cultivar – Cultivars that have been selected from warmer climates will be better survivors. An example here might be the Red November™ Maple, Acer ginnala 'JFS-UGA' which is a Zone 4 tree that is heat tolerant and evolved from the heat of Georgia. However, cultivars will likely be from a narrow genetic base and as such, will not have the diverse genetic ability to adapt.
Water needs
Naturalists, such as the National Audubon Society, claim that cultivars will require much more water use than native trees. However, a sugar maple tree and a cultivar of the same size sugar maple tree will both require the same amount of water assuming all other aspects and the physiology of the two trees is the same.
Whether you prefer natives or cultivars, keep in mind that natives define the region where you live and provide the ultimate in sustainability because they breed and survive without intervention. Cultivars provide the diversity of amenity and interest to the landscape. Another thought to consider deals with municipal budgets and that the cost to plant and maintain a cultivar is almost always less than the cost to plant and maintain a native tree because the cultivar was bred to survive without intervention. A cultivar can be selected as suitable for a site, instead of needing frequent maintenance and pruning that might be necessary to maintain a group of native trees in the urban environment.
Sources
* Special thanks to Jay Banks and David Lloyd-Jones for their comments on this subject.
* Audubon at Home, “Plant Native Species”, National Audubon Society, Inc., 2013.
* Lloyd -Jones, David, “Tree Morphogenesis Book 1 Reduction Via Thinning” Kindle Edition, Chapter 8, 2013.
* Ramstad, Kristin and Cynthia Orlando, “Tree Choices: Native? Non-native? Invasive? The terms can be relative”, Oregon Department of Forestry Urban and Community Forestry Assistance Program, 2009.
Editor's Note: If you know of other reasons to use natives or cultivars, please click here to send an email with your comments. lenphillips@on-line-seminars.com