Overstory #126 - Trees for Urban Planting: Diversity, Uniformity, and Common Sense
Our forefathers planted American elms throughout the towns and cities of eastern United States. Rather than being an unconsidered 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 consequent destruction of millions of city trees served not only spur the search for replacements for American elm, but also to focus attention on urban forests.
We will not, and indeed should not, replace American elm with any single tree species in the quantities previously allotted to American elm. Instead, 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 new sites and settings for tree planting.
If we are to plant and sustain city forests that will delight and inspire the residents and visitors in our urban centers, we need both diversity and uniformity of plant material to reduce the costs of maintenance and reduce the use of potentially dangerous pesticides. We need to plant more of the superior trees developed through genetic research and utilize the practical experience of practitioners of urban forestry. We have to plan the planting of city trees, and understand the problems and potentials of our actions.
The ten percent solution
In recent years, there has arisen a dictum that "Thou shalt not plant more than 10% of any species" in a particular area. Generally, that area is undefined, but for a municipal arborist or city forester it can be interpreted as being within the boundaries of his or her responsibility. I am not sure who first propounded the "10% rule", nor am I sure that anyone would want to take credit for it, but it is not a bad idea. However, in an example city of 100,000 trees, 10,000 trees each of 10 species still represents a large degree of uniformity.
The 10% rule is a reaction to the possibility that some major insect or disease pest could, at some point in time, virtually wipe out the trees in a city. In general, the rule is 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. More recently (although the jury is still out in regard to its origin), the continuing spread of dogwood anthracnose disease on our native Cornus florida has caused great alarm.
There are also many native insect and disease problems that we are well aware of and must consider as potential threats to the urban forest. While a complete listing of such pests is beyond the scope of this paper, a few examples may suffice: oak wilt and obscure scale on oaks, fire blight on trees of the rose family, borers in white and green ash, sycamore anthracnose on Platanus species and hybrids, and the elm leaf beetle on elms. Some of these pests can be lethal, but all pests may contribute to the suboptimal growth and appearance of host trees.
In addition, there are also many known pests, native and introduced, with such a broad host range that a diversity of species, or even genera, will not discourage them. Among these are the gypsy moth, "evergreen" bagworm, Japanese beetle, Armillaria root rot, Verticillium wilt, and various nematodes. Thus, 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 esthetics of the urban forest. The l0% rule alone, while reasonable, simply does not address the realities of host-pest relationships.
Monocultures, clones, and cultivars
To begin, let us deal with a few terms that must be properly understood if we are going to communicate our thoughts and results.
It is almost universally agreed that tree monocultures are bad, even though those who espouse this wisdom may not agree on what a monoculture really is. We can start with the premise that a monoculture consists of large numbers (hundreds, thousands, millions) of plants of the same species growing in a restricted area. From this perspective, we would have to conclude that monocultures are the fundamental basis of agriculture.
There are several reasons for agriculture's reliance on genetic uniformity. One of the principal reasons is that most crop plants grown on a large scale are the products of generations of genetic research to breed and select plants that are resistant to major pests and are adaptable to specific localities. The inherent superiority of these plants and the uniformity of reliability in sowing, culture, and harvesting suggests using monocultures. Also, most agricultural plants are annuals, and if pest problems do arise, an army of scientists is ready to battle the pest, usually successfully, with new genetic combinations, chemicals, or biocontrol agents.
The city forester is not as fortunate as the farmer. Few trees currently grown and sold as clones in the nursery trade have been purposely developed and thoroughly tested for pest resistance. The trees must endure for decades in frequently difficult situations where environmental and biotic stresses are continually changing. The development of a new "replacement" for a clone, with similar characteristics of growth and pest resistance may require decades of research.
Some of the clones (trees on their own roots) now available for city planting were originally selected for certain esthetic reasons, propagated by budding and grafting, and marketed as named cultivars. Clones may not be cultivars, and cultivars may not be clones: and the distinctions between clones and cultivars have been discussed in an earlier paper (Santamour, 1976). All it takes to make a clone a cultivar is the application of a name to that biological entity. All it takes to make a grafted cultivar a clone is to put it on its own roots.
A grafted cultivar is genetically uniform above ground, and it is likely that all trees of a given cultivar will possess the same degree of resistance or susceptibility to biotic or abiotic influences. However, the use of seedling understocks, whether of the same or a different species, introduces an element of diversity that might affect tree performance. Certainly, one of the major functions of a root system is the absorption and transport of water and mineral nutrients to the tree. Genetic variation among rootstocks must have profound effects on cultivar performance. Of course, those effects are seldom so drastic that the distinctive morphological characteristics for which the cultivar was originally selected are altered to the point that the cultivar is no longer recognizable.
In summary, tree monocultures may only pose major problems when the numbers of trees are large and the area occupied by the trees is restricted. Twenty to fifty trees of a single species, or even a single clone, planted along a few blocks of city streets do not constitute a "dangerous" monoculture. Genetic uniformity within a species is to be desired, especially when the clones, cultivars, or seedlings have proved to possess certain desirable characteristics. Genetic diversity is achieved by mixtures of uniformity, and will be discussed later.
Advantages of cultivars
Cultivars are named selections. In landscape trees, a cultivar is generally propagated by budding or grafting scions from a single plant species. The above ground portion of all trees will be genetically identical, but there will be genetic variability among rootstocks.
The most obvious advantage of cultivars is their reliability, especially those cultivars that have been in the nursery trade for 20 years or more. They can be counted on to develop the form, color, and growth rate for which they were selected. Their longevity in the trade and their widespread planting has provided the testing necessary to determine both their good and bad characteristics. The urban tree planter knows what to expect of such trees.
One other characteristic of most cultivars, especially those that had been traditionally propagated by budding and grafting, is their genetic capacity for strong wound compartmentalization. Our studies (Santamour 1984, 1986) have shown that every cultivar tested, in a wide range of genera and species, were strong compartmentalizers. The conclusion was made that the grafting and budding process constituted an inadvertent screening and only strong compartmentalizing trees would be amenable to long -term commercial propagation by these techniques. Some of the cultivars formerly propagated by budding and grafting are now propagated on their own roots and have, of course, retained this important trait. On the other hand, cultivars of genera or species that had traditionally been propagated from cuttings (e.g. poplars, willows) were not subject to the "screening" process and may be either weak or strong compartmentalizers.
Interspecific and intergeneric diversity
If we are planting and managing the urban forest to minimize potential pest problems, we must look at host-pest relationships. Pests tend to follow the taxonomic categories of host plants at the species, section, series, genus, or family levels. Let us consider the genus as the major taxonomic category. The fact that we refer to many pests with host-generic names (Dutch elm disease, oak wilt, bronze birch borer, maple anthracnose) indicates that many species of the host genus are susceptible to those pests. Thus, the 10% (species) rule offers little protection against potential epidemics. Could we amend the 10% rule to include genera?
In a way, we have already done this. In many genera, only a single species in widely planted in urban landscapes: Ginkgo biloba, Gleditsia triacanthos, Pyrus calleryana, Tilia cordata, Sophora japonica, Liriodendron tulipifera, Liquidambar styraciflua, and Zelkova serrata. This is intergeneric diversity. There are relatively few tree genera in which there are several species with proven value as urban trees, most notably maples (Acer) and oaks (Quercus). The maples are divided by taxonomists into about 20 botanical sections, and the oaks into five subgenera. With few exceptions, hybridization between species belonging to these different categories does not occur; thus there may be important genetic differences among such species, The three most widely planted maples (Acer rubrum--red maple, A. saccharum --sugar maple, A. platanoides--Norway maple) belong to three different sections, yet they are all susceptible in some degree to maple anthracnose disease. We know that red oaks (subgenus Ervthrobalanus) may be more susceptible to oak wilt than the white oaks (subgenus Lepidobalanus) and that white oaks may be more susceptible to gypsy moth than red oaks. But there are notable and important exceptions to this generalization. Therefore, the quantity of trees planted in any particular genus must also be limited.
The next taxonomic category above the genus is the family. Generally, in urban America, trees of one genus of a particular family are planted in preference to others, e.g.: more Quercus than Fagus (beech) in the Fagaceae, more Betula (birch) than Alnus (alder) in the Betulaceae. There are, however, two large families that must be considered, the rose family (Rosaceae) and the legumes (Leguminosae or Fabaceae).
Leguminous trees include Albizia, Cercis, Cladrastis, Gleditsia, Gymnocladus, Labumum, Maackia, Robinia, and Sophora. Actually these genera can also be classified in three subfamilies or, indeed, into three separate families and there may be limited similarity among genera in host-pest relationships. Still, both Gleditsia triacanthos and Albizia julibrissin (mimosa) are highly susceptible to the so-called mimosa webworm.
Tree genera in the Rosaceae include Amelanchier, Crataegus, Malus (mostly crabapples in urban planting), Prunus (mostly cherries), Pyrus (mostly E. calleryana), and Sorbus. It would be extremely difficult to apply the "species" rule to the various cultivated Crataegus, Malus, and Prunus since many (if not most) of the cultivars of these genera are really interspecific hybrids of unknown parentage. Of greater importance, however, is that trees of Amelanchier, Crataegus, Malus, and pyrus are all potentially susceptible to the bacterial disease "fire blight". It is, therefore, likely that new diseases or insect pests may find a wide range of hosts in this family.
Thus, we can see that genetic diversity within a species is no safeguard against potential pest problems, generic diversity is most important, and family diversity must also be taken into account.
THE l0-20-30 FORMULA
A broader diversity of trees is needed in our urban landscapes to guard against the possibility of large-scale devastation by both native and introduced insect and disease pests. Urban foresters and municipal arborists should use the following guidelines for tree diversity within their areas of jurisdiction. For maximum protection against the ravages of "new" pests or outbreaks of "old" pests the urban forest should contain:
- No more than 10% of any single tree species.
- No more than 20% of species in any tree genus.
- No more than 30% of species in any tree family.
Strips or blocks of uniformity (species, cultivars, or clones of proven adaptability) should be scattered throughout the city to achieve spatial as well as biological diversity.
For uniformity, use clones and cultivars that have been in the nursery trade for a long time and that have proven their reliability. Use some of the newer introductions that have been developed through scientific research and that have been selected for survival traits such as pest resistance or salt tolerance. Use somewhat cautiously and on a trial basis some of the untested new cultivars of unfamiliar species or genera.
For uniformity, use seedlings of known geographic origin (or, in the case of exotics like Tilia cordata, from proven seed sources) so that the plants will be able to tolerate the general climatic conditions in your area.
For diversity, use the best clones, cultivars, and seedlings of many species and genera either as scattered strips or blocks of uniformity distributed throughout the city or as mixtures of individual trees along parkways and in parks.
For the education of future generations, plant a wide variety of trees (e.g., catalpa, hickory, horse-chestnut, sassafras, and even thorny honeylocust) in park areas that can and should be used to stimulate an interest in the diversity of Nature.
Santamour, F.S., Jr. 1976. Clone vs. cultivar: the root of the problem. Amer. Nurseryman, 144 (4): 20, 36.
Santamour, F.S., Jr. 1984. Wound compartmentalization in cultivars of Acer, Gleditsia, and other genera. J. Environ. Hort. 2:126-128.
Santamour, F.S., Jr. 1986. Wound compartmentalization in tree cultivars: addendum. J. Arboriculture, 12:227-232.
Santamour, F.S., Jr. 1988a. Graft compatibility in woody plants: an expanded perspective. J. Environ. Hort. 6:27-32.
Santamour, F.S., Jr. 1988b. Graft compatibility related to cambial peroxidase isozymes in Chinese chestnut. J. Environ. Hort. 6:33 -39.
Santamour, F.S., Jr. 1988c. Cambial peroxidase enzymes related to graft compatibility in red oak. J. Environ. Hort. 6:87-93.
Santamour, F .S, Jr. 1989. Cambial peroxidase enzymes related to graft compatibility in red maple. J. Environ. Hort. 7:8-14.
This article was adapted with the kind permission of the publisher and sponsoring organization (USDA-ARS, U.S. National Arboretum) from:
Santamour, Frank S., Jr. 1990. Trees for Urban Planting: Diversity ,Uniformity , and Common Sense . Proc. 7th Conf. Metropolitan Tree Improvement Alliance (METRIA) 7:5765.
For more information about METRIA, visit ces.ncsu.edu/fletcher/programs/nursery/metria/
About the author
Dr. Frank Santamour was a Research Geneticist in the Floral and Nursery Plants Research Unit at the United States National Arboretum for 33 years, until his death in July 2000. Frank joined the United States Department of Agriculture as a Geneticist with the Forest Service in 1957, stationed at the Morris Arboretum in Philadelphia. He studied the morphological and cytological assessment of forest tree hybrids and provenances in a wide range of genera (pines, poplars, oaks, ashes, and spruces), and the genetic improvement of pines; he was particularly interested in resistance of soft pines to the white-pine weevil. From July 1964 to July 1967, Frank was Geneticist at the Morris Arboretum of the University of Pennsylvania. During this period his major research emphasis was on biochemical and cytological studies on magnolias and a wide range of other, non-forest, landscape trees and shrubs. On July 31, 1967 he rejoined USDA at the Agricultural Research Service's U.S. National Arboretum as a Research Geneticist, to establish and lead a research project on Cytogenetics Breeding, and Evaluation of Shade Trees. Frank was the first in this country (or, in the world) to initiate work with the major goal of developing genetically superior street trees, in many genera, to withstand the environmental and biological stresses of modern urban areas. Frank helped to establish the U.S. National Arboretum as the recognized national and international leader in landscape-tree breeding and development. He developed the concepts and methods to serve as a guide in this relatively new field of research, including the introduction of biochemical methods for selection. Frank initiated and/or completed basic and applied research studies dealing with genetic improvement in Platanus, Liquidambar, Liriodendron, Magnolia, Albizia, Gleditsia, Acer, Betula, Quercus, Carpinus, and Fraxinus.
Frank graduated from the University of Massachusetts with a B.S. in Forestry in 1953; from Yale University with an M.F. (Forestry) in 1954; from Harvard University with an A.M. (Biology) in 1957. He gained his PhD from the University of Minnesota in 1960 with a major in Forestry and minor in Plant Genetics. In over 40 years as a professional scientist, Frank authored or co-authored about 275 publications, with over 200 since he joined the U.S. National Arboretum in 1967. Among many other achievements, Frank used biochemical methods to examine graft incompatibility, insect and disease resistance, and to verify interspecific hybrids in several genera. His work on wound compartmentalization, and on the correct usage of nomenclature to describe landscape trees, was also well known. Frank often used his wit and humor to make points in delivery of his scientific papers talents so well integrated that he was frequently in demand as a speaker. Dr. Santamour was generally regarded as the world's leading authority on the genetics, breeding, and development of superior landscape trees.
The United States National Arboretum web site is usna.usda.gov/
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