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A Push-Pull Approach to Increasing Biodiversity

T. Davis Sydnor

Professor of Urban Forestry, School of Natural Resources,
The Ohio State University, Columbus, Ohio

Manuscript number OH184-96.

A paper from the Proceedings of the 9th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Columbus, Ohio, August 8-10, 1996.


The loss of the American elm during the first half of this century has underscored the potentially disastrous results of having a monoculture in our cities (Dirr 1990). Academicians, urban foresters, and nurserymen acknowledge the need for biodiversity (Gerhold, Lacasse and Wandell 1993). Hahn (1996) and Vild (1996) noted that their city inventories contain 58 and 45% respectively plants in the genus Acer. This is in contrast with the 20% suggested by a commonly used guideline governing biological diversity. The 10-20-30 rule suggests that no more than 30% of the trees be of the same family, 20% of the trees be of the same genera, and 10% of the trees be of the same species. Although we have been concerned about biodiversity for many years, the concept has been elusive and difficult to put into practice.

Urban foresters wanting to increase species diversity encounter many challenges. For instance, many useful species and seed sources are unknown to both the nursery producer and the urban forester; assets and liabilities of certain tree species to cities may not be known; initial demand for uncommon or new species is low and thus new species remain unsold; nursery inventories of new species tend to be low, reflecting the demand; production techniques for rare plants typically are unknown, or production cycles are longer or more costly than for commonly grown species or cultivars; little or no incentive exists for marketing plants that are not protected by patents or trademarks.

Since new species introduction faces many challenges, a multifaceted approach was deemed best by the author:

This paper outlines a multifaceted approach to new species introduction that is being developed at Ohio State University.

Increasing Awareness and Evaluating Performance

Nursery producers, landscape contractors, urban foresters and property owners are often unaware of potentially useful trees. Awareness of new plants must be increased at all levels. Both growers and users have a stake in increased diversity in Ohio's cities; thus they were the principal target groups. A paper, Protecting Your Community's Forest Through Biological Diversification or Let's Give the New Guy a Shot, was prepared to introduce urban foresters to infrequently used species. This publication accompanied a survey form sent to urban foresters.

Primary contact was through Ohio's regional state urban foresters who sponsor Tree City USA award ceremonies in their regions. These meetings included most of Ohio's urban foresters. I spoke on the topic of diversification and passed out the paper and survey form. At other meetings the regional foresters announced the program and passed out the publication and survey form.

Because established plants in the landscape are often the best method for introducing new plants, three Ohio sites were developed to introduce and evaluate new trees: The Ohio State University Columbus campus, the Ohio's Shade Tree Evaluation Project at the Wooster campus, and plantings in selected Ohio cities.

The Ohio State University campus in Columbus has long been used for evaluating new plant introductions. This long tradition of new plant evaluation results from continuing relationship between OSU's Buildings and Grounds Department and academic units. Many species and cultivars of the 120 taxa on the survey form are currently on campus. Students and visitors alike can evaluate the performance of these plants. The short fall of using campus sites for evaluation is that they are not often replicated plantings.

A second evaluation site is Ohio's Shade Tree Evaluation Project (STEP). The project, begun in 1965, evaluates shade tree performance under Ohio conditions. More than 180 species and cultivars have been evaluated on the 6.4 ha (14 A) plot. Currently it contains 1090 trees; each taxa is represented by eight single tree plots. The trees are available for study during daylight hours.

In STEP, trees performance is evaluated in more favorable environments than typically found in city streets. The STEP site has relatively undisturbed soil and an unrestricted root zone. The uniform environment allows studies of specific functional characteristics (Gallagher and Sydnor, 1983) as well as aesthetic features (Haserodt and Sydnor, 1983). For more than a decade, a group of green industry professionals evaluated the trees twice annually. The STEP plot has many 30 year old specimens which have outgrown the experimental plot. Currently the STEP plot is being renovated with the idea of regaining some of its former uses. Current plans are to evaluate trees up to 20 years of age and then reestablish new plots.

The third tree introduction and evaluation "site" is Ohio's cities. Many of Ohio's urban foresters are keenly interested in urban forest diversity. Annually they plant uncommonly grown trees for evaluation. Recently their efforts have been augmented by the American Electric Power (AEP) Smart Tree Program. AEP is donating uncommonly grown trees to cities in the AEP service area for evaluation. The only stipulation is that the trees must be sited responsibly to avoid interference with utility lines.

Tree performance in city (real world) environments is given particular credence by urban foresters. Urban foresters are quick to share experiences. The recent annual meting of the International Society of Arboriculture in Cleveland featured a field trip to evaluate unusual tree species in city environments.

Identifying Demand

City foresters and municipal arborists were asked to read the Protecting Your Community's Forest Through Biological Diversification or Let's Give the New Guy a Shot publication and identify the number of trees that they anticipated planting/purchasing in the year 2000. The survey's heading and the first ten plants on the survey form are given in Figure 1 (figure unavailable).

Survey forms were returned from twenty-five cities and villages in Ohio. Surveys were tabulated with a total of 17,965 trees requested for planting in the year 2000 (Sydnor and Struve 1996). There were 27 species in 20 genera requested more than 200 times. Celtis reticulata was the only example of a species that was introduced in the lectures and publications that was heavily requested. This tree has a mature height of less than 25' and high alkaline soil tolerance. These two characteristics are desirable for urban plants and may account for its high demand.

Acer was the most popular genus followed by Quercus, Fraxinus, Amelanchier, and Tilia. Gleditsia, Pyrus, Ulmus, and Syringa were requested heavily but less often than the first five. Amelanchier, Syringa/, /and Ulmus are increasing in popularity based on the number of recently released cultivars.

Results of the survey were conveyed to the nursery producers when the survey results were published in August 1996 edition of The Buckeye, official magazine of the Ohio Nursery and Landscape Association (Sydnor and Struve 1996). Nursery producers had an estimate of the demand for new plant introductions. Producers could order liners for Spring 1997 planting allowing three growing seasons for the plants to reach a saleable size. Fast growing trees can reach 1.5 to 2 inch caliper size within three to four years when starting from a liner. Slower growing trees would not be saleable in four years, but the plants would be in production. A source would have been identified and an urban forester could then incorporate the new plants into their plans. Currently the STEP plot is being renovated with the idea of regaining some of these uses. Trees were allowed to remain resulting in a mixed forest. Current thinking will retain plants for no more than twenty years in the future.

Production Research

Ohio State has been involved in nursery production research for years. The Ohio Production System (OPS) is a result of this effort (Struve et al 1987). Recently Dr. Dan Struve has teamed with American Electric Power and is using the OPS to produce AEP's Smart Trees. Most of the "SmartTree" candidate species are not commonly grown. One objective is to evaluate these species, a second is to develop production schedules. Small tree species are another particular interest for AEP since smaller trees can be planted beneath power lines without requiring regular pruning. Struve (1996) notes that more than 1400 Smart Trees have been donated for planting since Fall of 1994.

Marketing Incentives

Marketing is thought by many to be a dirty word but, in my judgment, it is a key to increasing diversity in the urban forest. The current popularity of clonally propagated trees is partially a result of a nursery being able to recover marketing costs. The key to recovering costs is to identify a characteristic that can be marketed. Most trees have been marketed for aesthetic characteristics such as fall color.

Seed sources can be marketed and protected as easily as clones if distinguishing traits can be identified. Patenting will protect the sources from unauthorized propagation. Seedlings can also be sold under a trademark that would protect the name but would not prevent others from propagating the plant. Both patenting and trademarking allow the seed source developers to recover marketing costs.

Defined seed sources can deliver a trait of significant value such as cold hardiness to an urban forest. Koelreuteria paniculata is produced mainly from seed in the US. Seedlings in Ohio are variably cold hardy. Many individuals were killed during the Winter of 1993-94 but occasional individuals suffered little or no damage (Sydnor 1994). Collecting seed from cold-hardy individuals would constitute a cold hardy land race. Identifying and marketing cold hardy seed sources would be a boon to an urban forester who could specify the proper seed source and reduce or eliminate winter injury. Koelreuteria paniculata is urban and alkaline soil tolerant and can be grown beneath utility lines but is not commonly used because of the danger of winter damage.

Selections of specific individuals from improved seed sources can lead to clonal introductions if the plant can be asexually propagated. Patents might be preferred in this situation since the patent holder controls propagation rights. Many aesthetic characteristics are best captured in this fashion.

New plant introductions are an excellent opportunity for industry-wide cooperation. Ohio is fortunate in that several nurseries are actively involved in introducing and marketing improved cultivars.


Increasing diversity requires many steps to insure success. All members of the Green Industry need to be made aware of some of the excellent plants that are available to increase the diversity of our urban forests. After increasing awareness, demand must be quantified and shared with the producers so that adequate quanties can be grown. Production techniques are being evaluated for production of plants that have been identified to assist producers. Sources of desirable trees that are available for sale must then be identified. The final step would then be to convey sources of desirable trees back to the urban forester so that the trees can be purchased and planted resulting in a more diverse planting.


Dirr, M. A. 1990. Manual of Woody Landscape Plants, Fourth Edition. Stipes Publishing, Champaign, IL. 1007 pp.

Gallagher, P. W. and T. D. Sydnor. 1983. Variation in Wound Response among Cultivars of Red Maple. J. Amer. Soc. Hort. Sci. 108:774-746.

Gerhold, H. D., N. L. Lacasse, and W. N. Wandell. 1993. Street Tree Factsheets. College of Agriculture, Pennsylvania State University, State College, PA. 393 pp.

Hahn W. H. 1996. City of Akron, OH. Personal Communication.

Haserodt, Heidi and T. D. Sydnor. 1983. Growth Habits of Five Cultivars of Gleditsia triacanthos. J. of Arboriculture 9:186-189

Struve, Daniel. 1996. Personal Communication.

Struve, D. K. M. A. Arnold, and D. Chinery. 1987. Red oak whip production in containers. Proc. International Plant Prop. Soc. 34:415-420.

Sydnor, T. D. and D. K. Struve. 1996. Your Customer's Needs for the Year 2000. Educational Update, The Buckeye, August 1996. 8pp.

Sydnor, T. D. 1994. Observations about How Specific Plants Performed Following the Infamous Winter of 1993-94. Educational Update, The Buckeye/, August 1994. 8pp.

Vild, C. 1996. City of University Heights, OH. Personal Communication.

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Format updated July 24, 2009