Dr Nina Bassuk, Professor at the Urban Horticulture Institute, School of Integrative Plant Science, explains creating urban tree biodiversity within a uniform street tree landscape
Street tree landscapes typically consist of uniform rows of a single species, generally selected for their high branch clearance from the ground, attractive appearance and high tolerance to urban stresses. However, the desire for uniformity, which can be seen in countries all over the world, has created a conflict between the cultural preference for visual uniformity and the practical need for species diversity.
Most cities exhibit a low species diversity when it comes to urban trees. This may be caused by historical convention combined with the assumption that only a few species would adhere to cultural and design norms. However, the over-planting of a limited number of species called monocultures, have brought about the decline of formerly common and numerous types of tree. When a few trees dominate, their diseases and insect pests can proliferate causing tree decline.
Elms, chestnuts, and ash trees have been decimated by the proliferation of insects (emerald ash borer) and diseases (Dutch elm disease, Chestnut Blight). It is clear that design objectives must be balanced against the practical need for species diversity in street tree plantings.
Faced with the difficulty of balancing aesthetic and ecological concerns, current designers all too often short-change or even abandon one or the other objective-genetic diversity or visual uniformity. Where they may have once planted an entire neighbourhood with the same species, those favouring uniformity over practicality might now plant a single species for one or two blocks of a given street. Although this sort of compromise may feel like a bow to diversity, it isn’t a true solution to the problem. Planting trees in somewhat smaller ‘same species’ blocks will not necessarily prevent the kinds of devastation associated with monocultures on a block by block basis, particularly if the species selected have already been heavily planted in the community.
For those favouring an ecologically sensible approach, the alternative to monocultures is sometimes to plant wonderfully diverse selections of trees that share no common characteristics whatsoever. The results of such efforts can be aesthetically disappointing and in a number of cases have led to public outcry.
Unfortunately, this type of plant selection has served to fuel the idea that the only way to achieve uniformity in design is through the exclusive use of one species.
The case for visual uniformity
So, what makes uniform plantings so appealing in the first place? What makes them so difficult to give up? The advantages to uniformity are primarily aesthetic and have a long-standing tradition over many centuries internationally. A street lined with rows of more or less identical trees brings to most observers a sense of order and tranquillity, even the domination of nature. In the most heterogeneous of neighbourhoods, a uniform row of trees can have a cohesive influence, tying together diverse elements and creating a sense of neighbourhood identity. Street trees can also soften the potentially jarring transitions from residential to commercial areas.
Moreover, marching rows of identical trees have been used as a symbol of power by military commanders as they marched their armies down those uniformly designed streets.
The case for species diversity
Unfortunately, the appeal of same species plantings is ultimately outweighed by its disadvantages. Even if aesthetics were the only consideration, the fact that unhealthy or dying trees are unattractive makes the need to diversify unavoidably.
Another factor that makes monocultures impractical is the tremendous diversity inherent in the urban environment. The challenges and stresses for trees can change dramatically within very small distances, often making it impossible for a single species to thrive uniformly throughout a given area. Variables, such as light, reflected temperature, drainage, soil compaction, limited rooting space, soil pH, availability of water, exposure to salt, and restrictions to crown development can vary tremendously even from one tree space to the next. A careful assessment of site conditions prior to plant selection rarely points to the selection of a single species. Even those who are aware of this fact often make the mistake of selecting one species that will purportedly survive under any and all difficult conditions. Such widely adaptable species dominate the aforementioned list of overplanted trees that have suffered decline, become unmanageable, or both.
To avoid similar problems in the future, it is clear that uniform plantings of a limited number of species must be avoided. But, is it possible to gain the practical advantages of diversity without giving up the aesthetic desire for uniformity? Fortunately, the answer is yes. Through the careful selection and grouping of plants, communities of trees can be created which, despite their genetic diversity can satisfy our desire for visual uniformity.
By breaking down the visual characteristics that distinguish one species from another into basic categories, we have selected a set of five criteria for putting genetically diverse species into aesthetically compatible groups. The first criteria concerns height to first branch. The distance from the ground to the first branches of the tree canopy creates the visual and physical envelope that we view or walk under. By keeping this space equal between trees, the walking or viewing experience appears alike between trees. The second and third criteria are tree size and shape. Nothing will create a uniform appearance less than a large tree planted next to a small tree or a narrow tree planted next to a broad spreading tree.
The other two criteria of less importance are branching density and foliage texture. They are given secondary consideration because they generally are not as obvious to the casual observer and can even become difficult to distinguish as the distance from the observer increases. By using these criteria, it is possible to have greater biodiversity in our cities while acknowledging the desire for visual uniformity.
Dr Nina Bassuk
Urban Horticulture Institute,
School of Integrative Plant Science
Cornell University, New York, U.S.
Tel: +1 607 255 4586
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