Trees for Urban Planting: Diversity Uniformity, and Common Sense

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: (1) plant no more than 10% of any species, (2) no more than 20 % of any genus, and (3) no more than 30 % of any 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. Terms such as “monoculture”, "cultivar" , and “clone” are discussed and a rationale is provided for the planting recommendations. The potential problems of graft incompatibility in some species are considered in the light of recent research. The widespread planting of American elms in the towns and cities of eastern United States by our forefathers was not a stupid idea. Rather, our early horticulturists were simply 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 to focus attention on urban forests but also to quantities previously allotted to American elm. 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 settings for tree planting. THE TEN-PERCENT sites and