Abstract

Elias, T.S. and H.S. Irwin. Urban trees. Scientific American 235(5): 111-118.

How can trees survive in an urban environment? The human artifacts we call cities first appeared less than 10,000 years ago. The ancestors of today’s trees are some 20,000 times older than the oldest cities. In view of the fact that trees have been exposed to more than 100 million years of selective pressure to adapt to natural environments, it borders on the biologically miraculous that any tree can occupy an environmental niche as hostile as a city street. The fact that urban trees manage to survive is certainly not the result of rapid evolutionary adaptations to the city’s hostile environment. Neither the 200 years that have seen the rise of the industrial metropolis nor the 100 centuries of coexistence between trees and city of any kind is a period long enough to have let selective pressures sort out the genes needed to give rise to a city-proof trees. It is man’s intervention, not nature’s, that has kept trees in the city. A good demonstration of this fact, on which we shall concentrate here, is the succession of street trees in the northeastern U.S. over the past 200 years.

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