As much as tree evolution and adaptability is affected by selective pressure from the environment, we must not forget that trees have evolved communication processes over hundreds of millions of years which are more forgiving than playing life’s deadly experiments alone. Each tree produces hormones, (with evidence of many being undiscovered) and these molecules act as signals and allow communication with their selves and fellows in the area. Common science talks about 5 major hormones: auxin, cytokinin, gibberellins, abscisic acid and ethylene. These hormones switch on or off chemical pathways and affect responses not only by the type of hormone, but also by their concentration. In this way the same hormone may induce two different responses. And some of these responses are very interesting.
In 1979, David Rhoades, a zoologist at the University of Washington, was investigating the effect of tent caterpillar attacks on willow trees. He monitored two groups of trees in a field in Seattle, one with no caterpillars as a control, and the other infested. Two weeks later he plucked leaves from the infested tree and fed them to caterpillars in a laboratory and found that they grew slower than usual. What was interesting was that plucked leaves from the nearby control group of willows also inhibited caterpillar growth. The willows flood their leaves with unsavoury chemicals (normally phenolics) which discourage insect growth, however, this defence tactic had also been used by the control group, suggesting that communication must have happened via chemicals in the air from the infested tree. Since then similar results have been found with poplars and by using isolation chambers to have a third control which prevents diffusing molecules from being in contact with other trees, and with no resulting phenolic increase.
Unfortunately trees in urban environments are exposed to many more chemicals and hormonal communication can be somewhat confusing. Thus root growth may be mismatched to foliage growth, or early blooming may occur from unnatural local ethylene concentration. This is the price trees pay living with us until we can sufficiently increase our air quality. And this is a price they pay while they absorb carbon dioxide and provide us with oxygen, shade, and the natural beauty of their presence.