STRESS MANAGEMENT FOR TREES1

Excessive stress is a common cause of tree mor- tality in the urban environment. People must take positive steps to minimize and avoid stress on trees if urban shade trees are to survive and grow. Stress kills trees. However, stress is also part of the environment of all trees and a stress-free con- dition for a tree can only exist in an environmental- ly controlled laboratory. Rather, it is the degree (the intensity and the duration) of stress that determines the health effects that will result. Stress can be defined as a detrimental force or influence. Trees have evolved to most successful- ly tolerate the low levels of stress that are usually found in the natural forest ecosystem. However, even in the forest an extreme of environmental stress, such as prolonged drought, or successive insect defoliations can result in tree mortality. Ad- ditional levels of stress have been shown to deplete the tree's energy reserves and thereby decrease the tree's ability to protect and repair itself (Wargo, 1975, Houston and Wargo, 1981). It is in the artificial ecosystem of urban shade trees, sometimes referred to as the "urban forest" that stress levels often reach a lethal magnitude. Unfortunately this is precisely the environment where trees are most needed to improve the quali- ty of life in towns and cities (Tattar, 1978). The problem, therefore, is how to grow trees and preserve their health in hostile environments where high levels of urban stress, such as restricted root space (figure 1) and construction injury (figure 2) are everpresent. If we examine a