Heat stress reduces a tree's ability to retain water. As leaf cells dry out and collapse, visible signs of heat and drought stress begin to appear.
pound one another. Aaron Dickinson, president and master arborist at Arborcology Consult- ing , said heat stress and drought stress often go hand-in-hand. “The heat stress itself leads to drought stress,” he stated. When trees are exposed to extreme tempera- tures, Dickinson explained, they move water in an effort to support themselves during periods of intense heat. That process can ultimately leave the tree dehydrated, much like what hap- pens to a person exposed to excessive heat. The resulting symptoms are not always easy to recognize. According to Kendra Wagner, re- search scientist and research and development manager at Rainbow Ecoscience , drought- and heat-stressed trees can be difficult to identify because their foliage may simply appear wilt- ed, drooping or discolored. She likened it to romaine lettuce forgotten in the refrigerator: The leaves wilt, droop, and gradually lose their color. When a tree is under heat stress, its leaves can- not retain as much water. As the cells lose wa- ter, they become soft and eventually collapse, creating some of the visible symptoms associ- ated with heat and drought stress. VULNERABILITY CAUSED BY HEAT Those visible symptoms are only part of the problem. Heat stress can weaken a tree inter-
nally, leaving it less capable of defending itself against other environmental pressures. “We see a decreased ability of the plant to de- fend itself,” Wagner said. That reduced defense can make heat-stressed trees more vulnerable to disease and insect activity. Canker pathogens, in particular, can become more problematic on trees that have experienced heat stress, and the resulting damage may not be immediately apparent. In the Northeast, Kevin Brewer, northeast terri- tory technical manager at Arborjet , said rising temperatures are also contributing to humid- ity-related problems. Seasonal drought com- bined with higher humidity creates conditions that can leave more trees vulnerable to patho- gens and insects. “A good symptom of climate-caused stress is when you see native insects attacking native trees,” Brewer explained. For arborists, these conditions create a difficult management problem. Properly timed fun- gicide applications can help address certain pathogen issues, but the larger challenge is determining how a changing climate should in- fluence the way trees are selected, planted and managed from the start. Before looking that far ahead, however, ar- borists must first help trees recover from the stress they have already experienced. That may
Under heat stress, a tree's leaves lose their capacity to hold sufficient moisture. As cellular water levels decline, the cells lose turgor, soften, and ultimately collapse, resulting in the characteristic symptoms of heat and drought stress.
62 | Fall 2026 ArborTIMES ™
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