When trees look wilted, scorched, or stressed in the summer, many people assume the culprit is simply “lack of water.” But tree stress isn’t always about drought. Heat stress and drought stress are two related but distinct challenges trees face. Understanding the difference can help us care for them more effectively.
Heat Stress: When It’s Just Too Hot
Heat stress occurs when temperatures climb beyond what a tree can tolerate, even if soil moisture is available. Trees cool themselves through a process called transpiration where they release water vapor through tiny pores in their leaves. On above-average heat days, trees increase transpiration, and can lose water faster than roots can replace it. They may then close their leaf pores (stomata) to conserve moisture.
Heat stress is especially problematic when hot days are paired with warm nights. Trees rely on cooler nights to recover, and without that break, they struggle to recover. Warmer summer nights are one of the biggest trends we are seeing in southern Wisconsin, so expect more heat events in the coming years. Repeated heat events cause accumulation of stress in trees.
Drought Stress: When the Soil Runs Dry
Drought stress, on the other hand, occurs when there simply isn’t enough water available in the soil. Even if temperatures are moderate, prolonged dry conditions deprive roots of the moisture they need to support leaf growth, nutrient transport, and cooling. Signs of drought stress often include wilting and drooping leaves, premature fall color or leaf drop, smaller-than-usual leaves or fruit, or branch dieback in severe cases
Drought stress builds gradually, and its effects may not be obvious until weeks or months later. Prolonged droughts can affect trees for a couple of years after. During prolonged droughts, trees use up sugar reserves that they store in their roots to survive. If they don’t get the opportunity to replace those reserves the following years, they may not survive. Trees stressed by drought are also more vulnerable to pest and disease outbreaks, since they have fewer resources to mount defenses. We frequently see larger outbreaks in certain pest issues during or after droughts.
How the Two Interact
Heat and drought stress often occur together, but not always. A tree can experience heat stress during a string of hot, humid days, even when soil moisture is adequate. A tree can also suffer drought stress during a cool, dry spring when the soil doesn’t recharge. When combined, such as during the summer drought of 2023 in southern WI, the two can cause severe damage. Hot weather accelerates water loss, and dry soil leaves no reserves to replace it. That’s why midsummer droughts are the most concerning.
Helping Trees Cope
Whether heat or drought is to blame, a few steps can reduce stress on trees. Most importantly, make sure to water your trees deeply. Long, slow soaks spaced apart weekly are ideal, and are more effective than shorter watering periods daily. See our section on watering for more details.
Maintaining a 2-3 inch thick layer of organic mulch goes a long way to slow water evaporation from the ground. Mulch is an effective root insulator and can keep soil and roots cooler during heat events. A tree’s ideal mulch is its own leaves, but many homeowners prefer the cleaner look of wood chip mulch. Both provide good benefits, just make sure to avoid landscape fabric or rocks, which can exacerbate heat stress.
Monitor your trees closely for any signs of new pests or diseases. Getting ahead of outbreaks and treating issues as they come up will alleviate any further stress on your trees. Heat stress and drought stress may look similar, but they come from different sources. By recognizing the difference, we can better understand what trees are experiencing and give them the support they need.