THE LEAFLET

Feeling the heat these days? You’re not alone. As it turns out, trees feel heat stress. Long hot summer days tax trees just as it does us. We talk a lot about how a lack of adequate, consistent water negatively impacts a tree’s health,  but research shows that heat is also starting to play a role.

High temperatures initiate a physical response in both humans and trees. Humans sweat while trees transpire. Fluids exit our bodies through skin pores to regulate body temperature – hello sweating. Plants transpire by moving water to evaporate from leaves, stems, and flowers. Trees even have lenticels (sort of raised pores) on bark, twigs, and stems and stomata (tiny pores) on leaves facilitate the movement of water.

The hotter the temperature, the more water a tree needs: Respiration actively breaks down sugars releasing both carbon dioxide and water. As it turns out, the more a tree needs to respire, the greater amount of water a tree needs to release, so small increases in temperature can greatly increase site water demands. Trees run into trouble though – as a greater share of water is physically used to dissipate heat,  there becomes less available for tree life functions. Trees under heat loads need extra water.  Hot temperatures damage many old, young, and soil-limited trees. 

So when our area is baking under prolonged heat waves and minimal rainfall, trees truly feel the heat. That is how hot air temperatures affect a tree, but what about soil? A soil’s surface can both reflect heat and absorb it. In full sunlight, soils can reach 140°F. Heat radiated to tree surroundings and wind cooling keeps tree temperatures near air temperatures. Without the water used for transpirational heat dissipation or “cooling,” sometimes radiated heat from surroundings add to a tree’s heat load and increase associated water demand beyond availability. 

How can you know a tree is under heat stress? All that excessive heat from higher air and soil temperatures within a tree causes large amounts of water to transpire and can generate heat lesions on a tree! Leaves and twigs also feel the heat. Under prolonged heat stress they wilt, then physically scorch, senesce or color prematurely, branches and stems sunburn, and shoot and root growth become inhibited. This is one of the reasons you see a lot of sad, browning older mature trees in the summertime.

Trees, hot temperatures, and water deficits are intimately bound together, and another potential way climate change may impact us and our environment. We need trees and a fit environment to keep everyone and everything healthy. Help us make that happen at caseytrees.org/donate.