Heat Stroke in Trees

People and animals are not the only creatures that suffer from intense heat.  During periods of extremely high temperatures, such as the temperatures that have recently occurred in Texas, trees can suffer from heat stroke.  High temperatures can severely stress, and many times, kill trees.  Depending on the health, size, and species of the tree combined with the duration of the hot period and the environment surrounding the tree, trees can reach a thermal death threshold at 115ºF.  The problem is even worse for trees with restricted or damaged roots and for trees that are near pavement or are surrounded by bare soil.  The temperature of bare soil can reach 150ºF.

Trees dissipate heat by radiation, convection, and transpiration.  Transpiration, which is evaporation of water from the leaf surface, is the major mechanism that trees use to moderate temperature.  Trees without adequate soil moisture are not able to transpire and efficiently cool themselves.  Trees can lose more water from the leaves than they can take up through the roots.  To compound the problem, photosynthesis, which is the process that trees use to make food, stops at about 100ºF.  In addition, trees use a lot more of this food when it is hot.  During hot weather, trees use a great deal of their stored energy reserves to stay alive.  Stressed trees with low energy reserves usually do not survive long periods of hot weather.  Trees that do survive are more susceptible to borers and pine bark beetles.

A few strategies can be utilized to help decrease the negative effects of heat on trees.  Trees should be adequately irrigated so that they do not become moisture stressed.  Generally, two heavy waterings per week is adequate.  Avoid overwatering, which can kill many of the small absorbing roots needed to take up sufficient water.  Covering bare soil with mulch can help reduce soil temperatures and the amount of heat reflected and radiated up on the tree.  Mulch also helps conserve soil moisture.  Many trees will also need precautionary borer and pine bark beetle treatments to help protect against these insects until the trees recover from the stress.  Avoid tree pruning that removes green tissue since the tree will need as many leaves as possible to make food.  To promote health and vigor once the temperatures cool down, trees should be fertilized with a low-salt, slow release fertilizer.