Trees and Drought

University Park, PA -- September 27, 2016 -- In Pennsylvania, we have had an extraordinarily hot and dry summer. Those who make their living from the land are well aware that rain is changing. When it occurs, it is more intense and has seemingly less value to crops. It seems that those less connected to the land celebrate the warm days without rain – another sunny day is not always the best day.

Imagine what it is like to have your roots anchoring you in one place and depending on rain from the sky to ensure there is adequate moisture in the soil to keep you working. What kind of work does a tree do, you ask? Well, trees use carbon dioxide from the air, water from the soil, and light from the sun to make sugar through work called photosynthesis.

Photosynthesis is a complex process that requires certain conditions. All of our trees have leaves where the magic occurs. Tree roots collect and move water, which is absolutely essential, along with minerals and nutrients through long soda straw like tubes in the tree’s bole to the leaves. Photosynthesis involves combining carbon dioxide, which enters the leaf through small openings called stomates, water, and light in special cells called chloroplasts which contain chlorophyll (the green color in leaves) to make sugars. Stomates are important part of the process as they have the ability to open and close and thus control photosynthesis.

Stomates open and close by monitoring the amount of water available and air temperature. If the temperature is too high, then water demand is too high, and the tree stops making sugars necessary for its growth. When that happens, trees have to respirate. That is, they use up sugars to carry out life functions. The relationship between water in the soil and leaves is critical. And, on a hot summer day without rain, a tree might spend more of its time using up its sugars than using them to make wood, seeds, new twigs and buds, repairing damage, and getting ready for winter.

There is a lot going on with trees even when they are not growing. If things get really hot and water is too scarce, trees and most other plants will wilt and loose turgor pressure in their leaves. You have seen those wilting leaves. If water comes soon enough or the air temperature drops as it does late in the day and through the night, plants can recover; however, the stress of inadequate water can take its toll.

Trees under stress are susceptible to many threats. Insects and diseases are often lurking in the environment to take advantage of tree defense mechanisms negatively affected by heat and inadequate water. Healthy trees are constantly restoring and repairing weakened or damaged defenses. For example, Armillaria mellia, a common root rot, is always present the soil. When roots struggle to find water, they may begin to decline as water is actually pulled from their fine roots by the soil itself. Re-establishing water movement processes from those points to the leaves takes resources, and the roots may lose their battle with the root rot fungus and as a result begin a slow process of decline and, perhaps, death.

Across Pennsylvania, trees are showing signs of stress. Already, as you look around the neighborhood, you might see some trees are having leaf loss at the tops of their crowns. Elsewhere in the crown, leaves are detaching and littering the lawn with green rather than autumn colors.

You may have also noticed trees on road cuts turning brown or showing premature yellow. These cuts where the soil is shallow or facing south or west are often quick to show moisture stress. When water is scarce, as it is now, it is common to see maple and birch shedding leaves or going brown.

Elsewhere, there are reports of patches of oak, red and sugar maple, and even tulip poplar changing color sooner than expected or even appearing dead. It is difficult to interpret what is happening in all cases, but in some, the site might be poor, with shallow soils, or oriented to receive more direct light and heat; trees are responding by casting leaves earlier than expected. 

Water is essential for plant growth. Heat and lack of rain make for difficult growing conditions. Over the next few years, based on this summer alone, expect trees to struggle even if conditions are better next year. As we approach the end of the growing season, there is not much we can do for individual trees showing stress responses, especially in the forests. Lawn trees might benefit from deep watering. Make sure they get at least two inches of water under their crown spread every 7 to 10 days until the soil freezes.

Written by Jim Finley, Ibberson Professor of Forest Management and Director, Center for Private Forests at Penn State