Drought could mean long-term plant damage

Photo by Mike Kay

Yellowing leaves on this tulip poplar, near Harmony, are the result of a lack of rain.

The recent hot, dry weather has already resulted in short-term drought damage to trees. If this drought continues, there could be long-term consequences to trees and other plants as well.

Typically, when dry conditions prevail, trees begin to loose more water than they take in. This water loss is mainly through the stomata of the leaves and is part of photosynthesis, the process by which trees obtains energy from the sun. When water becomes scarce, the tree will slow down its energy production and close its stomata to conserve water, and the tree begins living off its energy reserves.

Short-term effects of drought include a limp appearance to the leaf and scorching of the margins. As drought continues, buildup of hormones occurs in leaves, which results in premature coloration, winter bud development and early leaf fall. Some plants, like tulip poplar, are very sensitive to drought, and many open-grown poplars are now beginning to display yellow leaves.

As the drought continues, trees are unable to obtain and store the energy they need; as a result their growth and vigor decreases. The lack of water in the ground also causes the small fibrous feeder roots to die back, which further affects tree health since it is these roots that extract the water and nutrients that a tree needs to survive.

Trees under drought stress begin to emit volatile oils that attract insect pests such as borers, which can cause much harm. Some of the more common borers that are attracted to trees under stress are pine bark beetles, bronze birch borer, dogwood borer, ambrosia beetle and greater peachtree borer -- all of which can have fatal consequences to trees. If that weren't enough, a weakened tree is less able to produce natural insect and disease repellents like oleoresins, tannins and alkaloids that would ward off these pests. The result is that the tree's defenses are weakened and they are more vulnerable to attack. Drought-weakened trees are more susceptible to cankers, root rots and wilting diseases. Some of the more common diseases that can be the result of drought include bacterial leaf scorch, oak decline and nectria canker.

The combined effect of energy loss, root damage, and insect or disease attack can be the dieback of branches, entire sections of the tree or the demise of the tree.

The last couple of years have been pretty favorable for trees, in general; if rain resumes, along with winter snowfall, trees should be able to weather this early drought. Newly planted trees are more susceptible to drought since many of their feeder roots were cut during transplanting, so it would be a good idea to water them if we experience prolonged periods (two weeks or more) without any significant rain.

Nature Notes for 7/25/2010