September 29, 1999 | By: Aimee D. Heald
LEXINGTON, KY

This years drought has stressed many forest trees in Kentucky and regardless of any rain we might get this fall, the effects of the drought may be seen for several years.

"Each tree species has a different way of dealing with drought. Understanding how each tree species responds to drought, and our observations from past droughts, allows us to predict how trees will respond to this years situation," Jeff Stringer, a forestry specialist for the University of Kentucky's College of Agriculture, said. "Some trees growing in extremely shallow soils, such as rock outcrops, have already died."

When all of the leaves on a tree turn brown and stay on the tree during the middle of the growing season the buds and roots have also desiccated and the tree is dead. This is different from a tree which sheds its leaves during a drought. Some species such as river birch, sycamore, and if it gets bad enough yellow-poplar (or tulip poplar) loose their leaves. This is their way of avoiding the effects of the drought.

By dropping leaves they reduce their demand for water. This helps keep their buds and roots from desiccating. It also helps these species avoid some aspects of drought. Generally, drought avoiders will regain leaves after a rain or in most cases next year. Other species tough it out.

Species of oak and hickory for example, do not loose their leaves. They tolerate the drought. Their biology allows them to internally tolerate moisture stress. These drought tolerators continue to compete with one another for the limited amount of moisture in the soil.

"While tree species have different ways of dealing with drought, they can not totally avoid the effects of extreme or severe drought," Stringer said. "We may see these effects for the next several years, especially if trees are stressed next year."

Generally, trees can deal with one stress episode. If we have several years of drought, or if other things, such as a late spring frost or insect or disease-caused defoliation, occur in conjunction with this drought, the effects will become more pronounced.

One of relatively mild effects of drought is reduced growth. The drought this year could very well effect the diameter growth of trees next year. However, the most important or dramatic effect will be the death of large forest trees over the next several years, especially if next year is bad.

Research in the UK forestry department showed droughts in the mid- and late 1980s resulted in oak death for several years. While all species of oaks were affected, research showed red and black oaks were more susceptible to the drought than white oaks. If drought conditions continue into next year, this research indicates it is probable that some oak trees, particularly the red and black oaks growing along with white oaks, will die. In older woods, red and black oaks are closer to the end of their natural lives than the longer lived white oaks and are more susceptible to the effect of drought. However, it is important to note that no species is immune and mortality also can prematurely come to white oaks if they are stressed enough.

Generally, the drought does not directly kill these trees. Instead the weakened condition of the trees makes them more susceptible to insect and disease attacks which ultimately kill them. Unfortunately, there is little that can be done to help forest trees which have become weakened by drought.

"The best management option is to be ready to salvage these individual trees as they die," Stringer recommended. "The hilly or mountainous nature of Kentucky's landscape and the wide variety of forest tree species present ensures that whole forests will not die. Instead individual scattered trees will become weakened and die."

Small insects which attack and feed inside the bark of the upper branches, and fungi which infest the roots are not easily seen but are present throughout the Commonwealth and are drawn to weakened trees. They can overwhelm the trees, sometimes during one growing season, but often over the course of several years. When large branches of oaks or whole trees turn brown in late June, July or August, you know they are stressed and under attack.

Quickly salvaging these trees may be the forest owners only means of dealing with the situation. However, it is important to understand that dying oak trees can generally be left in the woods with little effect on surrounding trees. Dead trees or snags can have significant benefits for some species of wildlife and if the trees around the infected ones have remained healthy they should be able to withstand these drought related attacks.

Contact: 

Writer: Aimee D. Heald 606-257-9764

Source: Jeff Stringer 606-257-5994