Brown Oak Trees Dot California's Landscape
During August 1987, an unusual phenomenon occurred in California. Many oak trees began turning brown and started dropping their leaves. While most of the trees affected were deciduous species that normally do lose their leaves each year, this event was unusual in that it happened approximately three months ahead of schedule. During most years, deciduous oaks don't begin changing color until November, when short days and cold temperatures signal that winter is coming and it's time to go dormant.
The occurrence of brown tree was widespread throughout the foothills surrounding the central basin of California. However, not all trees were affected. In general, dense stands of trees were the hardest hit. Trees in clumps or thickets began changing color in early August, while most single, open grown trees remained green. It also appeared that trees in shallow, rocky soils, or on south facing slopes, were affected more than those in valleys or swales.
Because last winter was exceedingly dry with no effective precipitation in much of California since March, most observers felt that the reason for the trees changing color so early was drought. Since a large oak can drink up to 300 gallons of water per day, soil moisture rapidly becomes depleted under drought conditions. Oaks, like other trees, respond to drought with their own built-in defense mechanisms. Premature leaf drop is one of these techniques. Since it is the leaves of the trees that provide the demand for soil moisture, shedding foliage is a method trees use to reduce this demand and lessen the possibility of totally running out of water. Long-time residents of California recall that oaks also used this strategy a decade ago in response to the drought of 1976-1977.
What are the effects of drought on our oak trees? As in the past, most trees which changed color or dropped their leaves early this year will probably recover during the winter and leaf out as normal next spring. It should be noted that foresters in 1976-77 reported that some oaks remained leafless for an entire year after the drought and leaves did not appear until the subsequent year. We do not know how often this phenomenon occurs, however.
Since drought does bring on stress in trees, and stressed trees usually grow slowly and often become more susceptible to insect and disease attack, the long-term consequences of the drought could be harmful. To try to better understand these relationships, a study has been established at the University of California's Sierra Foothill Range and Field Station near Marysville. This project will keep track of what happens to both trees that turned brown this August, and to those that remained green. By monitoring growth, acorn production, and insect and disease attacks for the next five years, we hope to better understand how periodic droughts in California affect the death and vigor of our native oak trees.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc