The Effects of Drought on California Oaks
With springtime upon us it appears that the 1990-1991 rainfall levels will be far below normal in much of California in spite of the heavy March rains. Coupled with the fact that this will be the fifth consecutive dry year in many portions of the state, the effects of the drought promise to be extreme and far reaching. Most of the attention on the drought has focused on the impacts to crop and livestock production, and to domestic water supplies. But the drought will also affect wild plants including oak trees. Itís difficult to accurately predict what these effects will be, but a study initiated in the Sierra Foothill Range Field Station near Browns Valley four years ago can shed some light on what to expect.
During the 1986-87 and 1987-88 rainfall years, precipitation at the Station was also far below normal, only about 18-inches annually, compared to a long time average of 29 inches. In mid-August, 1987 and in early September the following year, many trees at the Station and on the surrounding foothills began turning brown prematurely and dropping their leaves. While most of the trees affected were deciduous species that normally do lose their leaves, the event was unusual in that it happened approximately 3 months ahead of schedule. During most years, deciduous oaks donít change color and drop their leaves until the short days and cold temperatures of November and December. One of the principal species affected was Quercus douglasii commonly called blue oak. This species grows in the foothills of the Sierra Nevadas and the Coast Range.
At the time the trees turned brown, it was believed that the leaf loss was directly attributable to exceedingly dry soil conditions in mid-summer. As the soil moisture became depleted, the oaks apparently dropped their leaves to reduce their moisture requirements, thus helping to offset the potentially disastrous consequences of dehydration. During previous droughts, including one in 1976-77, oaks also dropped their leaves early. Reports indicated that when rainfall levels returned to normal, however, most trees apparently recovered and leafed out the following spring. Although previous experience indicates that most oak trees that turn brown early, survive and recover the following winter, the effects of drought on trees are not known. It is reasonable to assume that severe moisture stress prompting leaf loss is not particularly good for them. Shedding foliage early eliminates the apparatus necessary for photosynthesis. As a result, growth is reduced and trees may become more susceptible to insect and disease attacks. Since current photosynthate is also used for acorn development, the loss of foliage before acorns are fully ripened (usually in October) probably retards acorn development and maturity. This may negatively affect wildlife species that rely heavily on acorns for food. It probably also reduces the number of acorns that will become sufficiently mature to germinate in the soil and develop into seedlings. The study initiated four years ago was undertaken to identify some of the effects of drought on blue oak trees. The goal was to document what happens to trees in the Sierra Foothills that turned brown so early in 1987 and 1988 by monitoring survival, bud burst and acorn production.
Two adjacent 100-tree plots of blue oaks were established in mid-August 1987. These plots were selected because the oaks in then varied greatly in degree of browning from healthy looking green trees with abundant foliage to those that had turned completely brown or were bare. There were no obvious site factors such as slope, aspect or soils to explain the differences in browness. Within each plot all trees larger than three inches diameter breast height (DBH) were tagged and assessed for degree of foliage browning and leaf loss, and given a defoliation rating. In September each tree was also evaluated for acorn production. Starting in February, 1988 each tree was evaluated twice a week to determine leafout date and survival.
All trees were evaluated again the following year. The 1987-88 rainfall season was also dry and many of the trees within the plots began turning brown again prematurely though several weeks later than during the preceding year. In early September 1988 each tree was rated for defoliation followed by an acorn evaluation later in the month, and a leafout assessment the following spring. In 1989 and 1990 normal rainfall patterns resumed, and all trees remained green into the fall.
All 200 trees survived both yearís defoliations and leafed out the following spring. Interestingly, after both drought years the trees which turned brown earliest tended to leaf out the earliest the following spring. At this time the reasons for this relationship or its implications are not understood.
Efforts to compare acorn production with defoliation were hampered by the fact that overall acorn production was relatively low each year, with fewer than 30% of the trees having any visible acorns. However, there was a significant, though relatively weak, negative correlation between acorn production and leaf loss the first year indicating that greater defoliation was associated with less acorn production. Itís possible that this relationship was even stronger than detected since the data may have been biased by the fact that it was easier to observed acorns on defoliated trees since the foliage did not obscure them. There was not a significant correlation the second year.
The results of this study suggest that summer defoliation of blue oaks has little short-term impact on growth, survival, or acorn production. However, it remains to be seen how longer-term drought periods will affect these trees. Perhaps some trees are able to survive a year or two of below normal rainfall but will be killed or damaged by prolonged and repeated droughts. Long-term weather records do indicate that 5-year drought periods are not uncommon In California. Since many of the trees on hardwood rangelands are a century or more old, however, it is reasonable to assume that the species in general have evolved mechanisms to withstand the adverse impacts of even long term periodic droughts. It therefore seems likely that while many oaks will again lose their leaves early this summer, most will survive and continue to grow.
This study will continue to monitor plot trees for the next several years to determine how trees respond to these longer-term drought periods.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc