Released July 16, 2008

BERKELEY, Calif. - Most years, deciduous oaks trees’ color change coincides with signs of autumn – Halloween pumpkins, Thanksgiving cornucopias, shorter days and cooler nights. But the 2008 drought already has some blue oaks in the Sierra Nevada foothills changing color and losing their leaves.

This is not the first time scientists have observed early leaf loss in California oak trees.

“In 1987, during a severe drought, many oaks in the Sierra Nevada foothills, as well as in the Coastal and Transverse Mountain Ranges, began turning brown and dropping their leaves in August.” said University of California Cooperative Extension oak specialist Douglas McCreary.

A survival mechanism

Oak trees’ ability to shed foliage early is a survival mechanism, he said. All plants and trees require moisture to survive. Their roots pull moisture from the soil, channel it through the trunk, branches and stems in water-conducting tissues called xylem, and into the leaves. Some of the moisture is released into the atmosphere through small pores in the leaves called stomata.

The pores allow carbon dioxide to enter the leaves and be converted into food through photosynthesis. When faced with low soil moisture, the trees can either keep their foliage and continue losing water through the leaf pores, or drop their leaves and conserve moisture. Shedding foliage does suspend photosynthesis, but in the long term it keeps the trees from drying out completely and dying, McCreary said.

“Trees in dense stands are particularly apt to turn brown since there is greater competition for soil moisture,” he said. “Trees in shallow, rocky soils or on south-facing slopes will be affected more than those in valleys or swales.”

No detectable long-term damage

Scientists at the UC Sierra Foothill Research and Extension Center in Yuba County researched oak trees during the 1987 drought and found that trees that lost leaves early suffered no detectable long-term damage.

“Drought does stress the trees, so they usually grow slowly and become more susceptible to insect and disease attacks,” McCreary said. “The long-term consequences of repeated droughts are probably harmful. But we expect that most of the trees that change color and drop their leaves early this year will probably recover during the winter and leaf out normally next spring.”

Early leaf loss may be more harmful to the ecosystem than to individual trees. Loss of leaves can hamper acorn development and maturity, reducing the number of acorns that will germinate and develop into seedlings. Reduced acorn production also adversely affects the many wildlife species that rely heavily on acorns for food.

--30--

http://news.ucanr.org/newsstorymain.cfm?story=1123

Contacts: Douglas McCreary, (530) 639-8807, mccreary@nature.berkeley.edu

Jeannette Warnert, (559) 241-7514, jwarnert@ucop.edu