BERKELEY - Because the past rainfall year was relatively dry, the leaves of many oak trees in the Sierra Nevada and Coast Ranges have been turning brown, and some trees have even begun losing their leaves.. Several native California oaks, including California black oak (Quercus kelloggii) and blue oak (Q. douglasii) have exhibited these symptoms. Both of these deciduous species lose all of their foliage in the fall, but in late -summer they are normally green and leafy. A number of landowners have contacted their University of California Cooperative Extension county offices to find out what is causing the problem and whether their trees are in jeopardy.
While early leaf drop is unusual, it has happened many times before during dry years. In the severe drought of the mid-70s and again in the late 80s, some trees lost all of their foliage by mid-August. For this reason, several deciduous native California oaks – especially blue oak are labeled as “drought deciduous”. This simply means that they lose their leaves early in response to extremely dry soil conditions. This is apparently an adaptive mechanism to prevent lethal desiccation by eliminating transpirational moisture loss that can occur as long as the leaves are present on the trees.
The immediate effects of this leaf loss may be startling, but there should be little long-term impact on tree health, according to Doug McCreary, a Natural Resources Specialist Emeritus.
During the coming months, trees may continue to lose their leaves, McCreary says. There is also a strong possibility that many trees will grow new leaves before the fall, especially trees that lost their leaves relatively early in the season.
“Tree vigor also helps determine the amount of refoliation, since trees with more energy reserves are better able to refoliate than weakened trees,” McCreary said. “Trees with pre-existing stress, or infected trees that lose their foliage relatively late in the season, may not refoliate as fully. They may also experience some dieback in the crown.”
The leaf loss reduces the tree’s ability to manufacture food through photosynthesis and over time, repeated defoliations could weaken trees. But since these events are often widely spaced, long-term tree health is usually not seriously impacted. “Next year,” McCreary says, “it will probably be very difficult to tell which trees lost their leaves early, and which remained foliated late into the season.”
For more information about oaks and oak management and the names of experts working with oaks by region, visit the University of California’s oak conservation website at: http://ucanr.org/oaks. For information about sudden oak death, visit the Web site of the California Oak Mortality Task Force at: http://www.suddenoakdeath.org/.
MEDIA CONTACT:
Douglas McCreary, University of California natural resources specialist, (530) 639-8807, mccreary@berkeley.edu