In spite of relatively abundant rainfall this past winter, the leaves of many oak trees in the Sierra Nevada and Coast Range are missing or turning brown. Douglas McCreary, UC Berkeley Cooperative Extension natural resources specialist, theorizes that early spring weather conditions are to blame and expects the trees to recover.
"Above all, landowners should not assume the trees are dead and cut them down just because they have lost their foliage," McCreary said. "Next year it will probably be very difficult to tell which trees lost their leaves early, and which remained foliated late into the season."
In recent weeks, people have reported that oak trees normally fully leafed-out at this time of year are almost completely bare or have just isolated tufts of new foliage. Others have observed oaks partially or completely covered with dead, brown foliage. In some cases, entire hillsides now have oak forests with few, if any, leaves.
McCreary confirmed that several native California oaks, including deciduous blue (Quercus douglasii) and valley oak (Q. lobata) have exhibited these symptoms, as well as interior live oak (Q. wislizeni), an evergreen species that normally has a full canopy of foliage throughout the year.
McCreary said the die-back is almost certainly not Sudden Oak Death because the symptoms are different, none of the affected oaks are SOD hosts, and the locations where this has been observed generally don't overlap with the SOD infestation zone.
He attributes the leaf death to this year's early spring weather conditions.
"In mid- to late-April, there were several nights with extremely cold temperatures for that time of year, falling below freezing in many foothill locations on April 20," McCreary said. "The problem for the oaks was that at the time the freeze occurred, many trees were just starting to leaf out. The recently emerged leaves were very succulent and tender and, consequently, vulnerable to the low temperatures."
When the freeze occurred, relative humidity was also very low, further contributing to the damage. However, only oaks in certain locales were affected. At high elevations, most oaks had not yet started to leaf out, so there was no damage. At many low elevation sites, temperatures weren't cold enough to freeze the foliage.
"It appears that the greatest amount of damage occurred in the foothills at mid-elevations or in isolated frost pockets where temperatures were lower," McCreary said.
Although experience with this type of damage is sparse, based on other conditions that cause defoliation — such as low-intensity fires or insect outbreaks — McCreary said it is likely that the oaks will recover and produce a new crop of leaves this year.
"When there are ground fires in oak woodlands, the leaves on many trees become so hot that they are killed," McCreary said. "As long as the cambium (the area immediately beneath the bark) has not been lethally heated, the trees will usually recover and produce a new crop of leaves that same year. Similar recovery happens following spring defoliation by insects such as tent caterpillars."
The long-term consequences of the defoliation are harder to predict, McCreary said.
"During some years there can be unusually high levels of foliar diseases such as Anthracnose, which can exacerbate freeze problems," he said. "Also during especially dry years, oaks tend to drop their foliage in late summer. This could also prevent trees from recovering fully."
In general, tree vigor at the time of defoliation influences the response. Trees with more energy reserves are better able to recover than weakened trees. Another likely consequence of the freeze will be a reduction in acorn production since many flowers — which emerge at roughly the same time as the new foliage — were also killed.
While the current situation may not lead to long-term damage or tree death, it is certainly not good for the trees, he said. Leaf loss resulting from freezing reduces a tree's ability to manufacture food through photosynthesis and, over time, repeated defoliations could weaken trees. But because these events are often widely spaced, long-term tree health is usually not seriously impacted.
For more information about oaks and oak management and the names of experts working with oaks by region, visit the home page of the Integrated Hardwood Range Management Program at http://danr.ucop.edu/ihrmp/. For information about sudden oak death, visit the Web site of the California Oak Mortality Task Force at http://nature.berkeley.edu/comtf/.