By Trish Grenfell, Master Gardeners of Placer County
Q: Our Blue Oak trees on the perimeter of our property are losing much of their leaves. I know they are deciduous trees but their leaves normally don’t drop until the fall. Do you know what could be the cause? I fear it is Sudden Oak Death.
A: The dry days of our late winter and spring have taken their toll on our native trees that depend on winter rains. (I assume you do not irrigate your trees.) Questions like yours are popping up in our county this summer from homeowners like you and from folks who have noticed early color change and defoliation in our oak woodlands. Due to the publicity about Sudden Oak Death, it is natural for tree owners to fear that this is the problem. Dave Rizzo, UC Davis plant pathologist & SOD expert, states that Blue Oak is not a host of SOD and SOD infections on Black Oak are limited to stem cankers.
UC Cooperative Extension oak specialist Doug McCreary explains that the oak trees’ ability to shed foliage early is a survival mechanism. When faced with low soil moisture, the trees can either keep their foliage and continue losing water through 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. Trees in dense stands are particularly apt to turn brown since there is greater competition for soil moisture. Trees in shallow, rocky soils or on south-facing slopes will be affected more than those in valleys or swales.
“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.