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Drought and Native Trees

Drew Zart
Drought and Native Trees

by Drew Zart

beetle attacks on drought stressed coast live oakWhether it’s referred to as prolonged drought or the effects of climate change, or both, there’s no doubt that Marin County is getting dryer and the native trees are suffering from it.  In both tree pathology and entomology, a common general statement is that native trees aren’t susceptible to native disease pathogens or insect pests because they’ve evolved various defense mechanisms.  However, when physiological stressors like drought are involved, this generalization starts to fall apart.

Two of the major health concerns with native trees under stress are boring beetles and canker pathogens.  Take the iconic coast live oak (Quercus agrifolia) as an example.  In the past several years, we have seen a significant increase in live oaks attacked and often killed by the native ‘western oak bark beetle’ (Pseudopityophthorus pubipennis).  Under normal conditions, these beetles can only attack and breed in trees that are dead and dying from other causes, but with widespread drought stress, most live oaks are susceptible and beetle populations have dramatically increased.  Another example is the fungus Armillaria mellea.  This fungus is widely present in forest soils of California but is only capable of infecting and ultimately killing oaks and other tree and shrub species when they’re under physiological stress.

armillaria cluster growing from roots of dead live oak pUnfortunately, there isn’t much to be done on a large scale to protect our native trees.  On an individual tree basis, things like mulching to preserve soil moisture and irrigating during ‘normally’ wet months that don’t get rain (like this past February) will help maintain healthy trees, however these actions aren’t feasible on a forest-wide scale.  In large part, we’re stuck hoping for rain and waiting to see what survives.

Drew Zwart, PhD

Bartlett Tree Research Labs

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