Threats to Oak Woodlands

Blue Oak Woodland Mortality

image: Emil Hochdanz / Public domain

Elevated levels of blue oak mortality have been reported in various parts of California over the past few years.

Reported oak mortality tends to occur in a scattered or patchy pattern across the landscape. In some places, other species, such as valley oak have been affected. Concerned landowners have wondered whether another introduced pest or pathogen could be the source of this elevated mortality.

View “Recent Increases in Blue Oak Mortality,” Swiecki,Ted and Elizabeth Bernhardt, Phytosphere Research, April 2020

View “Tree Mortalilty in Blue Oak Woodland During Extreme Drought in Sequoia National Park, California,” Das, Adrian J., Ampersee, Nicholas J., Pfaff, Anne H., Stephenson, Nathan L., Swiecki, Tedmond J., et. al., 2019

Disease: Sudden Oak Death

Sudden Oak Death, or SOD, was first observed in California in 1995. In 2000, UC researchers identified the causal agent as Phytophthora ramorum, a pest that had never been identified before.

UC provided initial seed money to assist in studying this new disease, and to establish the California Oak Mortality Task Force (COMTF), a multi-agency collaborative team focused on understanding this new pest and educating the public about how to manage infected stands and curtail the spread of this damaging disease. 

Invasive Plants: Barbed Goatgrass

Barbed goatgrass (Aegilops triuncialis) is an introduced annual grass that is spreading throughout California grasslands. It is listed as a noxious weed by the California Department of Food and Agriculture and as a wildland weed by the California Exotic Pest Plant Council. When mature, it is unpalatable for livestock. In grasslands, Barbed goatgrass reduces the abundance of native perennial bunchgrasses and competes with more desirable introduced annuals, as well as native forbs.

Download  publication here.

A Field Guide to Insects and Diseases of California Oaks

Thousands of species of microorganisms and invertebrates inhabit oak woodlands. This book focuses the relatively small number of these are capable of causing damage to oak trees. The more conspicuous of these, for example the California oak worm with may completely defoliate a large tree, draw the most attention because of the nuisance they create, especially in urban areas. However, structurally damaging fungi that decay heart wood often go unnoticed until the tree’s health and trunk and large branches are weakened and, especially in an urban area, may need to be removed. Furthermore, since 1995, sudden oak death disease and the goldspotted oak borer have invaded and killed tens of thousands of coast live oak trees in wildlands of northern and southern California, respectively. This publication provides information that will help tree-care professionals and the general public identify and assess insects and diseases of California Oaks. A much more comprehensive treatment of these agents is provided in California Oak Disease and Arthropod (CODA) database linked on this website (

Citation: Swiecki, Tedmund J. and Bernhardt, Elizabeth A. 2006. A field guide to insects and diseases of California oaks. Gen. Tech Rep. PSW-GTR-197. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture, 151 p.

Conifer Encroachment

The loss of deciduous oak woodlands to native conifer encroachment is a major conservation concern in northwestern California and across much of the Pacific Northwest, resulting in associated losses of wildlife habitat, cultural uses, biodiversity, and other ecosystem services. These losses have drawn increasing attention in recent years, and oak woodland conservation and restoration efforts have gained momentum throughout the ranges of Oregon white oak (Quercus garryana) and California black oak (Quercus kelloggii), the two species most afflicted by encroachment …  Read more

Pests: Goldspotted Oak Borer

Goldspotted Oak Borer

The Gold Spotted Oak Borer (GSOB) is a recently discovered insect that has been decimating some coast live oak (Quercus agrifolia) and California black oak (Quercus kelloggii) stands in Southern California, so far exclusively in San Diego County. It is believed that this non-native insect arrived in California during the last two decades, likely from firewood transported to the state from Arizona or Mexico where GSOB is known to live. The University is currently involved in a large educational effort to make the state’s residents understand the threat this insect poses and to advise them on how they can help contain … Read more

Oaks and Climatic Factors