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An outbreak of the tree disease sudden oak death is affecting large forested areas along the U.S. West Coast, including on this hillside in California’s Big Sur.
Nearly half of forest ecosystems around the world face "
stand-replacing disturbances" — hazards that threaten to kill all of the trees in a localized region, such as fires, extreme weather, and disease. The spread of nonnative insects and pathogens has also reshaped North American forests, and today,
a disease outbreak is sweeping along the U.S. West Coast.
Sudden oak death, caused by the plant pathogen Phytophthora ramorum, has reached epidemic proportions in California and Oregon since it first arrived in the San Francisco Bay area in about 1990. But the regional extent of both the disease and related tree mortality is not clear, hampering forest managers' responses to the epidemic and to other threats, as die-offs can increase fuel loads and fire severity, reduce forest productivity, and convert forests from carbon sinks into carbon sources.
Cobb et al. modeled the infection and mortality rate in forests facing
P. ramorum invasions by combining observations from plot networks on the ground, geospatial data, and existing data sets describing tree cover and pathogen distribution. The pathogen can infect the leaves and stems of more than 130 species of trees, shrubs, and ferns, but in the new study, the authors focus on the four most affected tree species: California bay laurel, tanoak, coast live oak, and California black oak.
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