A multi-year view of Sudden Oak Death

Sudden Oak Death (SOD) is a disease caused by the new pathogen Phytophthora ramorum (Frankel and others, 2013). This spore-producing water mold was first observed when tanoak trees began dying during the mid 1990s in California’s Marin and Santa Cruz Counties. SOD is now known to infect dozens of other species--hosts that help spread the disease--but they do not always succumb. While the SOD pathogen has been detected in nurseries throughout much of the US and in Europe the epicenter of this outbreak in the US, remains coastal northern California and southern Oregon where millions of tanoak and coast live oak have been killed. These forests exhibit a cool humid climate and frequent fog which may help the disease spread.

Tanoak have suffered the most from this infestation, as have the coastal forests of central and northern California that harbor them. Historically, tanoaks persisted in these forests due their habit of resprouting after frequent surface fires—a trait shared by Pacific madrone and huckleberries, among other competing hardwood trees and shrubs. The conifers Douglas fir and coast redwood are interspersed among these hardwoods so that large contiguous patches of canopy death from SOD are infrequent. Instead, mortality is scattered within mixed species stands (Meentemeyer and others 2008) . When viewed from above, this pattern of dispersed mortality makes mapping and even detection more challenging.

Detecting mortality in mixed species stands from satellite imagery is particularly difficult when decline is a multi-year phenomenon, when compensatory growth of surviving species is rapid, or when one or more constituent species are sensitive to climate variation. The detection of SOD in coastal California presents us with all these difficulties—tanoaks are typically a subdominant member of mixed evergreen stands, the tanoaks in a stand decline over several years, the ecosystem is otherwise productive, and remotely sensed grid cells often include drought-sensitive grass. Nevertheless, ForWarn’s multi-year time series of the Normalized Difference Vegetation Index (NDVI) captures the onset and decline of these early-hit stands along the Russian River along with apparent recovery in growth (from other species in the stand) in the last few years.