Stand level infection and mortality dynamics in California redwood-tanoak forests impacted by sudden oak death

This study examines rates and patterns of Phytophthora ramorum infection and the resulting disease sudden oak death in redwood forests. P.ramorum is a generalist pathogen with a broad host range. California bay laurel (Umbellularia California), a tree in which the disease is not fatal, facilitates stand level invasion. Pathogen spill-over from bay laurel to redwood (Sequoia sempervirens) and tanoak (Lithocarpus densiflorus) is widespread with infection usually leading to mortality in tanoak but not in redwood. The primary objective of this study is to evaluate climate drivers and vegetation structure on dynamics of infection and mortality. Disease incidence and tree mortality in redwood forests were determined by repeated sampling across a system of 120 plots at five long-term research sites from 2001 through 2006. Plots were located within the known geographic area of P. ramorum in California and ranged from Monterey to Sonoma counties.  Overall mortality levels were 28.8% for tanoak, 1.2% for bay laurel, and 0.5% for redwood over the five year period. Infection frequency varied among species and was greatest in bay laurel (88%), followed by tanoak (57%), and lastly redwood (0.6%). Infection in all species was greater during years with late spring rainfall suggesting altered rainfall patterns will influence pathogen transmission. Sudden oak death homogenizes the species composition of redwood forests and may eliminate tanoak from many stands in the next twenty years. Impacts of tanoak loss are undocumented, but functional roles of tanoak and continued presence of P.ramorum require consideration in future restoration of redwood forests.