Ecological niche patterns of three exotic forest pathogens in California coastal forest communities

Phytophthora ramorum (Pr), cause of Sudden Oak Death, is an exotic generalist pathogen established in over 450 km of California coastal forests. Two additional, potentially exotic Phytophthoras, P. nemorosa (Pn) and P. pseudosyringae (Pps), were recently discovered with similar host and geographic ranges and causing similar disease symptoms as Pr. However, Pn and Pps do not cause landscape level mortality of oaks (Quercus spp.) or tanoak (Lithocarpus densiflorus) as does Pr. A plot study was established to relate the presence of these three pathogens to community, structural, and environmental variables. Between 2001 and 2005, 499 1/20ha plots were established at 38 sites throughout central and northern coastal California. Four native forest alliances were surveyed: coast redwood, coast live oak, mixed oak, and Douglas-fir-tanoak forests. Over 21,000 trees and shrubs were measured and examined for presence of aerial Phytophthora species. Pr, Pn, and Pps were found in all forest alliances. Pr was found in 40% of plots, with Pn and Pps recovered from 18% and 13%, respectively. A classification and regression trees (CART) analysis was used to screen potential predictor variables. A general additive model (GAM) was then used to examine the characteristics of the partial response curves of selected models. Pathogen presence was highly related to forest structural, climate, and biophysical variables, although the patterns varied by species. Increased cover of bay laurel (Umbellularia californica) (a reservoir host that supports sporulation) was found to increase the probability of occurrence for all three pathogens. While the three Phytophthora species occupy similar host and geographical ranges, they differ in their specific ecological niches and impacts on coastal forests.