Exotic Plant Pathogens as Drivers of Forest Ecosystems
Plant pathogens can have dramatic effects on their host species and associated ecosystems, and in many cases the most destructive of these organisms are both exotic and highly invasive. One of the pathogens of greatest concern worldwide is Phytophthora ramorum, a highly invasive water mold that causes the forest disease known as Sudden Oak Death. Since 2002, Hall Cushman has been collaborating with a team of scientists from North Carolina State University, University of California (Davis) and Sonoma State University to understand the biotic and abiotic factors that predict the spread of this pathogen and to explore its long-term impacts on the forest ecosystem. We have established – and for over 15 years have sampled – a network of 200 plots randomly distributed across a 200 km2 forested landscape in northern California. Although this project is still ongoing, we have found strong evidence that human activity is associated with increased spread and prevalence of P. ramorum (Cushman & Meentemeyer 2008). In addition, through the use of aerial photographs and field surveys, we have shown that woodland expansion between 1942 and 2000, likely due to years of fire suppression, has facilitated the establishment of P. ramorum (Meentemeyer et al. 2008). Most recently, we have used long-term data to demonstrate that ecological heterogeneity across hierarchical levels of host-pathogen-environment associations influences infection dynamics of this forest disease (Haas et al. 2015). In general, our hope is that this research will enable us to understand the factors that drive the spread of this exotic forest pathogen and the ways in which pathogen-mediated tree mortality is altering the structure and composition of forest landscapes.