COS 93-8 - Wildfire influences forest disease dynamics through selective host mortality and pathogen suppression: sudden oak death in Big Sur, CA

Thursday, August 11, 2011: 10:30 AM
9AB, Austin Convention Center
Margaret R. Metz1, Maia M. Beh1, Kerri M. Frangioso1, Ross K. Meentemeyer2 and David M. Rizzo1, (1)Plant Pathology, University of California, Davis, Davis, CA, (2)Forestry and Environmental Resources, North Carolina State University, Raleigh, NC

Most disease ecology in natural plant communities has focused on shifts to community composition due to disease without consideration of the role of endemic or interacting disturbances. Sudden oak death (SOD), caused by the exotic pathogen Phytophthora ramorum, is an emerging forest disease associated with extensive tree mortality in coastal California forests. Wildfire is an important endemic disturbance in these forests, influencing community composition and resource availability in the absence of SOD. In 2008, California experienced the first wildfires in SOD-impacted forests, providing the opportunity to examine the feeedbacks and interactions between these two disturbances. Fire may impact SOD directly through suppression of P. ramorum or indirectly through mortality of important hosts for the epidemiology of the disease.  We established an extensive network of 280 monitoring plots examining the impacts of SOD on forest dynamics in Big Sur, CA in 2006-07; 121 of these plots burned in 2008, including areas where the pathogen was absent and present. Using surveys of burn severity, tree mortality, and regeneration, we ask (1) how wildfire affected the survival of the pathogen; and (2) how the trajectories of forest recovery differ under the separate or joint influences of SOD and wildfire.


The dominant tree species in coastal CA forests differ both in their susceptibility to mortality from SOD and from fire, such that both disturbances caused selective patterns of mortality. In two habitat types, the dominant hosts for pathogen sporulation suffered greater fire-caused mortality than other species, which should lead to disease suppression in burned, infested areas relative to unburned, infested areas. We observed such suppression because only 20% of sampled, previously infested burned areas were found to contain the pathogen immediately following the fire.  In many sites where P. ramorum was no longer recovered, we found other non-native Phytophthora species that we previously detected only in areas without P. ramorum, indicating the possibility that fire affected the competitive dynamics of the pathogen.  Outside of burned areas, forest composition has been shifting to dominance by sporulating species that do not die from pathogen infection, leading to positive feedbacks on disease prevalence in these areas and continued mortality of hosts suffering lethal infections.  The trajectory of post-disturbance recovery thus differs greatly among sites depending on the separate or joint influences of SOD and fire because of changes to host abundance by each disturbance.

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