CANCELLED - Interacting disturbances: Did Sudden Oak Death mortality in Big Sur worsen the impacts of the 2008 Basin Complex wildfire?

Background/Question/Methods Coastal California forests have experienced extensive mortality from the emergent generalist pathogen Phytophtora ramorum, causal agent of Sudden Oak Death (SOD). The forests of Big Sur are among the most impacted by P. ramorum, with 100% of hosts infected in some stands and hundreds of thousands of dead trees across the region. In late June 2008, the Basin Complex wildfire burned 240,000 acres in Big Sur, including areas that had been impacted by SOD in varying degrees of severity. We used pre-fire data on tree mortality and post-fire assessments of burn severity to ask whether this exotic pathogen worsened fire severity at the local scale. Prior to the fire, in 2006 and 2007, we established 280 intensive long-term monitoring plots randomly distributed across the Big Sur region and quantified disease incidence, levels of tree mortality, amount of coarse woody debris and various other biological and physical characteristics of the forest. Immediately following the fire, we conducted a rapid response survey of 61 monitoring plots to assess burn severity. We measured ash depth, soil damage, coarse woody debris consumption and bole/canopy scorching and torching, all of which are immediate indicators of burn severity likely to disappear with the onset of California's winter rains. Tree mortality and regeneration responses will be assessed in summer 2009, one year following the fire.

Results/Conclusions In the 61 plots surveyed for burn severity, pre-fire volumes of downed host logs in infected redwood-tanoak and mixed-evergreen stands >2 and >70 times greater, respectively, than those in uninfected stands. However, pre-fire log volumes for all species and the volumes of logs consumed by the fire did not differ between infected and uninfected stands in either forest type. Standing dead biomass also differed significantly among infected and uninfected plots, but measures of burn severity did not significantly differ. At the local scale, pre-fire SOD impacts do not appear to have influenced burn severity, but effects of SOD-caused fuel loads on fire spread and behavior are still unknown. Burn severity stems from a wide variety of processes driving fire behavior at larger scales. Interactions between SOD and wildfire are therefore complex and may be more apparent at the landscape scale. In the coming years, a full census of fire-caused tree mortality, which does not necessarily occur immediately post-fire, and forest regeneration will clarify the extent of local-scale fire impacts in relation to the distribution of SOD mortality.