Past disturbances can leave biological legacies that influence the likelihood, severity, or behavior of future disturbances. The dynamics of emerging plant diseases (which may act as biotic disturbances) depend primarily on three factors: host composition, pathogen occurrence, and the extent to which local environmental conditions foster pathogenesis. By causing selective mortality and changing forest structure, wildfire may alter plant disease dynamics. Since the 1990s, the fire-prone redwood forests of coastal California have been heavily impacted by the emerging disease Sudden Oak Death (SOD), which is caused by the pathogen Phytophthora ramorum and results in landscape-scale mortality of native oak species. Further, these forests are dominated by resprouting species, which often persist post-fire and heavily influence stand recovery. The 2008 Basin Complex fires in Big Sur, CA burned across SOD-impacted forest, impacting a previously established monitoring network and providing the basis for a natural experiment investigating fire’s impacts on disease dynamics. Previous surveys conducted immediately post-fire suggested that wildfire reduced the occurrence of P. ramorum but did not eliminate the pathogen (Beh et al. 2012). Following the Basin fires, burned and unburned plots were repeatedly sampled to assess forest structure, composition, microclimate, pathogen presence, tree mortality, and resprouting.
In this study, we assessed the influence of post-fire mortality, regeneration, and associated microclimates on disease recolonization six years post-fire, and we then used historical fire data to construct predictions of future pathogen persistence rates and disease intensity. Using Bayesian multilevel mixture models, we tested whether stand structure, composition, climate, and regeneration metrics influenced occurrence and intensity of P. ramorum infestation at the individual tree and stand level. Likelihood of post-fire infection at the individual tree level was positively correlated with host trees’ rates of post-fire resprouting. Resprouting vigor was, in turn, negatively related with stand density and positively related with pre-fire tree size and pre-fire mortality of neighboring trees. We hypothesize that as resprouting trees increase in size following top-kill by fire, these resprouting clusters generate microclimatic conditions that increasingly facilitate the survival and reproduction of P. ramorum, which exhibits negative responses to increasing temperatures and decreasing humidity in other laboratory studies. Temperature and humidity data collected by microclimatic sensors placed in a subset of burned plots support this hypothesis. Using data from stands impacted by older fires, we constructed a chronosequence of resprouting growth rates and predicted thresholds for post-fire persistence of P. ramorum.