Interactions among Sudden Oak Death, Fire Fuel Loads, and Climate in Sonoma County Oak Woodlands
Purpose of the Study: Understanding impacts of invasive pathogens and wildfire on forests is a major challenge facing scientists, land managers, and policymakers. In this study, I used a long-term plot network in eastern Sonoma County to disentangle the interactions among an exotic pathogen (Phytophthora ramorum), fire fuels loads and climate in oak woodlands. I investigated the interactions among: 1) sudden oak death (SOD) inoculum loads, disease prevalence and pre-fire surface and ladder fuel loads, 2) surface and ladder fuel loads and canopy condition, and 3) canopy condition and post-fire surface and ladder fuel loads. Procedure: From 2004-2016, measurements of disease prevalence, tree growth, tree mortality and tree survival were collected for the three most abundant SOD host tree species: coast live oak (Quercus agrifolia), California black oak (Quercus kelloggii), and California bay laurel (Umbellularia californica). In 2014, surface fire fuel loads were quantified, and airborne Lidar data were collected. Elevation, slope, average potential solar insolation, and topographic moisture index, and climate water deficit were also calculated for these plots to determine the interactions of microclimate and topography on pre-fire fuel loads. In 2019, I recorded tree mortality and survival of these same species and quantified surface and ladder fuel loads across 33 plots (15 x 15 m) with a range of burn severity following the 2017 LNU Complex Fires. To analyze the interactions among abiotic and biotic factors over time I created a hypothesized pathway to describe potential direct and indirect effects of oak mortality, disease load, microclimate, topography, climatic water deficit, pre-fire fuel loads, and canopy condition on post-fire fuel loads. I tested this pathway using a path analysis. Findings: I found that P. ramorum disease load had no effects on pre-fire fuel loading. However, oak mortality in 2014, indirectly affected by P. ramorum disease load, significantly increased the amount of large surface fuels, and decreased ladder fuels. Plot elevation and maximum temperature also had a positive correlation with pre-fire ladder fuel amounts. Pre-fire ladder fuels, in concert with drought intensity and slope interacted to effect canopy condition post-fire (in particular, percent canopy damage). Percent canopy damage was greater in less sloped plots with greater amounts of ladder fuels and increased drought severity. Pre-fire ladder fuels also had a significant direct effect on post-fire ladder fuels, whereas increased amounts of post-fire ladder fuels were observed in less-sloped plots with greater amounts of pre-fire ladder fuels and more percent canopy damage. Surprisingly, plots with increased fire severity due to increased fuels had increased re-sprouts and re-growth of U. californica trees immediately following fire. Conclusions: Current management practices in oak-woodlands include the removal of U. californica in the presence of oaks to reduce the potential P. ramorum inoculum load. My results support this practice, as SOD indirectly increases fire fuel loads, which may in turn increase both the future susceptibility of the ecosystem to fire and the severity of these future wildfires.