LB 23-204 - Assessing the effects of Sudden Oak Death on overstory community composition in mixed evergreen forest

The introduced disease Sudden Oak Death (SOD), caused by the pathogen Phytophthora ramorum, has killed millions of trees in California and Oregon since its emergence in the 1990s. A few species are of primary importance to the current epidemic: California bay laurel is the primary driver of SOD in mixed evergreen forests, spreading the pathogen aerially from non-lethal foliar infections. Stem infections on SOD-susceptible oak species are lethal. The differing harm that species face from SOD may be ecologically destabilizing as the competitive balance between bay laurels, susceptible oaks, and non-host tree species is disrupted. Additionally, much of SODs range is fire prone and we can anticipate that both forms of disturbance will increasingly co-occur in the future. Some of these dynamics may be mediated by stand characteristics. We asked how patterns of overstory species compositional change differed in relation to disease loads, stand composition, and wildfire. We utilized disease symptom and tree demographic data from a plot network in Sonoma County, California. This network was measured from 2004-2016, experienced wildfires in 2017 and 2020, and a subset was re-measured in 2022. We used hierarchical clustering, ordination, and PERMANOVA to compare the influence of P. ramorum and wildfire in tree community change across different stand types. We hypothesized that the relationship between disease loads and levels of overstory composition change would be strongest in stands characterized by susceptible oak species and that wildfire would drive composition towards susceptible oak species and away from less fire-resistant bay laurels. Preliminary results show evidence of compositional shifts in tree densities towards bay laurel and away from susceptible oaks. We observed differences in the relationship between disease symptoms and compositional change based on stand characteristics. In the plots monitored over the 18 year period we observed an overall decline in susceptible oak densities and an increase in bay laurel densities. Finally, we observed a greater drop in density for susceptible oaks than bay laurels post-fire. This study indicates that P. ramorum is driving long-term declines in susceptible oak species in mixed evergreen forests, but that the effects are unevenly exhibited throughout the landscape and are influenced by stand characteristics. Additionally, we found evidence that wildfire may accelerate some of these successional trends.