Assessing the Effects of Sudden Oak Death on Overstory Community Composition in Mixed Evergreen Forest

Presenter: Alex Martin

Presenter Status: Graduate student

Academic Year: 22-23

Semester: Spring

Faculty Mentor: Nathan Rank

Department: Biology

President's Strategic Plan Goal: Sustainability and Environmental Inquiry

Abstract:
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. While SOD infects 100+ regional plants, only a few species are important to the epidemic in Sonoma County mixed evergreen forests: non-lethal leaf infections, primarily on California bay laurel, spread the pathogen aerially, while non-reproductive trunk infections on several oak species, including coast live oak and California black oak, are lethal. The unequal harm that hosts face from SOD is likely to result in changes to woodland composition. In addition to SOD, these oak woodlands are prone to fire disturbance. Given differences in tree species wildfire resistance, this may induce different shifts in tree composition than SOD. I analyzed 192 15m2 study plots in Sonoma County that were established in 2004 and measured until 2016. I then assessed 19 plots which we re-measured in 2022, nine of which burned in the Sonoma County fires of 2017 and 2019. I ask how patterns of overstory species compositional change over a nearly 20 year period differed in relation to the level of P.ramorum symptoms, initial stand composition and whether these patterns are different in plots affected by wildfire. This work may provide a clearer picture of tree species shifts that have occurred in Sonoma County woodlands and assist stewards of public and private lands incorporate the effects of SOD into their long term planning.