Impacts of sudden oak death at Point Reyes National Seashore: Tanoak mortality, vegetation dynamics, and fuel loading

Background/Question/Methods   Sudden oak death, an emerging disease caused by the exotic pathogen Phytophthora ramorum, is impacting forests and woodlands throughout coastal California. Tanoak (Lithocarpus densiflorus), the most abundant broadleaf tree in the conifer-dominated forests of the infested area, is the most severely affected species; several lines of inquiry have concluded that sudden oak death could eventually drive tanoak to extinction, at least in certain parts of its current range. In this study, we used data collected in 2007 and 2009 to examine disease-induced tanoak mortality rates, vegetation dynamics, and fuel loading within the redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii) forests of Point Reyes National Seashore, a popular tourist destination approximately one hour north of San Francisco. In order to facilitate comparisons between forest types, our protocol imposed an equivalent minimum tanoak basal area across all sample plots. To detect differences between severely diseased and relatively intact areas, we used a randomized split-plot design (which was made possible by the stochastic and highly patchy local distribution of the disease), along with a mixed effects analytical framework. We investigated impacts of sudden oak death as well as baseline differences between redwood and Douglas-fir forests in our study area.

Results/Conclusions   We found that new tanoak mortality occurred from 2007 to 2009 throughout all sampling strata – redwood and Douglas-fir, “diseased” and “healthy” (as of 2007) – but that the rate of increase, as well as the cumulative impact, was much greater in Douglas-fir forests. In 2007, proportional mortality was higher in the redwood forest type, but by 2009 nearly 100% of tanoak basal area was dead in diseased Douglas-fir plots, as compared to approximately 75% in diseased redwood plots. Our analyses also revealed several indirect effects of this mortality. In redwood forests, tanoak mortality reduced canopy cover, reduced herbaceous cover, increased non-tanoak hardwood regeneration, and increased fuel loading (especially 1000-hour fuels). In Douglas-fir forests, tanoak mortality reduced canopy cover, increased herb species richness, and increased 1-hour and 10-hour fuels. We also detected important baseline differences between these two forest types; Douglas-fir forests exhibited higher tree diversity and greater abundance of non-tanoak hardwoods (i.e. greater functional redundancy), suggesting divergent responses to equivalent levels of tanoak mortality. Finally, we integrate these and other findings to explore potential long-term consequences of this novel disturbance, including interactions with historical disturbances, cascading community-level impacts, and shifts in the composition and structure of affected forests.