Disentangling disease and fire interactions in California mixed-evergreen forests

Background/Question/Methods

Disturbances are important causes of spatiotemporal heterogeneity in forests. Their ecological impacts vary depending on many factors, including species composition and abiotic conditions, which themselves are distributed heterogeneously across environmental gradients. One disturbance can influence the likelihood or severity of subsequent disturbances, leading to disturbance interactions with potentially unexpected outcomes. Understanding disturbance interactions thus requires a holistic approach that disentangles multiple interacting drivers of ecosystem change. We investigated the relative importance of biotic and abiotic drivers of forest change using a natural experiment presented by a large wildfire that occurred in a region heavily invaded by a destructive, non-native fungal pathogen. The mixed-evergreen forests of Big Sur, California have experienced widespread tree mortality from the exotic oomycete pathogen Phytophthora ramorum, causal agent of sudden oak death (SOD). These forests occur across wide climatic, topographical, and latitudinal gradients, and host a diverse assemblage of species differing in vulnerability to SOD and in contributions to epidemiological dynamics. A substantial portion of our SOD-monitoring forest plot network burned in 2008, encompassing a gradient of disease impacts. We used confirmatory path analysis with hierarchical models to examine direct and indirect influences of species composition and environmental gradients on disease impacts and fire-related tree mortality.

Results/Conclusions

We observed strong gradients in rainfall, solar irradiance, and temperatures that varied across the ~100-km latitudinal gradient of the study area as well as across the steep topographical gradient from the coast inland. We quantified tree survival to 2009 in more than half of the burned plots (25 invaded by P. ramorum, 13 pathogen-free). The probability of post-fire tree survival reflected direct and indirect impacts of these gradients (and resultant species compositional differences) because of variation in the abiotic stresses trees experience in the post-fire landscape and the abiotic and biotic environmental influences on tree size, also a predictor of post-fire survival. These same gradients were important in determining the abundance of epidemiologically important hosts and the abiotic conditions conducive to host-pathogen interactions. Both effects help govern disease impacts and related changes to fuel quality and abundance that result from SOD-related mortality. Changes to fuels also influenced fire severity, reflecting interactions between this novel biotic disturbance of an introduced pathogen and the endemic wildfire disturbance. Understanding the impacts of altered disturbance regimes in an era of global change requires holistic and multivariate approaches such as this, which examine direct and indirect influences of a number of variables simultaneously.