COS 29-5 - Wildfire and tree disease promote the expansion of a nitrogen-fixing shrub in coastal California forests

Abstract: Novel disturbances increasingly challenge forest resilience, making it critical to understand where and why forests fail to recover. The disturbance response of forests near their climatic boundaries is of particular interest, because these forests may have the greatest potential to transition into non-forested communities. Such a community transition may occur if disturbance causes substantial tree mortality while favoring the expansion of an alternative vegetation type. Here, we examine how the novel disturbances of repeated wildfire and sudden oak death, an introduced tree disease, affects the boundary between chapparal and forests in Big Sur, California. These forests are comprised of tree species near their southern range limit, restricting them to cooler and wetter portions of the landscape, and along the boundary edge of chaparral. The chaparral boundary appears dynamic – chaparral species tend to proliferate after fire, but can be excluded when over-topped by trees after long fire-free periods. However, it is unknown whether disease-related fuels and shorter fire intervals may favor the expansion of chaparral and result in a sustained loss of forests. Using data from long-term forest monitoring plots, we find that forest basal area declines following the introduction of sudden oak death and an increasing number of wildfires. We also identify a negative relationship between tree basal area and shrub area, but only for dominant chaparral species, suggesting a tension between trees and chaparral shrubs. The abundance of chaparral dominants is greater in hotter and more xeric parts of the forest landscape, and peaks within 2-3 decades following wildfire. This pattern is driven by species of Ceanothus, which are nitrogen-fixing, obligate post-fire seeders. Repeated wildfire and sudden-oak-death both promote the expansion of Ceanothus, such that it is present in cooler and more mesic forests. The response of Ceanothus to sudden oak death is only apparent when tree mortality precedes fire, and is related to fuels, and not basal area of live or dead trees. We parameterized a simulation model with empirical measures of Ceanothus, and estimate high rates of symbiotic nitrogen fixation, but not enough to replenish fire-driven nitrogen losses, especially under shorter fire intervals. Our findings suggest that increasing frequency and severity of wildfire due to sudden oak death create high light and nitrogen-poor conditions that favor the N fixer, Ceanothus. Whether the expansion of Ceanothus signals a regime shift may depend on the effect of chaparral shrubs on tree recruitment, disease dynamics, and the fire regime.