Altered Fire Regimes, Severe Fire and Forest Recovery in Mixed Conifer Forest

Understanding how altered fire regimes are shifting vegetation communities across the landscape will be critical for managing forested landscapes during a time of global change. Moreover, altered fire regimes can inform ecological theories on alternative stable states and ecosystem resilience. I investigated how increases in high severity fire are affecting ecosystem properties both directly and indirectly, in ways that can support both land management decision-making as well as broader inquiries into state transitions. In terms of the indirect effects of severe fire, I examine how the application of rice mulch for erosion control in severely burned mixed conifer forests affects native plant communities. I found that rice mulch is linked with higher nonnative richness, including numerous species that have total fidelity to mulched areas, suggesting the mulch may have introduced these species. I also observed that the mulch is homogenizing the plant communities; both of these findings could have persistent effects on ecosystem properties. I also studied the direct effects of fire severity on forest regeneration patterns in mixed conifer forests across California, building a spatially-explicit predictive model of postfire conifer regeneration from 24 wildfires. To build this model I used a novel approach to characterize seed availability from maps of estimated basal area. I calculated estimated annual seed production from basal area by species at each pixel using established equations and then smoothed these maps of seed production to simulate a neighborhood effect at a variety of scales. The most important predictors in this model were 30-year mean annual precipitation (mm) and seed availability, highlighting both the initial biological filter (seed availability) and site suitability (annual precipitation). Finally, I used a combination of remotely-sensed data and field data to empirically evaluate the potential for shifts to alternative stable states after severe fire in Sierran mixed conifer. Because positive feedbacks are often responsible for maintaining a given vegetation state, I evaluated the evidence for the initiation of a positive feedback in severely burned vegetation communities at two sites in Yosemite National Park. Areas that burned severely in the 1990s that were regenerating as montane chaparral tended to reburn severe in more recent (2009, 2013) fires. When comparing areas that have burned once versus twice at high severity, I found that areas twice-burned at high severity had a greater response of sprouting species, and of sprouting versus seeded individuals for the facultative seeder shrub species; twice-burned at high severity areas also had significantly higher cover of nonnative annual grasses. I also found that areas twice-burned at high severity had significantly fewer obligate seeder conifer seedlings. The once- and twice-burned areas also had distinct plant communities, though this difference was not driven by differences in homogeneity within the two communities. These data suggest that positive feedbacks are likely beginning to operate after the second severe fire event, and that this event causes a shift in the communities that may be indicative of alternative state. Collectively, these diverse projects contribute to our understanding of shifting vegetation communities under a time of global change.