The recent extreme drought in California drove a wave of tree mortality in Sierra Nevada forests. This mortality was concentrated in particular areas (dry, dense forests) and on particular species (especially Ponderosa pine). During the same time period, there have been several large fires that have included very large patches of nearly 100% tree mortality. It remains to be seen, however, what these short-term shocks to forest structure and composition means for the future biodiversity, composition and function of these forest stands. In past mortality waves, stands have often been resilient and regained density and composition through post-disturbance regeneration. Here we explore whether these recent disturbances mark the beginning of longer-term directional change, and whether pre-disturbance biodiversity and environmental diversity can help predict forest response. Explanatory variables include remotely-sensed and locally measured characteristics of forest stands (structural heterogeneity, forest density and species composition). We use species distribution models to project future forest composition and biomass under projected climate. We then ask whether areas projected by SDMs to experience major change in composition and structure this century correspond to areas that have recently experienced severe tree mortality.
Results/Conclusions
We have previously found that the distribution of tree mortality in 2015 was strongly associated with long-term climate and forest density, and was concentrated at low-to-middle elevation forests in the Southern Sierra. Here we found that forest structural heterogeneity has some (although weak) predictive power to explain the distribution of high-severity fire and thus extensive tree mortality. Together, this suggests that tree mortality rates are controlled by a combination of local biological structure and its interaction with larger-scale climatic factors.
Species distribution models project high levels of compositional turnover and reduction in forest basal area for most of these high-mortality areas by 2050-2070 when averaged across the suite of model projections for CMIP5. This suggests that forests in these areas are or will soon be out of equilibrium with climate conditions. Therefore, we expect that the community composition of regenerating trees will represent be different from extant stands and that the next generation of forest will support lower biomass.