The effects of bark beetle outbreaks on forest carbon and composition under future climate projections in the Lake Tahoe Basin

Background/Question/Methods

Understanding how climate change will alter forest composition and carbon in the western US requires knowledge of forest growth relationships with climate, long-term tree species dynamics, and past and future disturbances (primarily insect outbreaks and wildfires), and human management.  Integrating these diverse processes and their complex interactions requires the synthesis of many empirical data sources (both 'little data' and 'Big Data') into sophisticated forest landscape change models.  We examined forest change in the Lake Tahoe Basin, California and Nevada, given likely climate change and the potential for three extant bark beetle species to outbreak with increased Palmer Drought Severity Index (PDSI).  We used the LANDIS-II modeling framework to integrate these diverse processes and project future conditions.  

Results/Conclusions

Our simulations indicate substantial responses to climate change across the Lake Tahoe Basin landscape.  Although these forests will likely remain a C sink (absorbing more C than they release), regardless of climate regime, simulated bark beetles added large uncertainty and substantially increased the likelihood that these forests will become a C source in any given year.  Further, insect outbreaks substantially altered forest composition.  Bark beetles did not substantially interact with wildfire, however, and they may reduce the need for forest management to create fire resilience.  The future of western forested ecosystems will depend not only on climate change but also on disturbances, which will be shaped by climate change, future management decisions, and landscape legacies related to past land use and management.  Our results highlight the importance of focusing on long temporal and large spatial-scale processes for understanding climate change effects on forest succession, disturbances, and the Carbon cycle.