Past and future forests of the Lake Tahoe Basin: Understanding interacting effects from climate change, bark beetle outbreaks, wildfires, and forest and land-use management
Projecting future forest dynamics in a managed landscape requires knowledge of long-term forest succession as well as past and future disturbances, such as wildfires, bark beetle outbreaks, timber harvesting, and forest thinning. Wildfire and insect outbreaks are linked with changes in climate and require special attention when addressing long-term forest management goals. Our objective was to evaluate the emergent responses of multiple interacting processes: climate change, bark beetle outbreaks, wildfire activity, and fuels management on forest productivity, tree species interactions, and ecosystem carbon dynamics, within the regional landscape of the Lake Tahoe Basin, CA, and NV. We simulated forest change using the LANDIS-II model (www.landis-ii.org) and validated our productivity estimates with scaled Aboveground Net Primary Productivity data from tree-core data.
Results from our simulations indicated that widespread and intense clearcut logging in the 1880s caused legacy effects, wherein forest growth continues for a century or longer regardless of climate or future disturbance regime. We also found that although intensive fuels management may be imperative for creating a more fire resilient landscape in the future, the effectiveness of fuel treatments may be limited by the projected increases in climate-driven disturbances. For instance, climate change is projected to increase drought conditions that will exacerbate bark beetle outbreaks and wildfire activity. This may make it difficult to maintain fuel levels and reduce tree stress at the landscape level. Furthermore, we found that response of individual tree species to each disturbance may counterbalance the effects of climate change on landscape level carbon. For example, post-disturbance recruitment in forest gaps of nearby fire tolerant tree species or those not targeted by host-specific insects will, to an extent, restore live carbon lost from disturbance. However, direct climate effects on tree species (such as changes in forest growth and establishment) were less important for determining overall forest growth and carbon storage potential than the more immediate impacts from bark beetles and wildfires. In terms of future forest management, we found that if some insect species continue at projected levels and host-specificity overlaps with target harvest species, the need for fuels management may be reduced. This study highlights the importance of understanding legacy effects of past land-use and future climate-driven disturbances that will alter the way we manage our forested landscapes in the Sierra Nevada.