Climate change and landscape legacy effects on forest carbon dynamics and wildfires in the Lake Tahoe Basin

Background/Question/Methods

Forests are an important carbon (C) sink for the U.S. Future forest sequestration potential is unknown at the regional scale as forests continue to age, temperature increases, and natural disturbances respond to climate change.  Wildfire is of particular interest and area burned is expected to increase with expected decreases in snowfall, longer growing seasons, and changes in vegetative fuel characteristics. We posed the question: Under projected climate change scenarios, will reduced growth rates and enhanced wildfire activity in the Lake Tahoe Basin (LTB) of CA and NV increase net C emissions to the atmosphere? To answer this, we modeled the forests of the LTB using a spatially explicit landscape simulation model, LANDIS-II, coupled with the CENTURY soil model.  The coupled models simulated the effects of climate on succession, wildfire, forest growth, soil accumulation, and soil respiration.  Multiple C and nitrogen pools (e.g., live, detritus, soil organic matter) were simulated along with individual species and age cohorts, each with associated leaf and woody biomass and nitrogen characteristics. Cohort net primary productivity is limited by soil moisture, available nitrogen, soil temperature, and leaf area index. Three climate scenarios (high: A2 and low: B1 greenhouse gas emissions, contemporary climate) were used as model inputs.  

Results/Conclusions

The results suggest continued above and belowground C sequestration across the LTB, regardless of changes in climate.  This continued forest growth and C storage is due to the ‘rebound’ effect from extensive logging performed in the late 19^th century.  Future climate will, however, lower soil water and nutrient availability, reduce aboveground net primary productivity, and alter net ecosystem exchange as compared to baseline scenarios assuming a constant climate.  Climate change reduced the overall C storage potential in live (leaf, wood, fine & coarse roots) and dead (leaf litter, dead wood, fine & coarse root detritus) C pools, as well as soil organic matter (fast, slow, passive pools).  Differences between climate scenarios were most evident towards the latter half of the century, when temperatures clearly diverged and the forest and resulting C dynamics had a lagged response to changes in climate. Wildfire regimes responded to increased temperatures and increased fire ignitions, generating greater area burned than the historic average.  Changes in tree species composition and distribution will also be discussed.  This research illustrates the importance of considering the interacting effects of long-term disturbances, including landscape legacies, on shifts in species composition and analysis of regional scale forest C dynamics.