COS 31-1 - Impacts of increasing fire frequency and climate change on species composition and carbon dynamics in the boreal forests of Alaska

Abstract: The fast pace of climate warming and increases in fire frequency over the past few decades in northern latitudes has raised concerns about major shifts in vegetation and their ability to capture and store carbon. Although black spruce has been the dominant species in the boreal forests of Alaska for the past 6,000 years, unprecedented warming and wildfire over the past 60 years threatens to disrupt the dominance of black spruce and release globally significant amounts of C into the atmosphere through permafrost thaw. In some areas, the fire return interval has decreased from 100-120 years to only 10-15 years, favoring deciduous tree species and significantly altering the timing and magnitude of C sequestration rates. The goals of our “Reburns Alaska” project were to: 1) examine trends in short-interval burns using remotely-sensed data, 2) empirically measured above- and belowground C stocks, productivity, and heterotrophic respiration in the field, and 3) develop and validate physically-based permafrost and hydrology module for a widely-used, high resolution landscape simulation model (LANDIS-II) to forecast long-term dynamics of species composition and C sink strength given projected changes in climate (including thawing permafrost) and fire. We found that the proportion of short-interval fire events (< 20 years between fires) is increasing with time and although there is strong resistance to burning for the first decade after a fire, resistance to fire declines thereafter. At our 50 field plots, we found that black spruce presence in uplands declined sharply after two fires, while paper birch became increasingly abundant with each additional fire. After three burns, conifers were effectively eliminated. Areas with three burns also had a thin soil organic horizon and an increase in heterotrophic respiration. However, near-surface soil warming accounted for only 23% of higher respiration; increases in deciduous overstory vegetation and root biomass better correlated with respiration than soil temperature. Using DGS, our newly-developed extension of LANDIS-II, we found that conifer-dominated areas on the landscape that experienced greater numbers of fires more frequently shifted to broadleaf-deciduous dominance, which was exacerbated by climate change. We also found that fire rotation was significantly lower under climate change, driven by a greater frequency of large fires. Our work offers strong empirical evidence of the current and continued future divergence of boreal successional trajectories from previous historic norms, losses of soil organic carbon previously stored in permafrost, and ultimately the potential for large changes in this vast and fragile ecosystem.