Fire is an integral component of the disturbance regime in North American boreal forests. High latitude ecosystems are experiencing amplified climate warming, and recent evidence suggests concurrent increases in fire occurrence. Multiple observations in central Alaska have shown that severe burns consume a greater proportion of the soil organic layer, and that this in turn alters patterns of seedling recruitment. Specifically, with increased burn severity, indicated by a thin post-fire organic layer, aspen seedling establishment increases while spruce seedling establishment remains the same. These patterns of seedling establishment are directly responsible for post-fire stand composition. Quantifying differences in ecosystem carbon dynamics between forest successional trajectories in response to burn severity is essential for understanding potential changes in regional or global feedbacks between boreal forests and warming climate.
The objective of this study is to quantify differences in the carbon accumulation and vegetation productivity associated with alternate post-fire successional trajectories related to fire severity. To accomplish this we used the Biome Biogeochemical Cycling model (Biome-BGC). Observations of post-fire seedling establishment for moderate and severe fires are used to initialize a version of Biome-BGC with competing vegetation types. Aside from post-fire seedling recruitment, all other model inputs are held constant. Ecophysiological variables for aspen and spruce specific to central Alaskan forests are used.
Results/Conclusions
The model predicted a higher proportion of deciduous biomass after severe fires, whereas spruce comprised more of the biomass after less severe fire. These patterns persist for the entire 150-year post-fire simulation period. This result is in agreement with current theory and observations regarding the effects of burn severity on post-fire boreal forest composition. Additionally, we observed simulated differences in the rate and total amount of carbon accumulated in aspen and spruce-dominated forests. Model results match recent observations reasonably well, however inferring burn-severity of historic fires is challenging and this makes full validation difficult. Nonetheless, our results indicate that changes in boreal forest composition associated with a changing fire regime may alter ecosystem carbon dynamics in boreal landscapes. To this end we interpret modeled differences between aspen-dominated and spruce-dominated boreal forests in the context of net changes to regional and global carbon budgets. Additionally, we consider potential feedbacks related to disturbance and climate as a consequence of a transition from spruce-dominated to aspen-dominated ecosystems. We conclude that changes in boreal forest composition associated with post-fire successional trajectories may potentially alter the role of boreal ecosystems in the global climate systems.
