Environmental controls on soil CO2 flux following fire in black spruce, white spruce, and aspen stands of interior Alaska

Abstract

Boreal forests contain large amounts of stored soil carbon and are susceptible to periodic disturbance by wildfire. This study evaluates the relationship between post-fire changes in soil temperature, moisture, and CO2 exchange in paired burned and control stands of three Alaskan forest systems: Picea mariana (Mill.) BSP, Picea glauca (Moench) Voss, and Populus tremuloides Michx. In these systems, the environmental factor that most directly controlled rates of carbon exchange varied depending upon burn status and soil drainage. In mature unburned stands, CO2 flux was highly correlated with seasonal patterns of soil temperature. Following fire, these soils became significantly warmer, and carbon exchange became more sensitive to fluctuations in surface moisture conditions. The effect of fire on soil climate was most pronounced in the P. mariana stands, which experienced a mean seasonal temperature increase of 5–8°C in the upper 1 m of the soil profile, a 200% increase in the rate of active layer thaw, and a reduction in mean surface moisture potential. Evidence from soil CO2 profiles suggests that these environmental changes may have resulted in enhanced decomposition of carbon previously immobilized by permafrost, potentially transforming a landscape that was once a net sink for carbon into a carbon source.