How does increased fire frequency affect carbon loss from fire? A case study in the northern boreal forest

Fire frequency is expected to increase due to climate warming in many areas, particularly the boreal forests. An increase in fire frequency may have important effects on the global carbon cycle by decreasing the size of boreal carbon stores. Our objective was to quantify and compare the amount of carbon consumed during and the amount of carbon remaining following fire in black spruce (Picea mariana (Mill.) BSP) forests burned after long v. short intervals. We hypothesised that stands with a shortened fire return interval would have a higher carbon consumption than those experiencing a historically typical fire return interval. Using field measurements of forest canopy, soil organic horizons and adventitious roots, we reconstructed pre-fire stand conditions to estimate the biomass lost in each fire and the effects on post-fire residual carbon stores. We found evidence of a higher loss of carbon following two fire events that recurred after a short interval, resulting in a much greater total reduction in carbon relative to pre-fire or mature stand conditions. Consequently, carbon storage across disturbance intervals was dramatically reduced following short-interval burns. Recovery of these stores would require a subsequent lengthening of the fire cycle, which appears unlikely under future climate scenarios.