In boreal forest ecosystems natural wildfires have been the principal landscape disturbance for the last 6000 years. Historically, these forests have been carbon (C) sinks, accumulating C in the soil organic layer over multiple fire events. Recently, climate warming and drying has led to an intensification of wildfire disturbance. Increases in fire severity or more frequent burning have the potential to alter the structure and function of boreal ecosystems, shifting their long-term C balance from net accumulation to net loss, which will feed back positively to climate warming.
In this study, we synthesized data on pre-fire C pools, C emissions from fire, and the potential drivers of C emissions from 420 sites in six ecoregions of North America’s boreal forests after unprecedentedly large fires. We used structural equation models to identify interactions among the dominate drivers of C emissions and assessed the generalizability of these relationships between ecoregions. At a subset of sites, we used a novel application of organic soil radiocarbon dating to assess the impacts of fire frequency and severity on soil C combustion and its control on the long-term net ecosystem C balance.
Results/Conclusions
We found that time since last fire and landscape gradients in moisture affected tree species composition and pre-fire above- and below-ground C pools, which in turn drove C emissions. In all ecoregions, C emissions were not impacted by fire weather indices and were primarily limited by fuel, but the relative influence of above- versus below-ground fuels was ecoregion specific. Mature forests that burned were either neutral or a net C sink relative to the atmosphere since the previous fire, but forest that were young at the time of fire (representative of a shortened interval between fire events and increasing fire frequency) shifted to a net C source. Results from this study are crucial for modelling ecoregion specific C emissions, but we highlight that measuring C emissions alone is insufficient for detecting historically significant changes in C sink-source dynamics as the intensification of disturbance from boreal wildfires increases.