Model simulations driven by paleo-forcing data reveal large and rapid responses of carbon storage to boreal fire-regime shifts

Background/Question/Methods

      Climate warming is expected to increase the frequency and severity of natural fires in the boreal forest biome. Boreal forests represent >30% of terrestrial carbon stocks, and fire is a key component of the carbon cycle in these ecosystems. However, predictions of fire-regime change face substantial uncertainty, largely because complex fire-climate-vegetation interactions are poorly characterized in brief observational records. Furthermore, previous studies suggest that model projections of future carbon dynamics are sensitive to assumptions about the prehistoric fire regime. Paleofire reconstructions offer valuable insights to address these limitations.

      We collected 14 lake-sediment cores from the Yukon Flats, Alaska to elucidate patterns of long-term environmental change. We then converted fire-regime reconstructions from these data to input drivers for the Dynamic Organic Soils version of the Terrestrial Ecosystem Model (DOS-TEM). Combined with simulated paleoclimate from an Earth System Model, these “paleo-forcing” data allowed us to model past changes in ecosystem carbon storage in our study area to (1) assess the relative importance of climate vs. fire in driving carbon dynamics of the past millennium, and (2) evaluate the effect of assumptions about prehistoric fire regime on predictions of current and future boreal-forest carbon balance.

Results/Conclusions

      Fire-regime variations were the dominant control on simulated carbon storage, producing fluctuations of ~2 kg C/m² (~20% of total ecosystem carbon) on centennial timescales. By comparison, the direct effect of climate was minor. However, the most pronounced carbon loss occurred during the Medieval Climate Anomaly when fire frequency and severity were relatively high. This fire regime was likely due to warm, dry conditions, highlighting an important indirect influence of climate. That shifts in fire-regime were responsible for large and rapid losses of carbon in the past emphasizes the importance of incorporating fire into methodologies that estimate future boreal carbon dynamics.

      Because of the rapidity of carbon-storage responses to fire-regime change, legacy effects due to past variability were generally small. This finding builds confidence in predictions made in the absence of detailed fire-history information. However, our paleodata indicate that fire frequency has increased to unprecedented levels in the past century. If this dramatic shift was unknown from the sediment record, simulations would suggest a small net carbon gain since 1950 AD. In stark contrast, our results reveal rapid losses of >1 kg C/m². Thus, current estimates of the boreal C sink may be biased in this era of rapid change.