Concerns about the environmental and socioeconomic consequences of anthropogenic warming have motivated numerous investigations of climate change and its impact on ecosystems. High-latitude ecosystems are of particular interest because they are projected to be among the most sensitive to anthropogenic forcing and because biogeochemical and biophysical feedbacks to the global system can exacerbate climatic warming. The frequency and extent of boreal and tundra fires in recent years appear striking, possibly as a result of anthropogenic warming. Are these recent fires unprecedented? Or do they occur naturally, with observational and satellite records too brief to capture the true natural range of variability? Because fire return intervals are long in these systems, assessing the possibility of rising novelty from the complex effects of climate change on fire regimes requires a holistic perspective from paleo-analyses, field observations, and computer modeling. We synthesize recent peleoecological studies of fire-regime variability, place ongoing and projected future changes in the context of natural variability, and discuss their biogeochemical implications.
Results/Conclusions
Paleoecological and observational records indicate that fire regimes were highly variable across space and through time within the boreal-forest and tundra biomes of Alaska, with fire return intervals ranging from less than a decade to several millennia. At broad spatial and temporal scales, these systems are highly resilient to climate and fire-regime shifts. For example, paleorecords reveal that boreal forests have remained similar despite frequent burning in some regions over the past 6000 years, and model simulations suggest that tundra carbon stocks are insensitive to fire-frequency changes over the past 35,000 years. However, statistical modeling of fire-climate relationships reveals robust temperature and moisture-deficit thresholds in these biomes. Vulnerability to anthropogenic climate change and associated fire-regime shifts differs greatly across space, largely as a function of how close current climate is to these climate thresholds.
Despite numerous large fires in recent years, no unambiguous evidence exists that extensive novel fire regimes have emerged in Alaska. However, the rate of burning over the past few decades exceeded the limit of the past 10,000 years in some ecoregions, converting these ecosystems from a historical sink to a source of carbon. Models project novel fire regimes in some regions during the 21st century, with a myriad of feedback processes likely leading to "surprises." In particular, increased burning and permafrost thawing may interact synergistically, resulting in massive C emissions and other biogeochemical changes that will emerge over decadal to centennial timescales.