The influence of climate and land cover on boreal forest wildfires
Wildfire activity in boreal forests is projected to increase dramatically in response to anthropogenic climate change. Regional climate conditions are a major top-down control of fire regimes, but bottom-up factors, such as vegetation composition, can also strongly affect fire occurrence and spread by changing microclimate, fuel load, and the spatial arrangement of flammable fuels. Fires often burn out when switching between fuel types or encountering firebreaks such as water bodies, demonstrating how mosaics of land-cover classes can decrease fire spread across a landscape. Thus the spatial arrangement of fuels, land-cover composition, and land-cover configuration may play an equally important role to climate in driving fire-regime dynamics at landscape to regional scales. A rigorous assessment is needed to elucidate how land cover interacts with climate to influence boreal fire regimes and how the interactions may shift in response to climate or land-cover change. In this study, we used geospatial land cover data and historic weather station records to investigate boreal-forest fires in Alaska for the period of 2002-2011. Specifically we assessed how forest fires are influenced by climate, how and which vegetation classes burn, and how vegetation burning shifts under different climatic conditions.
Mean summer climate moisture index (precipitation minus potential evapotranspiration) was correlated with annual area burned (r = -0.68, p = 0.035), total number of fires (r = -0.85, p < 0.01), and the number of large fires (>500 km2; r = -0.60, p = 0.07). Area burned was related positively to percent cover of coniferous forest and woody wetland, and negatively to percent cover of shrub scrub, dwarf scrub, and fuel-free areas. Fires preferentially burned coniferous forest across large gradients of climate moisture index, but all land-cover types burned more under warmer and drier conditions. Land-cover contagion increased significantly with area burned, and land cover played a less important role in controlling fire regimes when weather conditions were more favorable for forest burning. These results suggest that reliable projections of boreal fire-regime change require consideration of the interactions between climate and land cover, as well as feedbacks from land-cover change.