Daily burned area and carbon emissions from boreal fires in Alaska

[摘要] Boreal fires burn into carbon-rich organic soils, thereby releasing largequantities of trace gases and aerosols that influence atmospheric compositionand climate. To better understand the factors regulating boreal fireemissions, we developed a statistical model of carbon consumption by fire forAlaska with a spatial resolution of 450 m and a temporal resolution of 1day. We used the model to estimate variability in carbon emissions between2001 and 2012. Daily burned area was mapped using imagery from the ModerateResolution Imaging Spectroradiometer combined with perimeters from the AlaskaLarge Fire Database. Carbon consumption was calibrated using available fieldmeasurements from black spruce forests in Alaska. We built two nonlinearmultiplicative models to separately predict above- and belowground carbonconsumption by fire in response to environmental variables includingelevation, day of burning within the fire season, pre-fire tree cover and thedifferenced normalized burn ratio (dNBR). Higher belowground carbonconsumption occurred later in the season and for mid-elevation forests.Topographic slope and aspect did not improve performance of the belowgroundcarbon consumption model. Aboveground and belowground carbon consumption alsoincreased as a function of tree cover and the dNBR, suggesting a causal linkbetween the processes regulating these two components of carbon consumption.Between 2001 and 2012, the median carbon consumption was2.54 kg C m^-2. Burning in land-cover types other than blackspruce was considerable and was associated with lower levels of carbonconsumption than for pure black spruce stands. Carbon consumption originatedprimarily from the belowground fraction (median =2.32 kg C m^-2 for all cover types and 2.67 kg C m^-2for pure black spruce stands). Total carbon emissions varied considerablyfrom year to year, with the highest emissions occurring during 2004(69 Tg C), 2005 (46 Tg C), 2009 (26 Tg C), and2002 (17 Tg C) and a mean of 15 Tg C year^-1 between2001 and 2012. Mean uncertainty of carbon consumption for the domain,expressed as 1 standard deviation (SD), was 0.50 kg C m^-2.Uncertainties in the multiplicative regression model used to estimatebelowground consumption in black spruce stands and the land-coverclassification were primary contributors to uncertainty estimates. Ouranalysis highlights the importance of accounting for the spatialheterogeneity of fuels and combustion when extrapolating emissions in spaceand time, and the need for of additional field campaigns to increase thedensity of observations as a function of tree cover and other environmentalvariables influencing consumption. The daily emissions time series from theAlaskan Fire Emissions Database (AKFED) presented here creates newopportunities to study environmental controls on daily fire dynamics,optimize boreal fire emissions in biogeochemical models, and quantifypotential feedbacks from changing fire regimes.

[学科分类] 地球化学与岩石