We evaluated and applied a mechanistic ecosystem model, ecosys, to disentangle the direct and indirect effects of wildfire on ecosystem and SOC stocks across the tundra and boreal ecosystems of Alaska during historical and future time intervals. We hypothesized that climate warming and increasing atmospheric CO2 will enhance plant carbon uptake, plant biomass, and thereby litter carbon inputs to the soil. However, accelerated soil decomposition and combustion losses from wildfire will offset the gains in litter inputs resulting in net SOC losses.
Projected increases in climate warming and wildfire may result in several ecological and climatic feedbacks that affect the carbon cycle. In particular, wildfire and its effects on the complex interactions between vegetation and soil carbon stocks may accelerate high-latitude soil carbon losses. Combustion losses of carbon will increase carbon sources to the atmosphere and thus feedbacks to climate warming, further increasing wildfire. Carbon losses from combustion may therefore slow or reverse the historical carbon sink of northern ecosystems.
Arctic and boreal permafrost soil organic carbon (SOC) decomposition has been slower than carbon inputs from plant growth since the last glaciation. Anthropogenic climate warming has threatened this historical trend by accelerating SOC decomposition and altering wildfire regimes. We accurately modeled observed plant biomass and carbon emissions from wildfires in Alaskan ecosystems under current climate conditions. In projections to 2300 under the RCP8.5 climate scenario, we found that warming and increased atmospheric CO2 will result in plant biomass gains and higher litterfall. However, increased carbon losses from (a) wildfire combustion and (b) rapid SOC decomposition driven by increased deciduous litter production, root exudation, and active layer depth will lead to about 4.4 PgC of soil carbon losses from Alaska by 2300 and most (88%) of these loses will be from the top 1 m of soil. These SOC losses offset plant carbon gains, causing the ecosystem to transition to a net carbon source after 2200. Simulations excluding wildfire increases yielded about a factor of four lower SOC losses by 2300. Our results show that projected wildfire and its direct and indirect effects on plant and soil carbon may accelerate high-latitude soil carbon losses, resulting in a positive feedback to climate change.