Deeper soil warming and increased root biomass drive soil C losses from heterotrophic respiration in high-latitude boreal forests experiencing short-interval reburning

Background/Question/Methods

Increasing wildfire frequency in the Alaskan interior boreal forest associated with climate warming over the last century has reduced the mean fire return interval in some areas to 25-50 years. This change has removed most of the surface soil organic layer (SOL) that insulates the mineral soil in the summer and shifted vegetation cover from black spruce to deciduous forest. Warmer soil temperatures can increase decomposition rates, particularly from the organic-rich upper mineral soil, potentially leading to soil C losses. In 2018-2019, we measured heterotrophic respiration (RH) rates, under/overstorey vegetation and root biomass, and soil temperatures at 10 cm depths (Ts10) in areas burned 1x, 2x, and 3x over 60 years and previously covered by spruce. We predicted soil temperatures at 45 cm depths (Ts45) for each plot using a Simultaneous Heat and Water Model (SHAW) based on air temperature, SOL depth, and solar radiation. We also estimated NPP for vegetation components using allometric equations and diameter measurements, and estimated peak seasonal EVI values using Landsat imagery from 2015-2020. Using a conditional permutation random forest (CPRF), we assessed an array of predictor variables to isolate potential underlying mechanisms responsible for higher RH with more frequent fires.

Results/Conclusions

Earlier research found that burn frequency substantially increased both Ts10 (+2-3°C) and RH rates (+28-80%), but that a higher Ts10 poorly predicted increases to RH with fire frequency. Frequently burned areas also had higher deciduous overstorey vegetation and root biomass. This indicates that other factors played a role in higher RH, such as deeper soil temperatures and turnover of roots and deciduous litter. The SHAW model predicted higher mean Ts-45 values (DOY: 135-285) in areas burned 2x and 3x (4-8°C) than 1x burns (2-3°C), and a larger number of days above 0°C (35-40). The relative contribution of the highest permutation importance scores for predictor variables for RH in the CPRF included: NPP of deciduous overstorey vegetation (~20%), root biomass at 15 cm depth (~19%), Ts45 (17%), root biomass at 45 cm depth (10%), EVI (10%), and Ts10 (4%). These results suggest that a shift to deciduous cover in boreal forests can stimulate the decomposition of previously inaccessible soil organic matter from deeper soil warming and the turnover of deciduous root biomass both near the surface and at depth. Consequently, an increase in fire frequency in the region with climate warming may lead to considerable soil C losses.