Wildfire is an important force of disturbance for Western forests, resetting the successional clock and temporarily altering regional carbon (C) cycling. However, climate change and management strategies have altered fire regimes in the southern Rocky Mountains, creating potential for more permanent shifts in C cycling if combustion and decomposition are not offset by vegetation regrowth. We took a chronosequence approach to examine post-fire soil respiration (SR) and aboveground net primary productivity (ANPP) trends in two pine forest types ubiquitous to western North America that have different post-fire regeneration strategies: lodgepole pine and ponderosa pine. In 2015 and 2016, we sampled 70 plots across six fires in different age classes (0-5, 10-15, 20-25, and 30-35 years old), testing the hypothesis that SR shows a short-term increase following high-severity stand-replacing wildfire and a secondary increase that lags behind the expected successional rise in ANPP, and that these trajectories differ by forest type.
Using mixed-effects linear models (treating plot as a random effect), we found differential long-term trends in SR for the two forest types that can be linked to their post-fire regeneration strategies. SR in ponderosa pine forests that burned at high severity (> 90% canopy mortality) was lower than in unburned (P=0.01), regardless of time since fire (TSF). In contrast, SR varied by TSF in high severity burns in lodgepole pine forests, with the highest respiration in the 21-22 year old fire, and respiration from burned areas exceeded that of unburned in the oldest two fires (21-22 and 33-34 years old; TSF x severity interaction effect P < 0.0001). SR covaried with seasonal changes in soil temperature and moisture, and these effects differed for ponderosa pine forests versus lodgepole pine forests. To assess effects on C cycle partitioning, SR and ANPP trends will be compared to each other.
Our findings suggest that the impacts of wildfire on C cycling vary between two forest types common to the Rocky Mountains. Efforts to assess and project disturbance impacts on C cycling need to consider this variability instead of generalizing across forest types.