Disturbance is an important regulator of biogeochemical cycles in terrestrial ecosystems. Given a trend of increasing wildfire frequency and intensity in the Western U.S., it is important to understand how the recovery and resilience of forest C pools are impacted by severe fire. We examine how state factors (i.e. climate and vegetation) influence postfire C cycling and soil development by measuring the quantity and quality of C stocks across environmental gradients. Study sites include four high-severity 2002 fires which burned in montane Ponderosa pine- Douglas Fir forest, upper montane Ponderosa Pine and Gambel Oak forest, subalpine mixed conifer forest, and subalpine Lodgepole pine forest. Sampling was conducted in June 2015 and 2016. Terrestrial C stocks and soil (top 10 cm) properties were measured for three 15x15m plots within each fire area, and referenced to nearby undisturbed sites. In addition, a proxy for relative bioavailability of soil organic matter (mg C-CO2respired per g soil C) was quantified through incubation experiments. We address: 1) How does wildfire impact SOM bioavailability? 2) How do recovery of C and N stocks vary across environmtental gradients? 3) What are the possible implications of postfire changes in C processing for forest C budgets?
Results/Conclusions
Fire resulted in complete tree death and combustion of the soil organic layer; only in one site was there reestablishment of conifer seedlings. As expected, burned soils had 13-65% reduced C and 25-55% reduced N relative to reference soils. Soil C bioavailability was 10-128% greater in burned soils relative to reference soils. However, only in sites with little revegetation (no live tree C) was the difference between burned and reference soils significant. Soil δ13C and δ15N were enriched in burned sites, suggesting that sources and/or processing of SOM was impacted by fire. In addition, in sites with high SOM bioavailability, respired δ13C-CO2 from burned soils was ~18% enriched relative to bulk SOM. We hypothesize that in sites with less regeneration, the sources of SOM were weighted toward microbial and grass/ forb C, which tend to be more bioavailable than tree (conifer) litter. This suggests a positive feedback: with less revegetation of woody taxa, more bioavailable soil C is turned over quickly, inhibiting the accumulation of SOM and creating a less favorable soil environment for seedling establishment. Thus, Ponderosa pine is sensitive to severe fire, while forests composed of quickly-regenerating species such as Lodgepole pine and Gambel oak are more resilient.