Ecosystem Carbon Remains Low for Three Decades Following Fire and Constrains Soil CO2 Responses to Precipitation in Southwestern Ponderosa Pine Forests

Abstract

The fire regime of ponderosa pine forests in the southwestern United States has shifted over the past century from historically frequent, low-intensity surface fires to infrequent, stand-replacing crown fires. We quantified plant and soil carbon (C) responses to this new fire regime and assessed interactions between changes in fire regime and changes in precipitation regime predicted by some climate models (specifically, an earlier monsoon rain season). We hypothesized that soil C pools and carbon dioxide (CO2) efflux rates would decrease initially following stand-replacing fires (due to low plant C inputs and the loss of the soil surficial organic (O) horizon), but then increase with time-after-fire (as plant C inputs increase). Water availability often limits soil biological activity in these forests, but we predicted that low soil C availability following fire would constrain soil CO2 efflux responses to precipitation. In a series of sites with histories of stand-replacing fires that burned between 2 and 34 years prior to sampling, burned patches had lower soil C pools and fluxes than adjacent unburned patches, but there was no evidence of a trend with time-after-fire. Burned forests had 7,500 g C m−2 less live plant biomass C (P < 0.001), 1,600 g C m−2 less soil total C (P < 0.001) and 90 g C m−2 less soil labile C (P < 0.001) than unburned forests. Lower soil labile C in burned patches was due to both a loss of O horizon mass with fire and lower labile C concentrations (g labile C kg−1 soil total C) in the mineral soil. During the annual drought that precedes summer monsoon rains, both burned and unburned patches had soil CO2 efflux rates ranging from 0.9 to 1.1 g CO2-C m−2 day−1. During the monsoon season, soil CO2 efflux in unburned patches increased to approximately 4.8 g CO2-C m−2 day−1 and rates in paired burned patches (3.4 g CO2-C m−2 day−1) were lower (P < 0.001). We also used field irrigation to experimentally create an earlier and longer monsoon season, and soil CO2 efflux rates at both burned and unburned plots increased initially in response to watering, but decreased to below control (plots without irrigation) rates within weeks. Watering did not significantly change cumulative growing season soil CO2 efflux, supporting our prediction that C availability constrains soil CO2 efflux responses to precipitation. This research advances our understanding of interactions among climate, fire, and C in southwestern forests, suggesting that climate-induced shifts toward more stand-replacing fires will decrease soil C for decades, such that a single fire can constrain future soil biological responses to precipitation regime changes.