Abstract:
Fire regulates the structure and function of savanna ecosystems, yet we lack understanding of how cyclic fire affects savanna productivity and carbon dynamics. Furthermore, it is largely unknown how predicted changes in climate may impact the interaction between fire and carbon cycling in these systems. This study utilizes a novel combination of prescribed fire, eddy covariance (EC) and statistical techniques to investigate carbon dynamics in frequently burned longleaf pine savannas that lie along a gradient of soil moisture availability (mesic, intermediate and xeric). Results over three years of EC measurement of net ecosystem exchange (NEE) show that the mesic site was a net carbon sink (NEE = -248.3 g C m^-2 yr^-1), while intermediate and xeric sites were net carbon sources (NEE = 157.5 and 146.2 g C m^-2 yr^-1, respectively), but when carbon losses due to fuel consumption were taken into account, all three sites were carbon sources (1077.9, 795.0 and 969.0 g C m^-2 yr^-1 at the mesic, intermediate and xeric sites, respectively). Nonetheless, rates of NEE returned to pre-fire levels 1-2 months following fire. Loss of leaf area drove the reduction in NEE following fire, but evolutionary adaptations to frequent fire allowed the ecosystem to quickly recover carbon uptake capacity. While losses due to fire affected carbon balances, drought conditions over the final two years of the study were a more important factor driving net carbon loss during the study. In this work, we found that cyclic fire in pine savanna ecosystems maintains structure and carbon dynamics, but also that complex interactions between water availability, ecosystem structure and fire influence carbon dynamics on multi-year timescales. Longer-term observations over greater environmental variability and multiple fire cycles and/or the development of process models would help to more precisely examine the complex interactions between fire and climate and make future prediction about carbon dynamics in these systems.
