The cyclic role of fire in the movement of carbon through longleaf pine ecosystems
The frequency and magnitude of fire determines structure and regulates function of savanna ecosystems worldwide, yet our understanding of prescribed fire impacts on carbon dynamics in these systems is rudimentary and becomes more uncertain with changing soil water availability. While we understand that fires occur cyclically and return at frequencies based on fire regime, the impact that fire has on carbon dynamics has often been viewed as a single event. We combined prescribed fire, eddy covariance (EC) techniques and fuel consumption plots over an ongoing study which currently includes 6 years of EC measurements and 3 dormant season burn cycles to examine the cyclic response of fire on longleaf pine forest carbon dynamics along an edaphic gradient (mesic, intermediate and xeric sites), in southwestern Georgia, USA. We estimated linear and nonlinear models, as well as used time series methods new to the EC community, to quantify drivers of carbon dynamics in these systems while addressing the questions: 1) How does fire influence net ecosystem exchange (NEE), gross ecosystem exchange (GEE), and ecosystem respiration (Reco), and how long do these variables take to recover post-fire? 2) Do environmental conditions or prescribed fire have a greater influence on these ecosystems’ carbon dynamics?
We determined from this study that our mesic site was a moderate sink of carbon, while the intermediate and xeric sites were carbon neutral during the course of this study. When loss associated with fire was combined with NEE rates, all sites became moderate carbon sources in the years that included a burn (2009, 2011 and 2013). Analyses of assimilation and respiration parameters (e.g., maximum photosynthesis, quantum efficiency, and daytime ecosystem respiration) showed a positive trend pre-fire and a negative trend post-fire for maximum ecosystem CO2 uptake rates, and the opposite relationship for daytime ecosystem respiration rates. Within 30 to 60 days following fire, ecosystem physiological activity was statistically similar to pre-fire and appeared to be driven by the pine canopy. Our results suggest that prescribed fire (low intensity, high frequency) does not re-set this ecosystem through its removal of litter and understory, but rather maintains the existing structure and function (in this case, carbon flux rates) because longleaf pine ecosystems have evolved with fire. While losses due to fire affected carbon balances on short time scales (instantaneous to a few months), environmental variation over the study was more important in determining the carbon dynamics of the longleaf pine ecosystem.