Effects of cyclic fire and water availability on longleaf pine carbon dynamics

Background/Question/Methods

Based on predicted changes in precipitation patterns, the average area burned annually in some forests could double by the year 2050, with most of the expected increases due to catastrophic wildfires in forests with long fire return intervals. These disturbances cause major changes in ecosystem carbon dynamics and forest structure, and recovery can take over a century. Longleaf pine forests in the southeastern United States are affected by frequent, low intensity fires that sustain woodland forest structure and the highest levels of biodiversity recorded in North America.  To date, only a limited number of studies have focused on prescribed fire's role in ecosystem carbon dynamics, particularly in frequent fire systems in the southeast. We used the eddy covariance method along an edaphic moisture gradient in longleaf pine forests at the Joseph W. Jones Ecological Research Center in Newton, Georgia to investigate how interactions between soil water availability and fire affect net ecosystem exchange of CO₂ (NEE).  We hypothesized that soil water availability would influence carbon dynamics more than fire. We collected data for three sites (xeric, intermediate and mesic) beginning in summer 2008, and in the winters of 2009 and 2011, prescribed fires were conducted on each site.

Results/Conclusions

Results over three years of measurement show that the mesic site was a carbon sink (NEE = -248.3 g C m^-2 yr^-1), while xeric and intermediate sites were sources (NEE = 146.2 and 157.5 g C m^-2 yr^-1, respectively).  We attribute this to larger total leaf area index (LAI) and a faster rate of recovery from disturbance at the mesic site.  NEE was reduced at all three sites during the month following fire, but recovered quickly due to a change in the diurnal source and sink relationships. Decreases in NEE appear to have been driven more by water availability associated with drought when the fires were conducted, than with the direct loss of photosynthetic capacity associated with the burn. The quick recoveries to typical NEE rates at both sites are associated with the high evolutionary adaptation of these plant communities to low intensity fire. These findings also suggest the need to characterize the fire-adapted ecosystem process rates as a function of a broad range of abiotic controls across fine temporal scales and larger regional scales. Future analyses will focus on the seasonal and inter-annual variability of climate and soil water availability on ecosystem carbon dynamics.