Concerns about global warming and greenhouse gas emissions have placed high priority on measuring influences of land management practices on ecosystem carbon storage.  The influence of wildland fires on carbon storage in forests is poorly understood.  Natural pine forests in the southeastern U.S. Coastal Plain represent a fire-maintained ecosystem, in which the herb-dominated native plant biodiversity depends on a median fire return interval of 2-3 years to prevent hardwood dominance.  However, questions remain about the influence of frequent prescribed burning on CO₂ release and long-term forest productivity.  We investigated the influence of fire frequency on carbon storage in an old-field upland pine (Pinus taeda, P. echinata) forest in northern Florida using experimental plots in a long-term fire frequency study (Tall Timbers Fire Ecology Plots).  Replicated plots 0.2 ha in area were treated with 1, 2, and 3-year fire return intervals since 1960, and additional plots were fire-excluded for 42-48 years.  In each plot, we measured soil % total carbon (0-20 cm depth).  Carbon stored in plants and detritus was estimated using diameter measurements and allometric equations for woody plants and destructive sampling otherwise.    

Results/Conclusions

Total ecosystem carbon increased with length of fire return interval, with averages of 84, 120, 125, and 180 tonnes C/ha in plots with 1, 2, 3, and 42+ year fire-free intervals, respectively.  Differences were mostly attributable to abundance of hardwood trees and saplings, which contributed 3.2-60.0 tonnes C/ha in 1-42+ year fire-free interval plots, respectively.  Soil carbon storage was highest in the 2-year burn interval treatment (13.1 tonnes C/ha) compared to the remaining three treatments (10.1-10.8 tonnes C/ha).  Herbaceous vegetation was all but eliminated in the 3 and 42+ year fire interval plots, attributable to increased competition with hardwood vegetation.  Results suggest that frequent prescribed fire in this forest type does not deplete soil carbon, which is a key indicator of soil fertility and long-term forest productivity.  Total carbon storage may be increased through fire-exclusion, but at the cost of most plant biodiversity and wildlife habitat values.  Frequent fire greatly reduces the severity of wildfires, which can kill trees, release stored carbon, and reduce soil fertility and long-term forest productivity.  We conclude that frequent (2-year average interval) prescribed burning in Coastal Plain upland pinelands promotes their function as a stable, ecologically-sustainable carbon sink.