Abstract
This study examined how fire frequency influences soil C and N dynamics in relation to spatial scale in two mixed-oak forest complexes in southern Ohio, U.S.A. We measured net N mineralization, net nitrification, total inorganic N (TIN) in the soil solution, and soil organic C in plots of two sizes (50 m2 macroplots and 1 m2 microplots nested within the macroplots) in sites burned once (periodic fire treatment), in sites burned three times (annual fire treatment) and in unburned control sites. N mineralization rate increased with increasing fire frequency and net nitrification was greater in soils given annual fire sites than in soils given periodic fire or no fire (control); in contrast, neither TIN nor soil organic C content were affected significantly by fire. Geostatistical analysis designed to detect spatial autocorrelation (patch structure) at sizes of 0.2–0.8 m (microplots) and 1.0–7.8 m (macroplots) revealed significant patch structure in only approximately 1/3 of the plots, and all of those had been recently burned. Comparison with studies done in these sites and others indicates that most spatial structure in soil properties occurs at ranges larger and smaller than those that could be resolved with this sampling design. Prior studies of the effects of fire on forests have suggested that fire enhances fine scale heterogeneity, though robust geostatistical analysis of patch structure has not often been part of such studies. The results presented here lend quantitative support that to the view that reintroduction of low intensity fire can restore native ecosystem spatial heterogeneity, and thereby help maintain overall plant species diversity and opportunities for the regeneration of diverse tree species.
Highlights
Forests dominated by oak (Quercus spp.) once covered much of the United States east of the Mississippi River as far west as eastern Oklahoma and northeastern Texas
The variance associated with plot size was not significant, there were significant interactions between plot size and fire treatment in determining net nitrification and total inorganic N in the soil solution (Table 1)
Total inorganic N was greatest in the annual fire treatment whereas at the microscale it was greatest in the control (Figure 4)
Summary
Forests dominated by oak (Quercus spp.) once covered much of the United States east of the Mississippi River as far west as eastern Oklahoma and northeastern Texas. Prior to extensive alterations of this landscape by European-american settlers in the 19th century, these forests were subject to frequent, low intensity fires (Guyette and Cutter, 1991; Sutherland, 1997) These fires typically occurred during the dormant seasons (especially late autumn and early spring) and were caused by a combination of lightning and human activities. Efforts to restore mixed-oak forest ecosystems to conditions more similar to those that existed prior to widespread landscape alteration have been initiated over the last decade in a number of states The goals of such projects include wildfire hazard reduction, facilitating regeneration and persistence of species which have come under threat as a result of fire suppression, countering the acidifying effects of atmospheric deposition rich in inorganic N, improving watershed condition, and increasing sequestration of C.
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