A twelve- or thirteen-month calendar?

Abstract

Prescribed burning is commonly used in forests to reduce fuel load, prevent encroachment of exotic species, maintain biological diversity and improve wildlife habitat. The low intensity burns that are conducted at intervals of 1–10 years may have long-term effects on the abiotic and biotic components of the soil. We assessed how prescribed burning altered litter, soil physical and chemical properties, and soil microbial communities in upland oak forests subjected to prescribed burning at frequencies of 0 (unburned), 2.5 and 5 fires per decade (FPD) over a 20 year period. Prescribed burning at 5 FPD reduced soil organic matter by 60% and soil organic carbon by 64% and increased bulk density by 20%. Compared to unburned controls, litter nitrogen was reduced by 20% and 21% and C/N ratio increased by 25% and 28% in sites burned at 2.5 and 5 FPD, respectively. Litter lignin/N ratio was reduced 28% at 2.5 FPD, compared to litter from unburned sites. We found no changes in soil pH, total N, NO3-N, NH3-N, plant-available Ps or K due to prescribed burning. Microbial biomass of Gram negative bacteria, as determined by phospholipid fatty acid (PLFA) analyses, decreased from 25% of biomarker abundance at the unburned sites to 20% at the 5 FPD sites. No other microbial functional group was significantly influenced by fire frequency. Our results indicate prescribed burning with low intensity fires in upland oak forests do not have major effects on the soils if these fires occur infrequently, i.e. less frequent than 2.5 FPD. However, frequent burning of these upland oak forests (5 FPD) resulted in important changes in the litter and soil that may adversely affect litter decomposition rates, soil carbon storage, soil water holding capacity, and soil erosion rates. Our findings indicate prescribed burning can be an important management tool, however, at high fire frequencies there may be tradeoffs between vegetation management goals and long-term adverse affects on the soil.