Interacting effects of wildfire severity and liming on nutrient cycling in a southern Appalachian wilderness area
Aims Wilderness and other natural areas are threatened by large-scale disturbances (e.g., wildfire), air pollution, climate change, exotic diseases or pests, and a combination of these stress factors (i.e., stress complexes). Linville Gorge Wilderness (LGW) is one example of a high elevation wilderness in the southern Appalachian region that has been subject to stress complexes including chronic acidic deposition and several wildfires, varying in intensity and extent. Soils in LGW are inherently acidic with low base cation concentrations and decades of acidic deposition have contributed to low pH, based saturation, and Ca:Al ratio. We hypothesized that wildfires that occurred in LGW followed by liming burned areas would accelerate the restoration of acidic, nutrient depleted soils. Because soils at LGW had extremely low concentrations of exchangeable Ca2+ and Mg2+ dolomitic lime was applied to further boost these cations. We evaluated the effectiveness of dolomitic lime application in restoring exchangeable Ca2+ and Mg2+ and subsequently increasing pH and Ca:Al ratio of soils and making Ca and Mg available to recovering vegetation. Methods Five treatment areas were established: severely burned twice (2000 & 2007) with dolomitic lime application (2xSBL); moderately burned twice with lime application (2xMBL); severely burned twice, unlimed (2xSB); moderately burned once (2000), unlimed (1xMB); and a reference area (REF; unburned, unlimed). In 2008 and 2009, we measured overstory, understory, and ground-layer vegetation; forest floor mass and nutrients; and soil and soil solution chemistry within each treatment area. Results All wildfire burned sites experienced substantial overstory mortality. However, understory biomass doubled between sample years on the most recently burned sites due to the rapid regrowth of ericaceous shrubs and prolific sprouting of deciduous trees. Burning followed by lime application (2xSBL and 2xMBL) significantly increased shallow soil solution NO3-N, but we found no soil solution NO3-N response to burning alone (2xSB and 1xMB). Surface soil base saturation and exchangeable Ca2+ were significantly affected by liming; Ca2+ concentrations were greater on 2xMBL and 2xSBL than 2xSB, 1xMB and REF. There was a smaller difference due to moderate burning along with greater soil Ca2+ on 1xMB compared to REF, but no difference between 2xSB and REF. Surface and subsurface soil exchangeable Al3+ were lower on 2xSBL than 2xSB, 2xMBL, 1xMB, and REF. Liming decreased soil acidity somewhat as surface soil pH was higher on the two burned sites with lime (pH = 3.8) compared to 2xSB without lime (pH = 3.6). Conclusions Liming resulted in decreased soil Al3+ on 2xSBL coupled with increased soil Ca2+ on both 2xSBL and 2xMBL, which improved soil Ca/Al ratios. However, the soil Ca/Al ratio response was transitory, as exchangeable Al3+ increased and Ca/Al ratio decreased over time. Higher lime application rates may be necessary to obtain a substantial and longer-term improvement of cation-depleted soils at LGW.