The Effects of Prescribed Fire and Thinning on Soil Properties and Surface Cover in Wisconsin Forests

Abstract

Prescribed fire frequency, thinning, and their combination, alter soil physical and chemical properties and lead to changes in surface coverages in mixed Red Pine ecosystems. Few studies have investigated soil properties under differing fire frequency regimes in a natural landscape. Fewer still have investigated the effects of thinning and prescribed fire together. The purpose of this study was to determine the effects of thinning along with prescribed fire usage at different frequencies on soils and surface cover. The treatments include a control (CONT); rough reduction burn with no thinning (RRB); thinning with no burning (TNB); thin and spring burns every one (TS1), two (TS2), three (TS3), and four (TS4) years; and a thin and fall burn every two years (TF2). Data analysis showed burning had a higher mean pH of 5.08 compared to not burning with a mean of 4.81 (p=0.041). The pH for the TS1 treatment was higher than the control with a mean of 5.17 compared to 4.80 (p=0.093). The combination of burning and thinning had a larger amount of Bray available P with a mean of 15.92 mg kg-1 compared to not burning and thinning with a mean of 11.52 mg kg-1 (p=0.03). Not burning and not thinning had a smaller amount of P with a mean of 14.89 mg kg-1 compared to 15.92 mg kg-1 for burning and not thinning (p=0.034). The combination of not burning and thinning had a larger amount of Al with a mean of 4.66 mg kg-1 compared to a mean of 2.11 mg kg-1 for burning and thinning (p=0.004). The combination of not burning and thinning had a larger amount of Fe with a mean of 1.71 mg kg-1 compared to burning and thinning with a mean of 0.83 mg kg-1 (p<0.001). Thinning had a smaller percentage of OM with a mean of 4.71% OM when compared to not thinning with a mean of 4.82% (p=0.003). The burn and thin combination had a smaller percentage of OM with a mean of 4.37% compared to the not burning and thinning main affect combination with a mean of 5.91% (p<0.001). There was an increase in OM for TNB with a mean of 5.91% compared to the thin and burn treatments. The thinning main affect had a larger amount of carbon with a mean of 3.26 g kg-1 compared to not thinning with a mean of 3.20 g kg-1 (p=0.013) Not burning and thinning resulted in a larger amount of carbon with a mean of 3.89 g kg-1 compared to burning and thinning with a mean of 3.10 g kg-1 (p=0.013). Total soil nitrogen had a larger mean of 0.24 g kg-1 for the combination of not burning and thinning compared to burning and thinning with a mean of 0.19 g kg-1 (p=0.002). Bulk density had a larger mean of 0.82 g cm3 for thinned treatments compared to a mean of 0.77 g cm3 for not thinned units (p=0.034). The combination of burning and thinning had a larger mean of 0.85 g cm3 compared to not burning and thinning with a mean of 0.70 g cm3. (p=0.071). There was also a larger percentage of bare soil for TS1 with a mean of 25.24% compared to all other treatments except TS3. Cryptograms had a larger mean of 6.78% in not thinned treatments compared to 1.75% in thinned treatments (p=0.016). Overall, there were few changes regarding soil properties and surface cover. Most of the effects seen in this study in regards to burning may be attributed to the addition of ash into the soil. The increase in pH, BD, and P may all be directly linked to the combustion of OM and the addition of ash into the soil and soil pores.