Restoration treatments in a Montana ponderosa pine forest: Effects on soil physical, chemical and biological properties

Low-elevation ponderosa pine ecosystems of the inland northwestern United States experienced frequent, low-severity fire that promoted open stands dominated by large diameter ponderosa pine (Pinus ponderosa). Fire exclusion has led to increased stand densities, often due to proliferation of less fire-tolerant species and an increased risk of stand-replacing wildfire. These fundamental changes have spurred interest in forest restoration treatments, including thinning, prescribed burning and thinning combined with prescribed burning. We examined the response of numerous soil physical, chemical and biological parameters to these treatments 1 and 3 years post-treatment, using a replicated field experiment. Individual restoration treatments were implemented in 9 ha units. We observed significantly lower C:N in the O horizon and higher O horizon and mineral soil NH4+ concentrations in both BURN and THIN/BURN treatments during year 1. Soil NH4+ remained elevated through year 3 in the THIN/BURN treatment. Net N mineralization, nitrification and NO3- concentration were significantly greater in the THIN/BURN than all other treatments during year 1 and net nitrification rates remained elevated through year 3. A high C:N substrate decomposed more rapidly in both BURN treatments relative to the unburned treatments. Treatments had no immediate effect on the soil microbial community; however, phospholipid fatty acid profiles differed 16-18 weeks following treatments due to higher actinomycetes in the THIN/BURN treatment. The large scale of our treatment units resulted in significant variation in fire severity among prescribed burns as a function of variation in fuel quantity and distribution, and weather conditions during burn days. Correlation analysis revealed that variation in fine fuel consumed was tightly correlated with net N mineralization and net nitrification. These differences in soil characteristics may influence stand productivity and understory species composition in the future.