| Prior to 1900, ponderosa pine (Pinus ponderosa Laws.) forests of western Montana experienced non-lethal fire at intervals of 13--50 years resulting in open, park-like, uneven-aged stands. Today, largely as a result of reduced fire frequency, many ponderosa pine stands are characterized by a dense interior Douglas-fir (Pseudotsuga menziesii var-glauca) understory, which gradually leads to Douglas-fir becoming a dominant species. Selection harvest and prescribed fire are commonly used to restore ponderosa pine ecosystems to their historic conditions. However, little is known about the effect of fire on soil microbial and biochemical processes. This dissertation was performed to investigate how prescribed fire influences N and C cycling and microbial activity under different management strategies and with or without the subsequent occurrence of wildfire. Both field and laboratory studies were performed to investigate the factors controlling soil C and N transformations with the return of fire to these historically fire-dependent forest ecosystems. Field studies demonstrated that prescribed fire improved the resistance of microbial communities to subsequent wildfire and aided in faster soil recovery. Of all restoration scenarios evaluated during the field study, prescribed fire had the least severe impact on the site with whole tree selection cutting and 50% tops returned. This site also appeared to have the highest labile N (potentially mineralizable N and NH 4+-N) both in mineral soil and organic horizon 9 months after fire. Finally, the laboratory studies simulated soil recovery after burning at low and high heating temperatures under moist and dry field conditions as can happen during spring and fall prescribed fires. The results demonstrated that dry heat (as during the late season fires) may physically protect active N pool by slowing N mineralization thereby potentially enhancing faster soil biological recovery. This research provides a more clear understanding of soil biogeochemical transformations under altered environmental conditions and forest ecological restoration scenarios. |
