Altered fire regime impacts on the soil biogeochemistry and microbial community structure of mixed conifer and ponderosa pine forests

Abstract

The practice of fire suppression and intensified grazing of dry western forests in the early 1900s has altered many aspects of forest structure and composition by increasing shade-tolerant species and creating a more continuous vertical and horizontal fuel structure. This has led to increased fire severity and extent throughout many western forests. Projections from climate change models suggest that fire severity and extent in many of the forests of the western United States will continue to rise. To mitigate this risk, managers are conducting prescribed burns when fuels are wetter and weather conditions are more suitable in areas with high fuel loads. The effects of these atypical fires on ecosystem structure and function are largely unknown. This dissertation assesses the impact of altered fire regime (season, severity) on the soil biogeochemistry and microbial community of two different forest types. To assess effects of altered fire season on soil systems, I studied sites subjected to early season prescribed fire, late season prescribed fire, and no fire (control) in a mixed-conifer forest in Sequoia National Park, CA. Results show that late season fires did have larger, more persistent impacts on the soil system than early season fires, relative to the control sites, altering several soil abiotic variables and decreasing biotic activity for 2-3 years post-burn. While these within-year treatment effects were significant for many soil environmental and biogeochemical parameters, the effect of year was significant for all variables tested. This suggests that certain climatological variables, such as precipitation, should be considered when analyzing treatment effects over several years. Post-fire nitrogen availability can be very important for re-colonizing organisms, both above and belowground. Inorganic nitrogen is very difficult to measure, however, due to the myriad of processes impacting net mineralization rates and methodological artifacts. To tease apart some of the problems associated with two different techniques and to assess their sensitivity to important environmental variables, I evaluated fire treatment effects on net nitrogen mineralization rates as measured by the soil core incubation method and the ion exchange membrane (IEM) method. The two methods were not significantly correlated. The core incubation method showed no change in net mineralization rate and a two-fold increase in net nitrification rate with fire while the IEM technique showed nearly a 3-fold increase in net mineralization rate and a 4-fold increase in nitrification rate with fire. The IEM method was also more sensitive to soil environmental variables than the core incubation method. To evaluate how varying severity fire impacts soil microbial communities, I established sites in dry ponderosa pine forests subjected to low severity fire, high severity fire, and no fire (control) in Pike National Forest, CO. The different fire severities did have different effects on the soil environment (pH, temperature, moisture). Soil microbial communities from low and high severity sites were not distinct, however, they were both significantly different from the unburned sites, according to canonical correspondence analysis of fatty acid (EL-FAME) biomarkers. Overall microbial biomass and richness were not significantly different among treatments, suggesting a quick recovery of a structurally distinct microbial community in both the low severity and high severity burn sites.