Fire disturbance effects on regional carbon cycling in a sub-humid woodland.

Fire disturbance affects many ecosystem processes, especially carbon (C) cycling. In addition, fire is routinely used as a management tool in wildland ecosystems. In this study, I measured the fire-affected C storage, vegetation composition, habitat suitability for an endangered bird species (golden-cheeked warble), and charcoal content in juniperoak woodlands with documented past surface fires within Balcones Canyonlands National Wildlife Refuge. Using this information, I modified a process-based model of ecosystem biogeochemical cycling (Biome-BGC) to simulate the effects of different fire types, the production and loss of fire-derived charcoal, and the charcoal effects on soil water availability. From my field measurements, I found significantly different total aboveground biomass C with average values of 5.25, 6.86, and 9.18 kg m-2 for 60 plots with known fire histories that categorized into the recent (<40 year), old (>=40 year), and no fire group, respectively. These data also showed that higher oak recruitment was associated with fires that occurred in years with low summer precipitation. I concluded that fire may have a dual effect on habitat suitability with catastrophic wildfire damaging potential habitat and significantly reducing regional C storage, and moderate intensity fires in dense young juniper stands promoting tree species diversity. From the laboratory analysis of soils derived from these same plots, I estimated that the regional average soil charcoal concentration to be 1.40 g C kg-1 soil (20.8 g C kg-1 SOC) based on the methods of 13C nuclear magnetic resonance and mid-infrared spectroscopy, with fire-affected sites having significantly higher soil charcoal concentrations than the non-fire sites. I calculated the regional annual soil charcoal loss rate to be 4.7%, potentially due to soilerosion on steep slopes. Finally, the modified Biome-BGC model was able to reasonably simulate fire-affected C and charcoal storage changes. The model also indicated that the effects of fire on the ecosystem properties (vegetation C, leaf area index, and net ecosystem exchange) were closely associated with the severity of fire. The fire-derived charcoal did not appear to have significant effects on the simulated ecosystem properties. My results provided detailed ecological information regarding fire-affected processes in these woodland ecosystems.