Biogeochemical response to fire in Mediterranean-type watersheds

Background/Question/Methods

Fire is a major restructuring force in Mediterranean-type ecosystems, inducing nutrient redistribution that is frequently invoked as a driver of ecosystem recovery. Fire severity is expected to increase with climate warming and associated droughts. To study ecosystem response to high intensity landscape fire, we used a combination of remote sensing, ground-based sampling of soil nitrogen dynamics and modeling in two burned, chaparral-dominated watersheds. These two watersheds, Mission Canyon and Rattlesnake Canyon, span the foothills of the Santa Ynez Mountains in Santa Barbara County, California, and large portions of both watersheds burned in November 2008 and/or May 2009. We used imagery from the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) to quantify burn severities across the two watersheds. Additionally, we established fifteen burned and three unburned plots in November 2009 and monitored them on a monthly basis through June 2011 for a variety of ecosystem properties including water content, soil and foliar carbon and nitrogen, soil pH, exchangeable inorganic nitrogen, and microbial biomass. The GIS-based hydro-biogeochemical model, Regional Hydro-Ecologic Simulation System (RHESSys) was used to integrate remote sensing and ground-based data to simulate water and nutrient fluxes across the fire-scarred watersheds. Soil nutrient dynamics were modeled under various fire regimes to evaluate the sensitivity of Mediterranean-type systems to intensified perturbations associated with climate warming.

Results/Conclusions

Burn severities were high across all sampling sites. Soil samples collected prior to the onset of rain were relatively enriched in ammonium, presumably due to ash residue deposition. Storm events then stimulated nitrification and additional waves of mineralization. Ephemeral herbs established quickly following the first post-fire rain events, thereby maintaining ecosystem nutrient capital as shrubs gradually returned. Nitrification was significantly enhanced in burned chaparral perhaps because fires increased soil pH, thereby raising the solubility of soil organic matter and stimulating nitrification, or perhaps because fires released nitrifying bacteria from competition with vegetation for ammonium. Overall however, nitrogen retention and export varied among plots, highlighting the complexity of ecosystem response to fire. Modeling results suggest that increases in fire frequency will alter soil biogeochemical processes and concomitant vegetation productivity, which may promote changes to long-term trajectories of ecosystem recovery.