Fire and Terrain Controls on Soil Carbon in Chaparral Watersheds

This project addressed the post-fire soil carbon distribution as a function of burn history and
terrain characteristics in two chaparral watersheds in the uplands of the San Gabriel Mountains,
Southern California. We hypothesized that (1) prescribed fire, because it does not burn as hot,
leaves a greater amount of soil carbon on the landscape than a wildfire, and (2) soil carbon
distribution is strongly linked to terrain characteristics that influence post-fire erosion. This
relationship was assessed for two watersheds; one subjected to a prescribed fire, the other a
wildfire. Terrain analysis using a 0.5-meter DEM defined terrain characteristics within each
watershed. Soil samples were taken within a predetermined grid and analyzed for soil organic
carbon (SOC). The mean SOC of the prescribed fire watershed, 4.44 kg m-2, was significantly
higher than that of the wildfire watershed, 3.47 kg m-2, which indicates that fire severity is a
major influence on the post-fire SOC. Terrain characteristics, such as elevation and curvature,
were correlated to SOC only in their roles as defining elements of the broader landscape
positions (shoulder, channel, sideslope). Slopes in this study were very steep, mostly near 60%,
and the correlations between slope gradient and erosion found in studies of gentler landscapes do
not occur here. The extreme steepness of these slopes may be such that substantial erosion
occurs on all points of the landscape. This study provides values for SOC in the post-fire
chaparral ecosystem and evidence of the differences that occur due to fire severity.