Effects of fire severity and post-fire climate on short-term vegetation recovery of mixed-conifer and red fir forests in the Sierra Nevada Mountains of California

Forest ecosystems in the Sierra Nevada Mountains of California are greatly influenced by wildfire as a natural disturbance, and increased fire severity and drought occurrence may alter the course of post-fire recovery in these ecosystems. We examined effects of fire severity, post-fire climate, and topographic factors on short-term (< 5 years) vegetation recovery in mixed-conifer and red fir forests in the Sierra Nevada. We hypothesized that short-term vegetation recovery patterns would be different among patches with varying fire severity, especially between low-moderate and high severity patches, and that post-fire climate would have differing impacts on short-term vegetation recovery in different ecological zones (lower montane forest vs. upper montane forest). 30-meter Landsat time series stacks were used to monitor short-term vegetation recovery following wildfire in mixed-conifer and red fir forest types. Changes in normalized difference vegetation index (NDVI) following thirty-five fires (> 405 ha) between 1999 and 2006 were examined. According to the modeling results provided by ordinary least squares (OLS) regressions including spatial variation coefficients, fire severity, post-fire wet eason precipitation, post-fire January minimum temperature, and topographic factors explain variations in short-term post-fire NDVI values (adjusted R-squared = [0.680, 0.688] for red fir forests; adjusted R-squared = [0.671, 0.678] for mixed-conifer forests). The modeling results indicated that burned mixed-conifer forest was sensitive to post-fire drought, while burned red fir forest, with higher summer soil moisture availability, was sensitive to post-fire temperature. We also found that differences in recovery related to fire severity disappeared more quickly in burned mixed-conifer forest than in burned red fir forest. Future efforts should focus on long-term recovery, including competition between forest and shrub species in previously burned areas.