Spatiotemporal Trends in Fire Occurrence and Severity
in the Sierra Nevada, California

We assessed temporal and spatial patterns in fire severity measured in 197 fires occurring on Forest Service lands in the Sierra Nevada, California, USA, between 1984 and 2004. We also examine climate causation of trends in fire occurrence during the 20th century. We measured vegetation burn severity by combining remote sensing methods (RdNBR, derived from pre- and one-year post-fire Landsat imagery) with field data collection (CBI, collected across multiple fires). Low- and middle elevation vegetation types characterized historically by high frequency, low severity fire regimes showed strong departures from the presettlement pattern, with significant contemporary skewing toward moderate and high severity fire. Higher elevation vegetation types showed little departure from expected presettlement fire regimes. Jeffrey pine, mixed conifer and white fir vegetation types all experienced increasing trends in the proportion of high severity fire. The average and maximum sizes of contiguous patches of stand-replacing fire within conifer types approximately doubled across the period of analysis. Maximum high severity patch size was positively correlated with fire size. Splitting the temporal fire occurrence record from 1908-2006 into early (1908-1956) and late (1957-2006) periods shows that a shift in climate correlations has occurred. For the fire size/area variables, the early period was characterized by a positive correlation with spring maximum temperatures, with the correlation shifting to summer maxima in the late period. In similar fashion, fire size/total area in the early period was negatively correlated with winter precipitation in the first half of the record, but with spring and autumn precipitation in the second half. We conclude that forest fuels are no longer limiting fire occurrence and behavior across much of the study area due at least in part to changing climate.