Fire Regime Attributes of Wildland Fires in Yosemite National Park, USA

Abstract

Past attempts to suppress all fires in some western forests have altered historic fire regimes. Accumulated debris and dense understories of shade tolerant species coupled with a warmer climate have led to catastrophic wildfires. Prescribed fires and wildland fire use fires are used by land managers to reduce fuels and restore natural conditions. Little is known about how wildfires, prescribed fires, and wildland fire use fires differ in their fire regime attributes. We compared the attributes of start date, duration, 95th percentile burning index, size, fire rotation and fire return interval, and fire severity of 144 fires >40 ha that occurred in Yosemite National Park from 1974 through 2005, and mean patch size and patch squareness (an index of complexity) of 106 fires that occurred from 1984 through 2005. We used fire history, weather records, and the Relative differenced Normalized Burn Ratio (RdNBR) derived from satellite imagery to make the comparisons. Prescribed fires started both earlier and later than wildfires and wildland fire use fires, and had the shortest return intervals. Prescribed fires burned during periods with a lower 95th percentile burning index (an estimate of potential flame length and fire line intensity), and resulted in larger patches that were unchanged or that burned with low severity. Wildfires were the largest in size and had the largest percent area burned at moderate and high severity. Wildfires also had the largest moderate and high severity patches, and the high severity patches were the most square. The significant differences in fire regime attributes suggest that land managers seeking to restore natural fire regimes and forest composition and structure must consider more than the metrics of area burned or fuel loading.