Data on the years in which fires burned, on fire frequency, and on intensity and areal extent of fires were gathered from 935 scars on 220 stumps of mixed conifer forest species in an 1800—ha study area in the Sierra Nevada, California, USA. Before 1875, fires scarred clusters of living trees every 9 yr on west—facing slopes at Redwood Mountain and every 16 yr on east—facing slopes. Mean fire—free intervals between 1700 and 1875 varied by habitat phase from 5 yr in ponderosa pine on a dry ridge to 15—18 yr in most moist sites with white fir. For most 1—ha sites, the maximum time without fire was 14—28 yr. From 1700 to 1875, fires on various sizes were found every 2—3 yr somewhere in a given drainage (not necessarily the same site) and every 5—9 yr in 3— to 16—ha sites. This compares with fires every 8—18 yr in 1—ha clusters and 11—39 yr on individual trees. Scar records of pre—1700 fires suggest intervals fairly comparable to those from 1700 to 1875. Evidence of fires diminished greatly after Indian burning was eliminated in the early 1870's, and such fire records became almost nonexistent after 1900, when fire suppression became more effective. Most of the pre—1875 fires were small and of low intensity. Even the larger fires were usually confined to 1 slope or 1 drainage area. The short mean intervals between fires suggest that pre—1875 mixed conifer forests did not usually have heavy accumulations of litter or dense thickets of understory trees. Instead, small—acreage, low—intensity surface first must have consumed accumulated litter at frequent intervals and at the same time killed most of the conifer regeneration which had become established since previous fires. Such frequent fires would have led to an intricate mosaic of age classes and vegetation subtypes which, in turn, insured that a subsequent fire would not burn large areas with great intensity. Intense fires which moved from crown to crown were absent in the study area for the past 400 to 2000 yr. If frequency of lightning ignition of fires over the past 50 yr is typical, ignitions by Indians must have augmented lightning—caused fires to yield the pre—1865 frequency of fires in the Sierra mixed conifer forest. Since 1900, the lack of frequent, low—intensity fires has resulted in a major increase in understory forest and fuels.

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