Fire history differences in adjacent Jeffrey pine and upper montane forests in the eastern Sierra Nevada

Fire history and forest structural characteristics of adjacent Jeffrey pine (Pinus jeffreyi) and upper montane forests was investigated in the eastern Sierra Nevada at the University of California Valentine Natural Reserve. Jeffrey pine forests had lower canopy cover, higher amounts of fine fuels, and higher shrub cover when compared to upper montane forest that were dominated by red fir (Abies magnifica). Fire dates were determined using standard dendrochronolgy techniques from fire-scarred Jeffrey pine, lodgepole pine (Pinus contorta var. murrayana), red fir, and western white pine (Pinus monticola) trees, snags, stumps, and downed logs. Fires were recorded from 1745 to 1889 and mean fire return intervals were 9 and 24.7 years for the Jeffrey pine and upper montane forest types, respectively. The median fire return interval was 9.0 years for Jeffrey pine and 24.0 years for upper montane forests. Significant differences were found in mean fire intervals and fire history distributions between the two similarly sized fire history plots even though they were only separated by approximately 100 m. This study suggests that fire regimes can vary over very fine spatial scales. Differences in fire regimes are likely due to differences in fuel beds and fire behavior. Additional keywords: fuel load, spatial fire history, tree canopy cover, flammability, California red fir, lodgepole pine. S.L. Stephens Fir e his tor y dif fer ences in adjacent f ores ts WF01008 Scot t L. St ephensI ntroducti on Introduction Fire has been an important ecological process in the forested ecosystems of the Sierra Nevada for thousands of years. The frequent, lowto moderate-severity fires that characterized pre-historical (the period before the influence of European settlement) disturbance regimes in many of these forests affected not only overall forest structure, composition, and fuel loads, but also a wide range of other ecosystem components and processes (Kilgore 1973; Weatherspoon et al. 1992; Agee 1993; Skinner and Chang 1996). Fire frequency was reduced in the Sierra Nevada late in the 19th century due to the introduction of livestock grazing, elimination of Native American ignitions, and fire suppression policies (Vankat 1977; Kilgore and Taylor 1979). The reduction in fire frequency has resulted in forests that are more susceptible to large, high-severity wildfires (McKelvey and Busse 1996). In response, fuels and silvicultural treatments (van Wagtendonk 1996; Stephens 1998) have been proposed at broad spatial scales to reduce fire hazard and increase ecosystem sustainability (USDA 1995). Past fire frequency may have been affected by fuel and vegetation characteristics but no studies have documented this because of the difficulty in determining if changes in fire regimes were due to differences in the environment (i.e. temperature, relative humidity, precipitation, windspeed) or due to differences in fuel and vegetation characteristics (i.e. fuel load, fuel bulk density, tree canopy cover). Fire scars can be assigned to a calendar year when crossdating techniques are used (Dieterich 1980; Swetnam et al. 1985). With this technique, a composite fire history can be produced and differences in mean fire return intervals (MFI) (Stokes 1980) over the sampling period can be examined. Statistical analysis can also be conducted spatially to determine whether fire regimes are significantly different between two or more locations (Grissino-Mayer 1995). Many Native American cultures on the west-side of the Sierra Nevada used fire to achieve specific land management objectives such as food and basket material production (Anderson 1993). However, the Owens Valley and Mono Lake Paiutes that inhabited the eastern Sierra Nevada did not use fire as an important land management tool (Fowler and