Simulated Effects of Two Fire Regimes on Bighorn Sheep: The San Gabriel Mountains, California, USA

Stephen A Holl,Bernard Bahro,Barry W Callenberger,Vernon C Bleich

doi:10.4996/fireecology.0803088

Stephen A Holl, Bernard Bahro + Show 2 more

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https://doi.org/10.4996/fireecology.0803088

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Abstract

AbstractAn isolated population of bighorn sheep (Ovis canadensis nelsoni) occupies fire-adapted chaparral ranges in the San Gabriel Mountains, California, USA. During 1976 to 2006, the amount of high-suitability habitat on bighorn sheep winter-spring ranges resulting from fires (HSF) ranged from 670 ha to 3392 ha, and population estimates for sheep, which were correlated with HSF, ranged from 130 to 740 individuals. During the past 100 years, the majority of changes in the HSF were associated with a fire regime dominated by periodic large, wind-driven, high-intensity crown fires, which resulted in high variability in the HSF. Prior to European settlement, the fire regime likely included smaller, variable intensity fires that burned during summer, but that also would have influenced the HSF. The size of those smaller fires today is effectively constrained by current fire management strategies, including exclusion and suppression. We predicted that smaller summer fires would increase the amount of high-suitability habitat and reduce the variability observed in the HSF during contemporary times, and the outcomes of our comparisons were consistent with those predictions. Small fires can be implemented by prescription, and can help to stabilize and maintain a self-sustaining population of bighorn sheep in the San Gabriel Mountains.

Highlights

In the San Gabriel Mountains of southern California, USA, bighorn sheep (Ovis canadensis) occupy winter-spring ranges dominated by chaparral vegetation for at least four months each year
Recognizing the management focus on bighorn sheep—and newly available information on the relationships between fire history, bighorn sheep demographics, and changes in the historical fire regime— our objectives were to: (1) describe the current fire regime in bighorn sheep habitat; (2) describe changes in the availability of suitable habitat during the twentieth century; and, (3) evaluate the potential for small, prescribed fires to be effective in increasing the amount of, and reducing variability in, HSF when considered in the context of the current fire regime
Fire history records for winter-spring ranges occupied by bighorn sheep in the San Gabriel Mountains indicated that fires burned into winter-spring ranges more often during fall when compared with summer (Table 1)

Summary

Introduction

In the San Gabriel Mountains of southern California, USA, bighorn sheep (Ovis canadensis) occupy winter-spring ranges dominated by chaparral vegetation for at least four months each year. Chaparral is a fire-adapted community characterized by moderate-to-long fire return intervals that are separated by intense crown fires; such fires reduce shrub canopy cover and result in an ephemeral (Biswell et al 1952, Hanes 1971, Keeley and Davis 2007) and highly nutritious herbaceous forage crop (Biswell et al 1952, Taber and Dasmann 1958) that enhances habitat quality for bighorn sheep by increasing nutrient availability and visual openness (Holl et al 2004, Bleich et al 2008). The population ranged from 130 to a high of 740 ±49 (95 % CI) (Holl and Bleich 2009). Bighorn sheep inhabiting the San Gabriel Mountains have been categorized as a Sensitive Species by the US Forest Service (USFS), with a management goal to maintain a self-sustaining population (USFS et al 2004)

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