Late-Holocene relationships among fire, climate and vegetation in a forest-sagebrush ecotone of southwestern Idaho, USA

Abstract

The forest-sagebrush ecotone is characterized by a more arid climate than forested regions; therefore, establishing fire histories using traditional methods (e.g. fire-scars from trees, charcoal in lake sediments) is problematic. This study uses radiocarbon dating of charcoal preserved in alluvial deposits to reconstruct a record of fire and geomorphic response in southwestern Idaho. Samples indicate three primary periods of fire-related activity: 4400—4000, 2000—1400, and 650—400 cal. yr BP. Charcoal macrofossil identification and comparison with other regional records indicate this area has likely alternated between a ‘fuel-limited’ system (fires limited by lack of fuels), and a ‘moisture-limited’ system (fires limited by too much moisture) with changes in Holocene climate. Over the past ~2000 yr, samples from this site indicate most fires occurred during wetter times than the record average. During overall wetter periods, (e.g. ‘Little Ice Age’ (LIA); 600—100 cal. yr BP) tree density may have increased, and fires occurred during intervals of relative drought. During times of prolonged drought (e.g. ‘Medieval Climate Anomaly’ (MCA); 1025—650 cal. yr BP) fire was recorded during a wetter interval. After ~600 cal. yr BP, fire activity was similar to alluvial charcoal records of low-intensity fires in a nearby ponderosa pine-dominated drainage, and sagebrush is common in charcoal samples. Both the ponderosa site and the Wood Creek site show low fire activity in ~6500—5000 cal yr BP; climatically, ~7—5 ka appears to correspond with regional records of drought. This work provides a unique record of fire in a semi-arid ecotone where (1) few records exist because of the paucity of dating sites, and (2) climatic sensitivity is likely enhanced.