Late Holocene Relationships Among Fire, Climate, and Vegetation in Rangeland Ecosystems of Southwestern Idaho

Rangelands are characterized by more arid climates than forested regions; therefore, establishing fire histories using traditional methods (e.g. fire-scars from trees or charcoal in lake sediments) is problematic. This study uses radiocarbon dating of charcoal preserved in alluvial fans and stream deposits to reconstruct a record of fire and geomorphic response in rangelands of 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 climate and fire records indicate this area has likely switched 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 rangeland site indicate most fires occurred during wetter times than the record average. During overall wetter periods, (e.g. 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. MCA 1025-650 cal yr BP) fire was recorded during a wetter interval. After ~600 cal yr BP, fire activity is similar to the record of low intensity fires in a nearby ponderosa pine-dominated drainage, and sagebrush is common in charcoal samples. Inferred shifts in the forest-rangeland ecotone are consistent with those reported elsewhere in the Great Basin. A comparison of OSL and C ages shows good correspondence between charcoal ages and the charcoal containing sediments, indicating fire-related sedimentation occurs soon after fire.