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Controls on Abrupt Forest Changes in the Snowy Range, Medicine Bow Mountains, Wyoming

Flaim, Andrew
Large-scale forest changes are currently taking place across the western United States as the result of insect outbreaks, fires, and drought, but the long-term consequences of these disturbances remain unknown. Permanent state-shifts may be possible if new climate conditions prohibit recovery to past forest states. Paleoecological records can help anticipate such outcomes by examining past effects of climate change and disturbance. This case study examines evidence that past events caused sub-alpine forests in the Medicine Bow Mountains in southeast Wyoming to shift permanently to open alpine meadows sometime in past centuries or millennia. Specifically, a high-elevation plateau known as Libby Flats today contains a large area of meadows covered with the logs from large trees killed in the past but never replaced by new forest growth. We analyze charcoal and needle concentrations in a sediment core extending to 6209 calibrated years before present (cal. yr BP) collected from a pond on the Libby Flats Plateau to determine the cause and timing of the death of the forests that once covered the area. Spruce (Picea) and fir (Abies) needle concentrations drop sharply to a minimum around 875 cal. yr BP immediately following a major increase in charcoal accumulation rates, indicating that fire played a critical role in the final loss of forest cover near the pond and creating a permanent state change on the landscape. This evidence supports the hypothesis that disturbance can trigger lasting changes in ecosystem states if they accelerate changes favored by long-term climate trends.
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University of Wyoming. Libraries