Forest disturbance history in the Sawtooth Mountains of Central Idaho and the Beaverhead Range of Western Montana
Gage, Joshua Albert
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Studies of disturbance history are important because they provide a framework for understanding the ecological response to past, present, and future climate change, and this information is useful for paleoecological researchers and land-use managers. Fire and insect outbreaks are common occurrences in western forests, and three studies were undertaken to increase our knowledge of their history in the northern Rocky Mountains. In the first study, sediment cores were sampled from 21 lakes located in forests experiencing mountain pine beetle infestation in the Sawtooth Mountains, Idaho. Cores were analyzed to determine whether or not mountain pine beetle remains were accumulating in the lake sediments in association with recent outbreaks. The study found that insect remains were sparse in the lake sediments, even in sites surrounded by heavily infested forests. These results cast doubt on whether paleo-beetle records can be reconstructed from lake-sediment cores. In the second study, one-meter-long sediment cores were taken from three lakes in Pinus contorta forests in the Sawtooth Mountains, Idaho, to reconstruct a historical record of fire spanning the last 2000 years. High-resolution charcoal analysis of the cores indicated changes in fire activity, but there was not a significant difference in fire occurrence during the relatively dry Medieval Climate Anomaly (1050 - 650 cal yr BP), the cooler Little Ice Age (750 - -50 cal yr BP), and the present day. Results suggest that the current fire regime has persisted for at least 2000 years, with little modification by humans or climate. In the third study, a high-resolution charcoal record was analyzed from Reservoir Lake in the Beaverhead Mountains, Montana to reconstruct the fire history of the last 15,000 years at the lower forest-steppe boundary. The charcoal record indicates relatively low fire frequency between 13,500 cal yr BP and 6000 cal yr BP and increased fire activity from 6000 to 1500 cal yr BP, suggesting increasing aridity in the middle and late Holocene. The fire-climate linkages observed in the paleoecological record provide insights that are useful in understanding future fire regimes with projected climate changes.