Postglacial vegetation and fire history of the southern Mission Valley, Montana

Abstract

Ecosystems shaped by mixed - severity fire regimes cover a large area of the Northern Rocky Mountains, yet relatively little is known about the historical variability and drivers of these ecosystems. The low - and mid - elevations of the Mission Range, Montana, are dominated by mixed conifer forests, and the area has been occupied by humans for at least 10,000 years, making it an ideal location for investigating how climate and humans may have affected vegetation and fire regimes during the late - glacial period through the Holocene. Pollen and charcoal records from lake sediment cores from a small closed - basin lake (Twin Lake) were used to reconstruct the vegetation and fire history of the southern Mission Valley, Montana, and compared to other sites in the region. During the late - glacial period, data show an abundance of Pinus (P. albicaulis or monticola) Artemisia, and Poaceae pollen prior to 13,000 cal yr BP, suggesting the site was dominated by an open landscape with shrubs and grass, cold relatively dry conditions, and minimal fire activity. Increased percentages of Pinus (P. Ponderosa or contorta), Picea, and Abies pollen at 13,000 cal yr BP mark the onset of a closed conifer forest, relatively cool and wet conditions and an increase in fire activity accompanying an increase in biomass. Large increases in Pseudotsuga/Larix and Artemisia pollen between 10,000 - 6000 cal yr BP suggest warmer and drier climatic conditions developed during this interval, consistent with other records from the northwestern U.S. Charcoal influx show this interval of warm and dry conditions led to low severity fires followed by high severity fires as forests of P. contorta or P. ponderosa became more dense between 7000 and 5000 cal yr BP. The mixed - conifer forests that dominate the site today began to develop ca. 6000 cal yr BP when fire frequency and severity became highly variable. Surprisingly, fire activity from ca. 5000 cal yr BP to present remained relatively high despite a cooling and wetting trend in the region. This departure of fire activity from climatic controls suggests other local factors influenced fire activity, and may suggest a greater role of human influence during the late Holocene.