Reconstruction of Holocene aridity,fire,and vegetation patterns using lake sediments from Dog Lake,Oregon,USA

Abstract

The Holocene history of vegetation and fire in the northern Great Basin (USA) is poorly understood, especially concerning forest-steppe ecotones. Dog Lake is a rare perennial freshwater landslide-dammed lake located near the lower elevation of conifer forests. The lake has a maximum water depth of 8 m which often lowers by several meters during late summer. A 13 m, 9600-yr sediment core revealed periods of increased aridity from indicators of lake levels (aquatic macrophytes, elemental analysis of carbon and nitrogen, and sediment composition), vegetation change (pollen assemblages) and fire (macroscopic charcoal). From 9200 to 8700 cal yr BP, 122 cm of dense Chara matting indicates that lake levels were periodically lower than ca. 3 m. From 8700 to 8200 cal yr BP, high detrital silt input and lower C:N suggests high levels of regional dust and/or erosion, consistent with increased drought, reduced terrestrial plant cover, and dust-fertilized aquatic productivity. Fire activity was also low between 8700 and 8200 cal yr BP, consistent with limited herbaceous fuel availability. Following 8200 cal yr BP, silt levels declined, and pollen assemblages show vegetation trending toward mesic conditions. Deposition of 12 cm of Mazama tephra had no discernable effect on vegetation or fire. Fires increased during the mid-Holocene between 6000 and 4000 cal yr BP during higher grass cover, consistent with cooler conditions relaxing fuel limitation on the fire regime. Vegetation changed little during the late Holocene, with Cupressaceae increasing at 5400 cal yr BP through to current day, and Pinus fluctuating, peaking at 800 cal yr BP and declining through to current day. Our findings matched other regional paleodrought studies and reveals the millennial-scale pattern of climate and fuel limitation on the fire regime at lower treeline.