Postglacial environmental change of a high-elevation forest, Sangre de Cristo Mountains of south-central Colorado

Continuous sediment, pollen, and charcoal records were developed from an 8.46-m-long sediment core taken from Hermit Lake in the northern Sangre de Cristo mountain range of Colorado. Presently, vegetation around the lake is upper subalpine forest, consisting of Picea engelmannii (Englemann spruce) with some Abies lasiocarpa (subalpine fir), and the lake lies >200 m below present tree line. We used several pollen ratios to reconstruct the relative position of the tree line and the occurrence of clay layers to infer landscape instability through time. Deglaciation of the Hermit Lake drainage began during the Bølling-Allerød interval. Between ca. 13.5 and 12.4 ka, high Artemisia (sagebrush) pollen abundance, low Picea/Pinus (spruce/pine; S/P) ratios, and sporadic occurrence of Picea macrofossils indicate alpine tundra-spruce conditions. Though the pollen record shows no transition to the Younger Dryas, the subsequent absence of Picea needle fragments suggests a lowering of tree line. By ca. 10.2 ka, a subalpine forest of Picea and Pinus grew there. Based on pollen ratios, tree line was higher than today from ca. 9.0 to ca. 3.8 ka, after which the tree line began to lower to its present elevation. Maximum expansion of the Picea-Abies subalpine forest, determined from both pollen and macrofossils, was coincident with the highest influx of charcoal particles and maximum deposition of postfire erosion (clay layers) into the lake. The period ca. 7.8-6.2 ka was the driest period, as shown by aquatic indicators, but pollen ratios suggest that ca. 6.2-3.8 ka was the warmest period of the Holocene, accompanied by high rates of burning, and consequently elevated erosion of clays into the lake. During the late Holocene, declining S/P ratios are interpreted as declining alpine tree line, while decreases in both Picea to Artemisia (S/Art) and Pinus to Artemisia (P/Art) ratios suggest climate cooling. Pollen evidence suggests expansion of the lower-elevation Colorado piñon (Pinus edulis), which has been documented as part of a widespread phenomenon noted by other studies.