SEDIMENT LAYERS PRESERVED IN ICE PATCHES: A NEW RECORD OF HOLOCENE ENVIRONMENTAL CHANGE ON THE BEARTOOTH PLATEAU

Temperature during the growing season is a primary control on treeline elevation at regional to global scales; however, relatively little information exists on treeline dynamics in response to long-term changes in temperature over the Holocene. Here we examine proxies of vegetation and treeline change from a permanent ice patch and wetland located above present-day treeline in the Beartooth Mountains, WY. We evaluate pollen, charcoal, and macrofossils preserved in organic layers within an ice core obtained from a 10,400-year-old ice patch and a 6000-year-old wetland sediment core to explore the relationship between seasonal changes in climate and shifts in treeline elevation. Pollen from the organic layers exhibited a low ratio of arboreal to non-arboreal pollen between 9000-6500 cal yr BP, suggesting downslope treeline movement during the warm and dry summer and cold winter conditions of the early Holocene. Increases in arboreal pollen dominated by pines and spruce suggest a potential upslope treeline expansion after the Holocene warm period (ca. 6500 cal yr BP), when summers became cooler and wetter than before. Pollen from the wetland shows grasses increased from 3000-1000 cal yr BP, suggesting another downslope shift in treeline elevation as winters and summers became cooler and wetter. Charcoal records indicate elevated fire activity during the early (ca. 10,000 cal yr BP) and middle Holocene (ca. 6000 cal yr BP), both periods of time with evidence of increased woody cover. However, charcoal accumulation rates were relatively low during the late Holocene until ca. 500 cal yr BP, signifying minimal fire activity at high-elevations over recent millennia. The records from these two sites indicate climate was an important driver of vegetation and treeline change during the past 10,000 years, though forests did not always respond to temperature in a linear fashion. While late Holocene treeline advanced with increases in growing season temperatures, early Holocene treeline retreated during periods of warmer temperatures, likely in response to moisture limitation. Predicted changes in effective moisture coupled with increased temperature could result in a retreat in treeline. These results add to a growing body of research highlighting ice patches as important records of past paleoenvironmental change.