Early postglacial vegetation development in the Greater Yellowstone Ecosystem

Abstract

The last glacial-interglacial transition in the western US (20,000-8000 years ago) was a period of rapid environmental change. In the Greater Yellowstone Ecosystem (GYE), much research has focused on postglacial vegetation changes; however, questions still remain regarding the relative trade-off between climate and nonclimatic factors, such as edaphic conditions, disturbance, and biotic interactions, in driving early postglacial vegetation development at finer spatial and temporal scales in the region. This study reconstructed vegetation development in the GYE from the time of ice retreat to the early Holocene insolation maximum (17,000-8000 years ago) at sub-regional and regional scales using fossil pollen data from three sites in the northern GYE and across a regional network of 13 previously published records. Fossil pollen data from lake sediments were compared to independent measures of climate (paleoclimate model simulations, stable isotope data), edaphic conditions (lithologic and geochemical data), and fire activity (charcoal data) to better understand climatic and nonclimatic drivers of early postglacial vegetation development. Climate was the primary driver of early postglacial vegetation development in the GYE. Increasing summer insolation and its direct effects on summer temperature and effective moisture directed changes in vegetation from pioneering herb and shrub communities to spruce parkland during the late-glacial period to subalpine forest and eventually open Douglas-fir forest by the early Holocene summer insolation maximum. Nonetheless, fire activity, site-specific edaphic conditions, and biotic interactions mediated vegetation responses to climate change. Elevated regional fire activity between 12,500 and 10,000 cal yr BP, driven by increasing summer temperatures and fuel biomass, facilitated important ecosystem changes from an Engelmann spruce and subalpine fir dominated system to one dominated by whitebark and lodgepole pine. Site-specific edaphic conditions, namely erosional processes associated with newly deglaciated terrain, inhibited early conifer expansion, and important competitive interactions between lodgepole pine and whitebark pine after the early Holocene limited the range of whitebark pine at middle elevations in the GYE. This research provides new insight into how ecosystems and plant species have responded to past climate change and is critical for better understanding local responses to regional climate change predicted in the coming decades.