Past vegetation dynamics in the Yellowstone region highlight the vulnerability of mountain systems to climate change

Aim: Reconstruct the long-term ecosystem dynamics of the region across an elevational gradient as they relate to climate and local controls. In particular, we (1) describe the dominant conifers' history; (2) assess changes in vegetation composition and distribution; and (3) note periods of abrupt change versus stability as means of better understanding vegetation responses to environmental variability. Location: Greater Yellowstone Ecosystem (GYE; USA). Time period: 16.5 ka bp-present. Major taxa studied: Juniperus, Picea, Abies, Pinus, Pseudotsuga. Methods: The vegetation reconstruction was developed from 15 pollen records. Results were interpreted based on modern pollen–vegetation relationships estimated from a suite of regression-based approaches. Results: Calibrated pollen data suggest that late-glacial vegetation, dominated by shrubs and Juniperus, lacks a modern counterpart in the area. Picea, Abies and Pinus expanded at 16 ka bp in association with postglacial warming and co-occurred in mixed-conifer parkland/forest after 12 ka bp. This association along with Pinus contorta forest, which was present after 9 ka bp, has persisted with little change at middle and high elevations to the present day. This stability contrasts with the dynamic history of plant communities at low elevations, where shifts between parkland, steppe and forest over the last 8,000 years were likely driven by variations in effective moisture and fire. Main conclusions: The postglacial vegetation history of the GYE highlights the dynamic nature of mountain ecosystems and informs on their vulnerability to future climate change: (1) most of the conifers have been present in the area for >12,000 years and survived climate change by adjusting their elevational ranges; (2) some plant associations have exhibited stability over millennia as a result of nonclimatic controls; and (3) present-day forest cover is elevationally more compressed than at any time in history, probably due to the legacy of the Medieval Climate Anomaly and the Little Ice Age.