VEGETATION RESPONSE TO WILDFIRE AND CLIMATE FORCING IN A ROCKY MOUNTAIN LODGEPOLE PINE FOREST OVER THE PAST 2500 YEARS

Wildfire is a ubiquitous disturbance agent in Rocky Mountain subalpine forests, and the frequency of large fires is projected to increase in the 21^st century due to warming climate and increased drought. Lodgepole pine (Pinus contorta), a dominant species in subalpine forests of western North America, is largely resilient to high-severity fires. However, recent studies suggest that the resilience of lodgepole forests may be compromised with predicted changes to climate and moisture availability. While much is known about modern post-fire dynamics of these systems, less is known about post-fire responses of lodgepole forests over longer temporal scales and in varying climate settings. This research investigates fire occurrence and post-fire vegetation change in lodgepole forests over the past two millennia to understand ecosystem responses to changes in wildfire and climate.

This research uses paleoecological proxies to reconstruct the impacts of wildfire on a lodgepole forest over the past 2500 years, a period including significant changes in regional climate. Chickaree Lake, Colorado, U.S.A., in Rocky Mountain National Park was chosen for the study site because of its small catchment size and existing paleoecological datasets. Pollen samples (n=51) were analyzed over multiple high-severity fire events previously reconstructed through charcoal analysis. A change in overall pollen composition was identified around 1,100 years ago when climate shifted towards modern conditions and Artemisia and Rosaceae pollen increased. Pinus was the dominant species throughout the record, however, there were visible decreases in Pinus pollen after fire events and small (<4%) but potentially ecologically significant quantities of Alnus and Sarcobatus pollen, which could replenish nitrogen stocks in post-fire successional sequences. These results help uncover the mechanisms that confer resilience within lodgepole forests to fire activity and reveal that variability in post-fire vegetation recovery can be important in understanding the long-term ecosystem dynamics of Rocky Mountain lodgepole pine forests.