TERRESTRIAL ECOSYSTEM RESPONSE TO ABRUPT CLIMATE CHANGE DURING THE LAST DEGLACIATION AROUND MONO LAKE, CALIFORNIA

Wildfires, aridification, and rising temperatures threaten semi-arid terrestrial ecosystems in western North America. The last major natural reorganization of the water-limited ecosystems in the eastern Sierra Nevada of California occurred during the last deglaciation from ~16 – 9 ka. As climate generally warmed and dried at the beginning of the Holocene, lowland Juniper woodlands contracted as megafauna diversity greatly declined. Gradual deglaciation was punctuated by the abrupt onset and termination of the Bølling–Allerød (14.6-12.9 ka) and Younger Dryas (12.9-11.7 ka) events which were agents of major terrestrial ecosystem change throughout much of North America. However, in the eastern Sierra Nevada, our understanding of mechanisms and rates by which ecosystems responded to past abrupt climate change events have been hampered by poor chronological control, low sampling resolution, and few studies extending back to ~16 ka. Here, we document vegetation, fire regimes, and megafauna populations dynamics during the last deglaciation using a well-sampled record of pollen, microcharcoal, and Sporormiella from Mono Lake, CA. We place our palynological results into the context of a recent paleo-shoreline record of Mono Lake and modern surface pollen samples collected from throughout the region. During the late Pleistocene, canopy structures of Pine-Juniper-Sage communities surrounding Mono Lake rapidly altered during abrupt hydroclimate changes (14.7, 13.8, 12.5, and 11.5 ka) via rapid increases in wildfires. The greatest turnover in pollen taxa corresponded to major warming events near the onset of the Bølling Oscillation (14.7 ka) and the Holocene (~11.5 ka). Pleistocene megaherbivore populations declined in two phases at ~14.7 and 12.5 ka before their extirpation from Mono Basin at 11.2 ka as xeric steppe expanded at the beginning of the warmer and drier Early Holocene. However, vegetation communities remained resilient to burning while hydroclimate remained sufficiently wet to sustain lowland wooded shrublands. By analogy, sustained warming and aridity predicted for the rest of the current century risks a rapid decline of similar scrub and woodland ecosystems in the western Great Basin and elsewhere initiated by intense wildfires.