A ~50 KA RECORD OF TWO JUNIPER SPECIES AT THE LA BREA TAR PITS AND IMPLICATIONS FOR DROUGHT IN THE LATE PLEISTOCENE

Anthropogenic climate change is causing increasingly severe and prolonged periods of drought in the American Southwest. These extended dry periods are impacting plant communities across the region, threatening the persistence of drought-sensitive and drought-resistant species, alike. A 2018 drought saw dramatic dieback of drought tolerant Juniperus utahensis (Utah juniper) in southeastern Utah. Similar periods of extended drought have occurred in the recent geologic record, and these may serve as analogues to allow predictions of how plants will behave under modern climate change. However, tracking the subtleties of vegetation response to these events can be challenging with the coarser spatial and taxonomic resolution of traditional palynomorph proxies. A ~55 ka record of juniper macrofossils from the La Brea Tar Pits (California, USA) allows for the reconstruction of interspecific relationships through time within the genus Juniperus. We radiocarbon dated 56 fossil Juniperus seeds, including 34 of Juniperus scopulorum (Rocky Mountain juniper) and 22 of Juniperus californica (California juniper). Comparison of radiocarbon dates between the two juniper species reveals co-existence of the two taxa prior to 50 ka, then a pattern of reciprocal occurrences throughout the terminal Pleistocene. While J. scopulorum occurs during wetter and cooler intervals, the more drought-tolerant J. californica is locally abundant during warm and dry intervals, such as a previously identified “mega-drought” in southern California from ~29.3 to 25.2 ka. Another period of J. californica dominance occurs from ~48-44.5 ka, indicating a previously unidentified period of extended drought in the area. This alternation of juniper species presence suggests at least local, if not regional stand replacement in response to warming and drought. However, after ~12.3 ka, both juniper species were extirpated from the Los Angeles Basin altogether. We hypothesize that the combined impacts of deglacial warming (e.g. the Bølling–Allerød), changing fauna (megafauna extinction and introduction of humans), and increased fire frequency are the likely causes of these extirpations. The La Brea Tar Pits plant macrofossil record preserves valuable insights into past species-specific response to environmental pressures analogous to those presented by anthropogenic climate change.