Identification of fossil juniper seeds from Rancho La Brea (California, USA): drought and extirpation in the Late Pleistocene

Abstract

Juniperus spp. are keystone shrubs in western North America and important climatic indicators in paleo-records. However, a lack of taxonomic resolution among fossil species limits our ability to track past environmental changes. Plant macrofossils at Rancho La Brea (RLB) allow for reconstructions of juniper occurrence to species across 60 000 yr. We use microscopy, image analysis, species distribution modeling (SDM), and radiocarbon dating to identify an unknown Juniperus species at RLB and put it into chronological context with fossil Juniperus californica at the site to infer past environmental conditions. We identify the unknown taxon as Juniperus scopulorum Sargent, 1897. The Pleistocene occurrence of this species in California expands its known distribution and documents its extirpation. Temporal ranges of the two fossil junipers alternate, revealing a pattern of differential climatic sensitivity throughout the end of the Pleistocene. Occurrence patterns suggest sensitivity to temperature, moisture availability, and the presence of two mega-droughts at c. 48–44.5 ka and c. 29.3–25.2 ka. Extirpation of both taxa by c. 13 ka is likely driven by climate, megafaunal extinction, and increasing fire. The extirpation of fossil junipers during these past climatic events demonstrates vulnerability of juniper species in the face of global change.