A 55,000-Year Reconstruction of Vegetation in the Los Angeles Basin: Insights into Future Change and a Better Guide for the Present

The California Floristic Province is a biodiversity hotspot. Endemic flora is threatened by climate change, habitat fragmentation and destruction. It is also host to the La Brea Tar Pits (LBTP), which in addition to its famous megafauna, preserves a rare long-term plant macrofossil record with taxonomic resolution to genus and species. The LBTP flora has the potential to provide a comprehensive vegetational history for the Los Angeles Basin and a dynamic baseline for modern conservation efforts. We used accelerator mass spectrometry (AMS) radiocarbon dating to establish a 55 ka timeline of plant presence in Los Angeles, California. We identified and radiocarbon dated 188 plant macrofossils from the La Brea Tar Pits collections. Eight distinct phases of vegetation were identified based on the loss or appearance of key taxa in the timeline. Pairwise similarity was calculated between each identified phase to compare changes to phase species makeup through time. The transition periods between phases were compared to existing climate proxy records to better understand forcings behind vegetation shifts. Two transition periods in the record are of interest to modern conservation efforts. First, fog-dependent closed cone pine species, Pinus muricata and Pinus radiata, become extirpated from the Los Angeles Basin at 48 ka. This disappearance corresponds with a period of extended drought. Second, repeated patterns of juniper species replacement during periods of megadrought, and their eventual extirpation at 12.8 ka, indicate the importance of land management choices in the ability of juniper populations to rebound after drought-induced die-off. Understanding the long-term dynamics of plant community structure in a region is crucial to managing landscapes in the context of global change, and these data are now being leveraged to inform local conservation efforts in the city of Los Angeles.