Scientists at the La Brea Tar Pits have identified a previously unknown species of juniper, Juniperus scopulorum, or Rocky Mountain juniper, in the area. This identification, along with the first-ever radiocarbon dating of fossil plants in Southern California, enhances our understanding of past environmental shifts and underscores the vulnerability of junipers and the ecosystems they shape in the face of modern climate change. Published in the journal New Phytologist, the study offers new perspectives on the megafaunal extinction at the Tar Pits and helps inform our understanding of future climate scenarios.

The mammoths and saber-toothed cats that once roamed Ice Age Los Angeles inhabited juniper woodlands. Junipers were not only a food source for these giant herbivores but also played a key role in shaping the local landscape for at least 47,000 years, before vanishing in the same extinction event that wiped out much of the megafauna.

Researchers were aware of two juniper species at the Tar Pits—California juniper (J. californica) and a smaller-seeded, unidentified species. Fossilized junipers offer valuable insights into the changing climate of the last Ice Age and how these plants may adapt to future climate conditions. The identity of the mystery species, however, remained unknown until now.

"We set out to identify this unknown juniper, and along the way, we uncovered several exciting findings," said Dr. Jessie George, postdoctoral researcher at La Brea Tar Pits and lead author of the study. "We identified it as Rocky Mountain juniper, which is now extinct locally and no longer found in California."

As part of the research, the team radiocarbon dated both juniper species, leading to another significant discovery. The dating revealed a pattern of reciprocal presence—either California juniper or Rocky Mountain juniper, but not both—during certain periods, offering a unique glimpse into climate fluctuations. The presence of each species acts as a climate proxy, with California juniper being more drought-tolerant than Rocky Mountain juniper.

"This back-and-forth occurrence of the two species provides a fascinating record of aridity and drought, which was previously undetected," said George. "California juniper is far more drought-resistant and better equipped to handle moisture deficits than Rocky Mountain juniper."

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The small size of the unknown juniper seed, about the size of Lincoln's forehead on a penny, made identification challenging, especially since DNA could not be extracted from the fossils. Instead, George compared seed and branchlet structures using advanced microscopy, image analysis, and species distribution modeling to arrive at a definitive identification.

While climate change played a significant role in the extinction of this juniper species, the team believes that the sudden disappearance of Ice Age megafauna and human-started fires also contributed to the juniper's local extinction. In a hotter, drier climate, even drought-resistant plants struggled to survive additional stress from wildfires. Unlike many other conifers, junipers have low tolerance for fire, which further endangered their survival. This finding highlights the ongoing threats junipers face due to human-caused climate change and may inform future conservation efforts.

"Today, we're seeing dramatic declines in juniper populations in the Southwest due to warming temperatures and increased wildfire activity linked to climate change," George explained. "By understanding how these events unfolded in the past, we can gain valuable insights into the factors driving plant and animal extinctions and apply this knowledge to contemporary conservation strategies."