Scientists have made a significant discovery in Southern California by identifying a new species through fossilized seeds, which sheds light on the intricate relationship between two juniper species and offers insight into the region’s climate challenges.
Researchers from the La Brea Tar Pits have unveiled a previously unidentified juniper species known as Juniperus scopulorum, or Rocky Mountain juniper. This identification, along with the first-ever radiocarbon dating of these fossil plants in Southern California, enhances our understanding of historical environmental shifts and emphasizes the vulnerability of junipers and the ecosystems they support amid current climate changes. The findings are detailed in the journal New Phytologist, contributing valuable knowledge regarding the megafaunal extinction at the Tar Pits and our climate’s future.
In the Ice Age, creatures like mammoths and saber-toothed cats roamed the juniper woodlands of Los Angeles, relying on these plants for nourishment. Junipers were not only crucial food sources for massive herbivores, but they also played a critical role in shaping the landscape for around 47,000 years before disappearing during the same extinction event that wiped out much of the megafauna.
Previously, researchers recognized two juniper species at the Tar Pits: the large-seeded J. californica (California juniper) and a smaller, unidentified juniper. These fossil junipers, with varying tolerances for heat and drought, help us understand the climatic transitions of the last Ice Age and how junipers may withstand future climate challenges; however, the identity of the unknown seed remained a mystery – until now.
According to Dr. Jessie George, a postdoctoral researcher at La Brea Tar Pits and the study’s lead author, “Our goal was to determine the identity of this mysterious juniper, and we discovered several intriguing things along the way. First, we identified it as Rocky Mountain juniper, which is a stark example of a plant that has gone locally extinct—it no longer exists anywhere in California.”
As part of their research, George and the Tar Pits team conducted radiocarbon dating on both juniper species, leading to another exciting discovery: “In dating these junipers, we observed a unique pattern of reciprocal presence—either California juniper or Rocky Mountain juniper appearing alone.”
Because each plant thrives in certain conditions, its presence offers clues about past climate. The researchers found that the alternating appearances of the two junipers coincided with prolonged periods of drought and warmer, drier weather that might have otherwise gone unnoticed in the fossil record. George explains, “California juniper is much more drought-tolerant and can better handle a lack of moisture than Rocky Mountain juniper. The patterns between the two species provide us with a fascinating record of aridity and drought that was previously hidden.”
The tiny size of the unidentified juniper seed—comparable to the size of Lincoln’s forehead on a penny—made it a challenging subject for study, especially since DNA has not been extracted from Tar Pits fossils yet. Instead, George analyzed the seeds and branchlets’ structures against those of other juniper species. This meticulous process involved using advanced microscopy, image analysis, and species distribution modeling (SDM) until a clear identification was reached.
While climate significantly impacted the local extinction of these junipers, the team suggests that the sudden loss of Ice Age megafauna and human-induced fires may have played roles, similar to what happened to those iconic large mammals. In a hotter, drier environment, even drought-resistant plants may struggle against the added stress from human fires. This is particularly true for juniper, which has low tolerance for wildfires compared to other conifer species. This finding points out the ongoing risks that junipers face from climate change driven by human activity and can guide future conservation efforts.
George notes, “We are currently observing a dramatic decline of these trees in the Southwest, exacerbated by rising temperatures and increased wildfires linked to modern climate change. Understanding how this trend unfolded in the past, what factors influenced these events, and where boundaries were drawn is vital.” She emphasizes that obtaining a clearer understanding of historical climate and environmental baselines can help contextualize changes in other plant species and the fauna we see amid significant shifts in the past. Improved capabilities for dating fossils will yield richer insights into ancient life at La Brea.”
The study titled “Identification of fossil juniper seeds from Rancho La Brea (California, USA): drought and extirpation in the Late Pleistocene” was penned by Jessie George, Monica Dimson, Regan E. Dunn, Emily L. Lindsey, Aisling B. Farrell, Brenda Paola Aguilar, and Glen M. MacDonald and published in New Phytologist on December 10, 2024.
