La Brea Tar Pits scientists have identified a previously unknown juniper species to the La Brea Tar Pits as Juniperus scopulorum, commonly known as the Rocky Mountain juniper. The successful identification, along with the first-ever radiocarbon dating of these fossil plants in Southern California, expands our ability to track past environmental changes and highlights the vulnerability of junipers and the environments they shape in the face of modern climate change. Published in the journal New Phytologist, the study unlocks a key finding to understanding the megafaunal extinction at the Tar Pits and better understanding our own climate future.
The mammoths and saber-toothed cats that shape our imagination of Ice Age Los Angeles browsed, grazed, and hunted in juniper woodlands. More than just a source of food for giant herbivores, junipers were keystone trees and shrubs in the region, in turn shaping the landscape for at least 47,000 years before completely vanishing from the region in the same extinction event that erased most of the megafauna.
Researchers have long known that there are two different species of juniper found at the Tar Pits — the large-seeded J. californica (California juniper), and the small-seeded, mystery juniper. With distinct tolerances for temperature and drought, fossil junipers play a crucial role in understanding the changing climate of the last Ice Age, and how junipers can survive our climate future, but the identity of the mystery seed remained uncertain — until now.
“We set out to identify this mystery juniper, and in the process, we found a number of exciting things,” says Dr. Jessie George, postdoctoral researcher at La Brea Tar Pits, and lead author on the study. “Number one, we identified this juniper as Rocky Mountain juniper, and it is one of the most extreme examples of a plant going extinct locally. It’s not present anywhere in California today.”
As part of the study, George and the other Tar Pits researchers radiocarbon dated the two species of juniper, which led to the second exciting finding: “In the process of radiocarbon dating these juniper species, we found this really interesting pattern of reciprocal presence — either California juniper only or Rocky Mountain juniper only.”
Because each plant survives in specific conditions, its presence acts as a proxy for climate. George and her colleagues found that this dance between the two junipers coincided with long periods of drought and warm, dry weather that would otherwise be hidden in the fossil record. “California juniper is a much more drought tolerant species. It withstands moisture deficit way better than Rocky Mountain juniper,” says George. “Through these back-and-forth occurrences of the two species from the Tar Pits, we have this really fascinating record of aridity and drought that was previously undetected.”
The small size of the unknown juniper seed — about as big as Lincoln’s forehead on a penny — made it a difficult subject, especially since DNA has yet to be extracted from Tar Pits fossils. Instead, George compared the structure of seeds and branchlets to other juniper species — the only way to uncover its identity. It required careful comparison using advanced microscopy, image analysis, and species distribution modeling (SDM) until the team reached a definitive answer.
While climate definitely played an important role in their local extinction, the team thinks that the abrupt disappearance of Ice Age megafauna and fires started by humans may have also contributed, much like in the case of those iconic giant mammals. In a hotter, drier climate, even plants well-adapted to drought couldn’t survive the extra stress of human fires. This is especially true for plants that are not adapted to wildfire-unlike many other conifer species, juniper has little tolerance for surviving or re-growing following fires. The finding highlights the threat junipers continue to face from human-caused climate change and could inform conservation efforts going forward.
“We’re seeing events of really dramatic decline of these trees in the southwest today because of warming temperatures and increased wildfire caused by modern climate change. So a direct record of how this might have occurred in the past, what factors were at play, and where those boundaries occurred is incredibly important,” says George. “It gives us a better framework to understand a baseline of climate and environment to contextualize changes in other plant life and the fauna that we see during these periods of significant change in the past. As our ability to precisely date fossils improves, better and more detailed information is revealed from ancient life at La Brea.”
“Identification of fossil juniper seeds from Rancho La Brea (California, USA): drought and extirpation in the Late Pleistocene” was authored by Jessie George, Monica Dimson, Regan E. Dunn, Emily L. Lindsey, Aisling B. Farrell, Brenda Paola Aguilar, Glen M. MacDonald and was published in New Phytologist on December 10, 2024.
