The study of plant fossils makes it possible to precisely reconstruct past interactions between climate and ecosystems. A team of researchers affiliated with the Natural History Museum in Los Angeles and several academic institutions, including UCLA, recently published in New Phytologist an analysis of fossil seeds found in the La Brea Tar Pits, California. These fossils, attributed to a locally extinct juniper species, Juniperus scopulorum, reveal major climatic changes at the end of the Pleistocene.
The alternation of dominance between this species and Juniperus californica illustrates the impact of prolonged droughts and fires on vegetation. These discoveries shed essential light on the links between past ecological collapses and current environmental challenges, particularly in the face of climate disruption.
Identification of an extinct species: a careful scientific investigation
THE Juniperus scopulorumcommonly known as Rocky Mountain juniper, was identified through rigorous analysis of fossil seeds found in the La Brea Pits, California. This is the first ever radiocarbon dating of these fossil plants from southern California. These fossils are then found to be several tens of thousands of years old. Using electron microscopy and morphological analysis, the researchers compared the characteristics of fossil seeds. They paid attention to their shape and structure, along with those of modern junipers. They wanted to establish their identity.
This process also included modeling ancient climatic distributions to reconstruct environments suitable for the survival of this species. Unlike the Juniperus californicawhich currently dominates in the arid areas of California, the Juniperus scopulorum thrived in cooler, wetter climates. This makes it a valuable indicator of past climatic conditions.
Map. © J. George et al., 2024
“ During the process of radiocarbon dating these juniper species, we discovered this really interesting pattern of reciprocal presence: either only California juniper or only Rocky Mountain juniper », Explain the researchers. This dynamic is directly linked to the climatic variations of the end of the Pleistocene. So the data shows that the Juniperus scopulorum thrived during cooler periods. While the Juniperus californicabetter adapted to arid conditions, became widespread during periods of drought. According to Jessie George, head of the study, this alternation testifies to mega-droughts and significant climatic fluctuations previously invisible in the fossil record.
The combined role of climatic and human conditions during the Pleistocene
Extreme climatic variations alone are not sufficient to explain the local disappearance of Juniperus scopulorum. Researchers believe that the activities of early humans, including the use of fire, played a key role. Unlike other better adapted conifers, the Juniperus scopulorum does not have the ability to regenerate after forest fires. This vulnerability to fires, combined with periods of warming and prolonged drought, would have created a particularly hostile environment for this species.
Illustration of morphological measurements used to analyze juniper seeds. Blue highlights the measured regions: (a) major axis and minor axis of the ellipse/best fit roundness (b) area (c) perimeter (d) solidity (e) major axis and minor axis of the ellipse /roundness of best fit of the basal perspective (f) diameter of Feret and MinFeret. © J. George et al., 2024
These anthropogenic fires occur in a context already marked by arid conditions. They would have added additional stress to local ecosystems and thus accelerated the extinction of junipers.
Periods of domination of Juniperus californica thus coincide with episodes of intense drought. Particularly those recorded between 48,000 and 44,500 years ago, then between 29,300 and 25,200 years before our era. These intervals are also marked by rapid warming. This will have modified the ecosystems of the southwestern United States. These extreme climatic conditions favored the establishment of Juniperus californicamuch more resistant to lack of water than the Juniperus scopulorum.
Implications for modern ecosystems: a lesson from the past
THE Juniperus scopulorum fossil highlights striking parallels between Pleistocene environmental changes and current challenges. Today, juniper trees in the southwestern United States face similar conditions. We find in particular global warming, increasing aridity, and the intensification of fires. These factors are causing a significant decline in these species, threatening their long-term survival. The past illustrates to what extent junipers, despite their relative resistance, have limits in the face of combined disturbances. “ By studying the responses of fossil junipers to past environmental stresses, we obtain a road map for anticipating the impacts of modern climate change on these trees. “, explains Jessie George.
In addition, the study reveals the crucial importance of key species such as junipers in structuring and maintaining ecosystems. During the Pleistocene, these trees played a fundamental role in providing habitat. But they also provided a food source for megafauna, such as mammoths and saber-toothed tigers. Their local disappearance has contributed to major ecological imbalances. They then illustrate how the loss of a single species can disrupt an entire ecosystem. The challenges junipers face today are not limited to direct climate impacts. But they also include the cascading consequences on associated animal and plant communities. The preservation of key species remains essential to maintain the resilience of modern ecosystems in the face of environmental crises.
Source: Jessie George et al., “Identification of fossil juniper seeds from Rancho La Brea (California, USA): drought and extirpation in the Late Pleistocene”, New Phytologist (2024)
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