The Tibetan Plateau, often referred to as the “Roof of the World” and the “the Third Pole of the Earth”, is home to a distinctive flora, with over one-third of its species being endemic to the high mountain regions. This makes the plateau a global hotspot for biodiversity.
The question of what drives species diversity was identified as one of the 125 key scientific frontiers by Science journal. Understanding the factors and mechanisms behind the formation of endemic species diversity in the high mountains of the Tibetan Plateau has long been a topic of significant interest.
On February 25 2025, the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), announced that a research team led by Prof. Tao Wang from ITPCAS, in collaboration with partners from the University of Basel, Xishuangbanna Tropical Botanical Garden, and Peking University, has recently completed a study published in the Nature Communications. The study reconstructed the temporal evolution of treeline on the Tibetan Plateau since the Last Glacial Maximum (22,000 years ago) and found that fluctuations in treeline elevation have shaped the spatial pattern of endemic alpine species diversity over the Tibetan Plateau.
This research has important scientific implications for more accurately predicting future shifts in alpine endemism diversity under a rapidly warming world, and formulating corresponding conservation strategies.
Prof. Tao Wang, the corresponding author of the paper, noted that it is widely accepted that the uplift of the Tibetan Plateau played a key role in the origin of its flora, while periodic climate fluctuations during the Quaternary Period (2.6 million years ago) have driven species diversification. Previous studies have shown that warming-induced upslope tree expansion could compress the habitat of endemic alpine species in high mountain regions, threatening their survival. However, in addition to the Quaternary climate fluctuations, the fluctuations in paleo-range limits have also influenced the diversity of endemic alpine species in alpine areas, a factor that had not been explored before.
In this study, the researchers integrated satellite-derived treeline elevation data, dendrochronological records, fossil pollen data, and a paleoclimate dataset into a climate-driven predictive model to reconstruct paleo-treeline dynamics since the Last Glacial Maximum. The study then explored the impact of paleoclimate and paleo-treeline changes on the spatial distribution of endemic alpine species diversity across the Tibetan Plateau.
The study found that the average elevation of the paleo-treeline on the Tibetan Plateau was lowest during the Last Glacial Maximum (LGM) (~3426 m) and highest during the Holocene Thermal Maximum (~4187 m), approximately 180 m higher than the present-day treeline (~4009 m). The fluctuation in treeline elevation between glacial and interglacial periods reached up to 850 m, causing the habitat area for endemic alpine species during interglacial periods to be about 50% of the habitat area during glacial periods.
Furthermore, the study revealed that the amplitude of paleo-treeline fluctuations reflects the degree of stability in alpine environmental conditions, which, in turn, shaped the spatial pattern of beta-diversity in endemic flora, defined as the variability in species composition across communities. Regions with greater fluctuations in environmental stability exhibit lower beta-diversity, indicating a homogenization of endemic alpine species composition across communities.
Dramatic environmental fluctuations trigger a filtering mechanism that adaptively selects species, resulting in more homogeneous species compositions across communities; conversely, stable environments allow species to develop higher specificity, leading to more diverse species compositions, explained by Jinfeng Xu, from the team of researchers.
While, temporal fluctuations in paleo-treeline are more important than those in paleoclimate in affecting the current spatial pattern of beta-diversity of endemic flora. This is because the heterogeneous terrain could provide micro-refuges that allow endemic alpine species to track suitable habitats over short distances, thereby buffering the negative impacts of climate change on their survival.
Prof. Tao Wang suggested that alpine endemics are not equipped to respond to the dual force from both accelerated tree expansion and drastic warming. This likely would accelerate spatial homogenization of endemic alpine species diversity, threatening the total endemic species pool on the Tibetan Plateau. The research team highlights that future work should incorporate the importance of tree expansion in developing more process-based species distribution models for projecting endemic species diversity and informing conservation strategies.
Figure: Paleo-changes of upper range limit of trees om the Tibetan Plateau over the past 22 thousand years at a 100-year interval.