Late Quaternary fluctuation in upper range limit of trees shapes endemic flora diversity on the Tibetan Plateau.

The influence of paleoclimate in shaping current biodiversity pattern is widely acknowledged. However, it remains unclear how the upper paleo-range limit of trees, which dictated the habitat of endemic alpine species, affects the variability in endemic alpine species composition across space over the Tibetan Plateau. We integrated satellite-derived upper range limit of trees, dendrochronological data, and fossil pollen records with a paleoclimate dataset in a climate-driven predictive model to reconstruct the spatio-temporal upper range limit of trees at 100-year intervals since the Last Glacial Maximum. Our results show that trees distributed at the lowest elevations during the Last Glacial Maximum (~3426 m), and ascended to the highest elevations during the Holocene Climatic Optimum (~4187 m), a level ~180 m higher than the present-day (~4009 m). The temporal fluctuations in paleo-range limits of trees play a more important role than paleoclimate in shaping the current spatial pattern of beta-diversity of endemic flora, with regions witnessing higher fluctuations having lower beta-diversity. We therefore suggest that anthropogenic-caused climate change on decadal-to-centennial timescales could lead to higher fluctuations in range limits than orbitally-forced climate variability on centennial-to-millennium timescales, which consequently could cause spatial homogenization of endemic alpine species composition, threatening Tibetan endemic species pool.