Testing multiple hypotheses for the high endemic plant diversity of the Tibetan Plateau

Aim: The Tibetan Plateau harbours the highest alpine and endemic plant diversity in the world, attributed to rapid diversification during the plateau uplift and Quaternary climate fluctuations. However, there is little understanding of which hypotheses associated with these geological and climatic processes garner strong support as explanations for the observed diversity patterns. Here, we test support for hypotheses related to uplift and climate changes that could account for the high endemicity and phylogenetic diversity of the world’s highest plateau. Location: Tibetan Plateau (TP). Time period: Neogene, Quaternary and current period. Major taxa studied: Tibetan endemic seed plants. Methods: We collated data on endemic seed-plant distribution based on county-level mapping from published monographs and online databases. We calculated species richness (SR) and phylogenetic diversity for endemic herbs, shrubs, trees, and all plants for 0.5-degree × 0.5-degree grid cells covering the TP. We derived environmental and evolutionary predictors to evaluate eight biogeographical hypotheses associated with plateau uplift and climate fluctuations, and used partial regression analysis and mixed conditional autoregressive (CAR) models to assess the relative contribution of each predictor to the extant diversity of the TP. Results: We found plateau uplift independently explained more variance in diversity than climate fluctuations, but there were also strong interaction effects. The full CAR models including all predictors explained 37%–75% of the total variation across diversity measures and life forms. The predictor representing the montane museum hypothesis explained the most variation (c. 25%), but each predictor explained at least 6%. Main conclusions: These results demonstrate that both the plateau uplift and Quaternary climate fluctuations had large impacts on current patterns of species diversity, but the influence of plateau uplift was more pronounced than that of climate changes. Our study suggests that plateau uplift and climate changes are the original drivers of complex biogeographical processes accounting for the biodiversity of the TP.