CONTRASTING PATTERNS OF C3-C4 EVOLUTION IN THE TIBETAN PLATEAU VERSUS SURROUNDING LOWLANDS

The high Tibetan Plateau and climatic effects (enhanced drying and seasonality) have beenargued to drive the ecological transition on the Himalaya foreland from deciduous forests (C₃) to grasslands (C₄) during the interval of 8-5 Ma. Studies also reveal the decline of C₄grasses in the Tibetan Plateau region. How the high Tibetan Plateau impacts regional ecology remains elusive, but the answer is crucial to understanding the heterogeneity in C₄-grassland expansion that demonstrated a significant difference in timing, trend, and magnitude. New carbon isotopes and two-end member analysis reveal the dominant C₃plants with increasing C₃grasses on the plateau since the middle Miocene, contrasting with the prominent C₄-grassland expansion in lowlands south and north of the plateau. The contrasting paleoecology patterns imply that the C₄plants, typically growing in warmer regions (low latitudes and low altitudes),failed to occupy the cold and high Tibetan Plateau which had risen high before the C₄-grassland expansion.