Vegetation responses to late Quaternary climate change in a biodiversity hotspot, the Three Parallel Rivers region in southwestern China

The late-Quaternary vegetation history of a biodiversity hotspot-the Three Parallel Rivers region (TPRR)-was palynologically studied based on a series of fluvial terrace sediments along the upper Lancang valley, southwestern China. A parallel study on the modern pollen rains in the region shows that the contemporary distribution of species along elevational gradients are distinguished by characteristic pollen assemblages, providing good analogs for the reconstruction of the vegetation history. By analyzing the sediments and pollen recovered from the terraces, a brief history of the region since the late deglaciation is revealed. During the late deglaciation (11-14 ka), the pollen assemblages were particularly rich in Abies, Picea and Betula, indicating a range expansion of the alpine forest during a relatively cold time interval. Meanwhile, co-occurring pollen of Castanopsis, Myrtaceae, Caprifoliaceae, Araliacae, Anarcadiaceae, etc. suggests these tropical/subtropical trees, which are distributed in today's evergreen forests to the south of this region, once extended into these valleys under a wetter climate. However, since the mid-Holocene, the evergreen tropical/subtropical forests have given way to savanna, as revealed from the corresponding pollen assemblages of the second terrace (4-7 ka). The alpine vegetation might have simultaneously contracted to higher elevations under a warmer climate, and subsequently retreated to its present distribution in response to the late-Holocene cooling. Our first glimpse into the Quaternary vegetation history of the TPRR provides new insights into the mountainous biotic response to climatic forcing, and therefore has important implications for diversification in these steep gorges.