A warm-temperate forest of mixed coniferous type from the upper Pliocene Sanying Formation (southeastern edge of Tibetan Plateau) and its implications for palaeoecology and palaeoaltimetry

Abstract

Coniferous forests represent one of the most important vegetation types in the southern Hengduan Mountains on southeastern edge of the Tibetan Plateau, but their evolutionary history remains poorly understood. Here, we report an upper Pliocene charcoal assemblage from the Sanying Formation at Fudong in the Lanping Basin, which represents remains of a mixed coniferous forest. Taxonomic studies indicate that the charcoal assemblage is dominated by Pinus armandii with a subordinate component of Cupressoideae, a few other conifers (Abies, Picea, Tsuga) and some angiosperms (Quercus, Rhododendron, Rubus, Sambucus, Zanthoxylum, Aralia, Amaranthaceae, Polygonum, Carex). Based on the present-day climate association of P. armandii, we consider that the reconstructed mixed coniferous forest to be of warm-temperate affinity. Some altitudinal variation of plant communities likely existed, with cool-temperate elements (Abies, Picea) occurring in higher altitudes on the mountain slope, warm-temperate elements (P. armandii, Tsuga, Quercus, Rubus, Rhododendron, Sambucus) in mid-altitudes, and warm elements (Zanthoxylum, Aralia) in lower altitudes. Due to the occurrences of Abies and Picea, we infer the presence of mountains >3000 m altitude near Fudong, which further constrains the timing of significant uplift prior to the late Pliocene. Together with fossil pollen data, our findings indicate that the southern Hengduan Mountains was increasingly dominated by conifers during the late Pliocene, particularly in its subalpine to alpine ecoregions, probably due to the progressive onset of cool and dry climates caused by regional uplift. Synthesising vegetation types, we conclude that the region showed large-scale vegetation zonation during this epoch, which generally ranged from evergreen broadleaved forests of warm affinity in the south to mixed coniferous forests of more temperate affinity in the north. The vegetation zonation can be interpreted as being shaped by large south-to-north altitudinal gradients due to uneven rates of uplift.