Spatiotemporal dynamics of forest arbor layer along an elevational gradient in southern Gaoligong Mountains

A better understanding of the spatiotemporal dynamics of species composition and biodiversity provides significant insights into the distribution patterns of biodiversity and the prediction of biodiversity fate in a global change context. However, how species and phylogenetic diversity change along elevational gradient with temporal scale in mountain ecosystems is still unknown. In this study, we used census data (2004, 2008 and 2013) of an elevational transect (elevational range 960 - 2 878 m) covering eastern and western slopes of Gaoligong Mountains, SW China, to investigate the spatiotemporal dynamics of forest arbor layer (DBH≥5 cm) species composition, species diversity and phylogenetic diversity. The results were as follows: (1) Species diversity was hump-shaped along the elevational gradient, while the phylogenetic diversity increased with increasing elevation. Clustering phylogenetic structure was observed at low elevations, and over dispersed or random structure was detected at high elevations. (2) Overall, temporal species and phylogenetic beta diversity analysis showed no significant difference among these three investigation time points. However, we found a higher clustering of phylogenetic structure along the temporal scale. (3) Spatial beta diversity showed that there was a significant loss of species at low elevations (960 - 1 381 m) on the eastern slope, where the vegetation had been disturbed or completely converted into farmland. The main lost species including Terminalia chebula, Quercus acutissima, Pistacia weinmanniifolia, Hovenia acerba and Ilex wattii. In contrast, the change in the western slope was mild at low elevations, only observing species abundance increasing, which including Cyclobalanopsis oxyodon, Symplocos chinensis, and Taiwania cryptomerioides. Therefore, we speculated that the dynamic changes in species and phylogenetic diversity of the arbor layer of subtropical evergreen broadleaf forest might be related to communities succession process and climatic change in middle and high elevations, while the communities at low elevations were seriously impacted by human activities in Gaoligong Mountains. The results deepen the understanding of the mechanisms of plant community dynamics in Gaoligong Mountains, and provide new insights to guide precise conservation strategies in the region.