Transition patterns across an evergreen-deciduous broad-leaved forest ecotone: The effect of topographies

Question: Topography has distinct effects on local-scale vegetation distribution by creating heterogeneous habitats. In ecotones, which are sensitive to environmental variation, this effect on vegetation differentiation may be magnified at a regional scale. Here, we evaluate vegetation transition patterns across an evergreen-deciduous broad-leaved forest ecotone (EDF ecotone) in eastern China, to investigate whether the transition pattern differs between two topographic positions with different habitats, and identify underlying environmental mechanisms driving variation in species- and trait-based community structure. Location: Anhui Province, China. Methods: Across an EDF ecotone, community data were collected in 47 20 × 20-mplots situated on upper and lower hill slopes. Trait-based community structure was constructed based on seven functional traits and weighted by species abundance. Environmental variables were grouped into three categories: thermal conditions, water availability and edaphic variables. Trends in species richness and dominance along latitude were evaluated for both evergreen broad-leaved species (EBT) and deciduous broad-leaved species (DBT). Variation partitioning analysis was used to distinguish the roles of the three environmental variables in determining transition patterns within the two topographies. Results: For both EBT and DBT, species richness showed similar transition patterns along the latitudinal gradient on both the upper and the lower slope. However, species dominance changed more abruptly on the upper than on the lower slope. Compared with edaphic variables, both thermal conditions and water availability had a larger impact on transitions across the EDF ecotone, but their relative roles differed between the upper and the lower slopes. Thermal conditions primarily explained the variation in species- and trait-based community structure on the upper slope, while water availability primarily explained variation on the lower slope. Conclusions: Our results suggest different roles of thermal conditions and water availability in determining transition patterns between the two topographies. Evergreen Castanopsis trees, having less cold tolerance, make transitions on the upper slope more sensitive to thermal conditions. Cyclobalanopsis trees, which prefer wet habitats, make the transition on the lower slope more sensitive to water availability. Based on the present findings, we predict that the EDF ecotone will become narrower and there will be a sharper boundary between the evergreen and deciduous broad-leaved forests of eastern China under warmer and drier climate conditions.