Elevational gradient pattern of woody plant community structure and species diversity in the Qinling Mountains

Aims: Plant community structure and diversity patterns research is valuable for revealing the mechanism of biodiversity maintenance. The Qinling Mountains are in the transition zone at the north-south climate dividing line where the flora and fauna meet. The flora has a complex community structure with diverse vegetation types of ancient origin. This region is an important platform for studying the plant community structure and diversity distribution pattern in the transition zone. Methods: According to the Center for Tropical Forest Science (CTFS) standards at the Smithson Institute for Tropical Research, we establish ten 1 ha (100 m × 100 m) long-term positional monitoring plots from 800 m to 2,600 m in the middle part of the southern slope of the Qinling Mountains. We conduct community surveys using our plots with the Qinling Huangguan forest plot as the core. Initially, we analyze the elevation gradient pattern of the species composition, systematic characters, diameter class structure, spatial distribution of dominant species, and species diversity of woody plants with diameter at breast height (DBH) ≥ 1 cm. Results: (1) We investigate a total of 208 seed plant species belonging to 50 families and 109 genera. The genera in the northern temperate zone account for 69.7% of the total number of genera, and the temperate zone is obvious. With increasing elevation, rare and occasional species and the composition of families, genera and species of the plant community show a unimodal pattern of increasing first and then decreasing. The dominant species of the community has obvious turnover along the elevational gradient, and the middle elevation shows the transitional nature of broad-leaved oak and coniferous fir forests. (2) The overall structure of DBH size class of all woody plant individuals generally shows an inverse “J” type. In terms of elevation, except for the bimodal distribution of woody plants at 1,600 m and 2,000 m, the rest of the elevation showed an inverted “J” shape, and the community structure is generally more stable. The spatial point pattern of dominant species at different elevations shows an aggregated distribution within the 1-10 m scale, which gradually changes to random and uniform distribution as the scale increased. With increasing elevation and removal of habitat heterogeneity, the scale range of aggregated distributions decreases and the scale range of random distributions increases. (3) In increasing elevation species richness shows a significant unimodal distribution pattern with the peak occurring between 1,200 m and 1,400 m. Changes in α-diversity (Shannon-Wiener diversity index, Simpson dominance index, Pielou evenness index) along the elevational gradient shows a significant bimodal pattern, with both peaks occurring between 1,000 m and 1,200 m, and 1,900 m and 2,100 m. β diversity (Bray-Curtis index and Jaccard index) shows a unimodal trend along the elevational gradient. Conclusion: Our study forms the foundation for the construction of long-term monitoring and a basic database of forests in the subtropical-warm temperate transition zone. This research provides a theoretical basis for the conservation of biodiversity in this transition zone and for sustainable forest management.