Abstract: Aims The community assembly process based on the ecological niche theory and neutral theory is crucial to the biodiversity maintenance mechanism, which was one of the hotspots in forest ecology research. Mountainous areas were rich in biodiversity, but there were relatively few studies on the functional diversity and pattern of changes in the community assembly of mycorrhizal plant along the altitudinal gradient. Methods In this study, based on eight 1 hm2 dynamic monitoring plots established at an altitude of 600-2100 m in Fanjing mountain national nature reserve, we divided 261 woody plants with diameter at breast height (DBH) of ≥ 1 cm into three functional groups: arbuscular mycorrhizal (AM) plants, Ectomycorrhizal (EcM) plants, and Ericoid mycorrhizal (ErM) plants. The change patterns of their community assembly process and functional diversity along the altitude gradient was analyzed, and the potential role of the assembly process in maintaining functional diversity was revealed. Important findings The study showed that the functional diversity of the different mycorrhizal plants varied significantly with altitude, among which, the functional richness and functional dispersion of AM and EcM plants showed a significant decreasing trend with altitude, the community weighted means of leaf area and specific leaf area of AM and EcM plants showed a decreasing trend with altitude. The leaf dry matter content, leaf nitrogen and phosphorus content of EcM plants showed an increasing trend with altitude, while the leaf area, leaf nitrogen and phosphorus content of ErM plants showed an increasing trend with altitude. The community assembly of three mycorrhizal plants were dominated by stochastic processes, in which the drift of ErM plants contributed more to community assembly than AM and EcM. The βNTI (Beta Nearest Taxon Index) of AM plants had no significant effect on the functional diversity, whereas it had a significant effect on the community weighted mean of functional traits of EcM and ErM plants. The βNTI had a significant positive effect on functional diversity of EcM and ErM plants, which maintained their functional diversity. In addition, soil nutrients (soil organic carbon, total nitrogen and hydrolysable nitrogen) had a significant positive effect on the functional diversity of AM and EcM plants, but had a significant negative effect on the functional diversity of ErM plants. Altitude had a significant negative effect on AM and ErM plants, and a significant negative effect on EcM plants. The results of the study provide scientific basis for revealing the mechanism of biodiversity maintenance in the southwestern mountains, which is of great significance for the protection and restoration of natural forests in the central subtropics.

Key words: ericoid mycorrhizal plants, arbuscular mycorrhizal plants, ectomycorrhizal plants, stochastic processes, deterministic processes, community weighted mean