Objective To explore the spatial pattern changes of plant species alpha diversity and lineage structure in dry-hot river valleys at altitude gradients, quantitatively identify and analyze the key environmental factors driving the vertical distribution of vegetation, with the aim of providing important references for theoretical innovation in vegetation research and ecological environment protection and restoration in dry-hot river valleys in China.

Method Seven vertical altitude gradient sample zones were set up along 1 200 to 1 800 meters in the typical dry-hot river valley area of Shikanzi Daqing in the Yongren section of the Jinsha River. Through the investigation of plant communities and habitats in the typical sample plots, combined with long-term positioning observation data such as meteorology from the Yongren Comprehensive Observation and Research Station of Natural Resources Elements in Yunnan Province, this study investigated the altitudinal distribution patterns of plant diversity in the dry-hot valley from three perspectives: species composition, community α-diversity, and phylogenetic structure. Furthermore, correlation analysis and redundancy analysis were employed to explore the response mechanisms of plant diversity and phylogenetic structure to climatic, topographic, and soil factors.

Result (1) There were a total of 34 plant species in the sample plot, belonging to 23 families and 33 genera. The Poaceae and leguminous families occupied an important position in the plant distribution of the study area and were significant across all altitude gradients.(2) The total community coverage, shrub layer coverage and altitude were positively correlated, while the herb layer coverage showed the opposite trend. This indicated that drought-tolerant herbs were dominant at low altitudes, while at higher altitudes, the habitat improved and the proportion of shrubs increased. (3) The species richness index, Shannon-Wiener species diversity index, Simpson ecological dominance index and Pielou community evenness index all showed a trend of increasing first and then decreasing with the increase of the altitude gradient. Overall, they presented a “mid-peak model”, with peaks appearing at 1 500-1 700 m for each index. The mid-altitude habitat was the best and had the highest diversity. (4) The net relatedness index (NRI) was positive in the altitude range of 1 200- 1 700 m and negative at 1 800 m, indicating that the community phylogenetic structure showed a trend of aggregation and divergence with the increase of altitude. Accordingly, the dominant driving force shifts from habitat filtering to interspecific competitive exclusion. (5) The annual temperature range was the dominant factor, explaining 21.3% of the α species diversity and phylogenetic structure. In addition, soil pH and soil thickness were also important factors affecting α species diversity and phylogenetic structure.

Conclusion The vegetation in the dry-hot valley area is generally scarce. Among them, leguminous species in woody plants are widely distributed at all altitudes and can be used as pioneer tree species for the restoration of dry-hot valley vegetation. The phylogenetic structure is aggregated in the low-altitude dry-hot valley area, indicating that the construction of dry-hot valley vegetation communities is controlled by habitat filtering. It is recommended to pay close attention to the changes of key factors such as the annual temperature range, soil pH and soil thickness to serve the restoration of vegetation in the dry-hot valley area of the Jinsha River.

Conclusion Vegetation in the dry-hot valley area is generally scarce. Among them, leguminous species of woody plants are widely distributed at all altitudes and can be used as pioneer tree species for the restoration of dry-hot valley vegetation. The phylogenetic structure is aggregated in the low-altitude dry-hot valley area, indicating that the construction of dry-hot valley vegetation communities is controlled by habitat filtering. It is suggested to pay close attention to the changes of key factors such as the annual temperature range, soil pH and soil thickness to serve the restoration of vegetation in the dry-hot valley area of the Jinsha River.