Abstract: Aims Species richness patterns along altitudinal gradients in mountain ecosystems have been a central theme in ecological research. While past studies often investigated species richness on individual mountains, regional-scale analyses have typically relied on samples collected along roadsides or foothills, potentially neglecting the crucial aspect of vertical zonation in mountain biodiversity. Methods Eight mountains along the Rongbu River with varying base elevations (ranging from 3 785 m to 5 155 m) were selected for the study. Employing transect and quadrat methods, species richness was comprehensively surveyed across the vertical zones of these mountains. Important findings (1) A total of 83 plant species were recorded in this survey, including species such as Elymus nutans, Poa tibetica, Arenaria bryophylla, and Astragalus strictus, all of which were documented across all eight mountains in the basin. (2) On a watershed scale, species richness per unit area (1 m2) followed a unimodal curve with respect to elevation. However, on individual mountain ranges, the distribution patterns of species richness per unit area along the elevation gradient varied, showing U-shaped distributions, positive correlations, or no significant relationships. (3) Overall, the cumulative species richness in the lower parts of the mountains was lower than in the middle and upper parts. Additionally, the species richness per unit area was higher on the shady slopes of the mountains compared to the sunny slopes. This study revealed a significant disparity between species richness in the base zone of the mountains and the overall species richness of the entire mountain. This finding indicates that plant surveys conducted solely along roadsides or at the mountain base significantly underestimate the species richness of the entire mountain by 13.5% to 54.8%. Therefore, it is imperative to pay particular attention to the distribution patterns of species richness across mountain spatial scales and their key influencing factors in mountain ecosystems, especially in the Qingzang Plateau. Methods Eight mountains along the Rongbu River with varying base elevations (ranging from 3 785 m to 5 155 m) were selected for the study. Employing transect and quadrat methods, species richness was comprehensively surveyed across the vertical zones of these mountains. Important findings (1) A total of 83 plant species were recorded in this survey, including species such as Elymus nutans, Poa tibetica, Arenaria bryophylla, and Astragalus strictus, all of which were documented across all eight mountains in the basin. (2) On a watershed scale, species richness per unit area (1 m2) follows a unimodal curve with respect to elevation. However, on individual mountain ranges, the distribution patterns of species richness per unit area along the elevation gradient vary, showing U-shaped distributions, positive correlations, or no significant relationships. (3) Overall, the cumulative species richness in the lower parts of the mountains is lower than in the middle and upper parts. Additionally, the species richness per unit area is higher on the shady slopes of the mountains compared to the sunny slopes. This study reveals a significant disparity between species richness at the base zone of the mountains and the overall species richness of the entire mountain. This finding indicates that plant surveys conducted solely along the roadside or at the mountain base significantly underestimate the species richness of the entire mountain by 13.5% to 54.8%. Therefore, it is imperative to pay particular attention to the distribution patterns of species richness across mountain spatial scales and their key influencing factors in mountain ecosystems, especially in the Qingzang Plateau.

Key words: Rongbu River watershed, mountain species richness, vertical vegetation spectrum, spatial patterns, Himalaya region