Yak and Tibetan sheep mixed grazing enhances plant functional diversity in alpine grassland

• Mixed grazing at a 1:2 ratio enhances plant functional diversity. • Grazing at moderate intensity increases species richness and β-diversity. • Soil nitrogen, potassium, and bulk density drive functional diversity. The response of plant functional diversity to external disturbances not only effectively predicts changes in the ecosystem but it also reflects how plant communities use external environmental resources. However, research on how different herbivore assemblages affect plant functional diversity is limited. Therefore, this study systematically explored the effects of three typical herbivore assemblages (yak grazing, Tibetan sheep grazing, and mixed grazing by yaks and Tibetan sheep) on species richness, plant functional diversity, and soil physicochemical properties in alpine grasslands on the Qinghai-Tibet Plateau, China. This study further investigated the primary mechanisms driving the changes in plant functional diversity. The results indicate four key aspects of this system: (1) Grazing significantly enhanced plant functional diversity, particularly when the mixed grazing by yaks and Tibetan sheep was applied at a ratio of 1:2. This ratio showed the most substantial improvement in the functional dispersion index and Rao’s quadratic entropy index. (2) Compared to enclosed treatments, grazing increased species richness and β-diversity, contributing to higher plant functional diversity. (3) Grazing treatments affected various plant traits, such as reducing plant community height and leaf thickness while increasing specific leaf area. However, the impact on plant functional diversity was most pronounced under the mixed grazing by yaks and Tibetan sheep at a ratio of 1:2. (4) Species α-diversity was positively correlated with plant functional diversity. Changes in plant functional diversity were primarily regulated by variations in soil physicochemical properties. Specifically, increases in soil available nitrogen significantly promoted changes in plant functional diversity, while increases in soil available potassium and bulk density had a significant inhibitory effect on these changes. Long-term grazing significantly reduced the height of plant communities in alpine meadows, while a balanced mixture of yak and Tibetan sheep grazing, especially at a ratio of 1:2, enhanced plant functional diversity the most. This suggests that, under these conditions, the use of external environmental resources by the plant community is optimized.