Assessing vegetation changes in alpine arid and fragile ecosystems is imperative for informed ecological restoration initiatives and adaptive ecosystem management. Previous studies primarily employed the Normalized Difference Vegetation Index (NDVI) to reveal vegetation dynamics, ignoring the spatial heterogeneity alterations caused by bare soil. In this study, we used a comprehensive analysis of NDVI and its spatial heterogeneity to examine the vegetation changes across the Three-River Headwaters Region (TRHR) over the past two decades. A random forest model was used to elucidate the underlying causes of these changes. We found that between 2000 and 2022, 9.4% of the regions exhibited significant changes in both NDVI and its spatial heterogeneity. These regions were categorized into six distinct types of vegetation change: improving conditions (62.1%), regrowing conditions (11.0%), slight degradation (16.2%), medium degradation (8.4%), severe degradation (2.0%), and desertification (0.3%). In comparison with steppe regions, meadows showed a greater proportion of improved conditions and medium degradation, whereas steppes had more instances of regrowth and slight degradation. Climate variables are the dominant factors that caused vegetation changes, with contributions to NDVI and spatial heterogeneity reaching 68.9% and 73.2%, respectively. Temperature is the primary driver of vegetation dynamics across the different types of change, with a more pronounced impact in meadows. In severely degraded steppe and meadow regions, grazing intensity emerged as the predominant driver of NDVI change, with an importance value exceeding 0.50. Notably, as degradation progressed from slight to severe, the significance of this factor correspondingly increased. Our findings can provide effective information for guiding the implementation of ecological restoration projects and the sustainable management of alpine arid ecosystems.

Increasing precipitation variability has been tightly coupled to livestock grazing through direct impacts on vegetation productivity in rangeland ecosystems. However, where and to what extent such impacts occur has not been quantified systematically at the regional scale; thus, adaptive grazing management is plagued by this knowledge gap. Hence, using 20 years of precipitation, rangeland productivity, and livestock density data across Tibet, we assessed long-term precipitation variability impacts on alpine rangeland grazing, specifically highlighting variations in intra- and inter-annual precipitation variability. We showed that the precipitation concentration index (PCI) and coefficient of variation in precipitation (CVP) both increased significantly across Tibet over the past two decades, especially in the western region. On the contrary, grazing intensity (GI) in most rangeland areas markedly declined over the same period. Moreover, we found that GI is highly responsive to PCI and CVP for the alpine steppe, but interestingly, only PCI is significantly associated with GI for the alpine meadow. Furthermore, the Granger causality test indicates an extremely significant causality between GI and PCI, further highlighting that PCI was a remarkable determinant of rangeland grazing over the last two decades. Notably, we statistically identified rangelands with higher precipitation variability that experienced intensive livestock grazing, specifically, GI responded positively to CVP and PCI. In conclusion, our findings provide novel support for the increasing precipitation variability impacts on rangeland grazing over time across Tibet, especially the intra-annual variation. Thus, we advocate the implementation of adaptive grazing management, such as excluding and minimizing grazing for the alpine rangeland ecosystem under higher precipitation variability.

Cover crops have been recognized as a sustainable practice to improve soil structure and fertility in intensive agroecosystems, yet quantitative evidence on their effects and underlying mechanisms remains limited in high-altitude cereal cropping systems. This study investigated the effects of diverse cover crops on soil physicochemical properties during fallow in southern Tibetan cereal cropping systems, quantified their effect sizes relative to bare fallow, and explored the associations among key soil properties. Results showed significant variations in 0–20 cm soil physicochemical properties among cover crop species. Specifically, the oat (Avena sativa)–common vetch (Vicia sativa) mixture significantly reduced soil pH and increased water-stable aggregate rate (WSAR), total carbon (TC), and C/N ratio compared with turnip (Brassica rapa var. L.) and common vetch monocultures, whereas bulk density, total nitrogen, and soil organic matter (SOM) did not differ among cover crop treatments. Effect size analysis indicated that cover crops significantly reduced soil pH by 3.39% and increased WSAR by 32.85%. Principal component analysis and the piecewise structural equation model revealed that soil pH, WSAR, TC, and SOM collectively explained 86.18% of the observed variation in soil physicochemical properties, with pH significantly and positively affecting TC but negatively influencing WSAR, which in turn strongly promoted TC and thereby greatly enhanced SOM. These findings demonstrate that cover crops substantially improve selected soil physicochemical properties, providing critical evidence for their integration into bare fallow to enhance soil health in high-altitude cereal cropping systems.