Altitude restricts the restoration of community composition and vegetation coverage of quarries on the Qinghai-Tibet Plateau

It is challenging to restore Tibetan Plateau vegetation once it has been damaged. Highway construction, necessary for economic development, have significant negative impacts on the composition, structure, and functioning of the Qinghai-Tibet Plateau alpine grassland ecosystem. However, there is uncertainty regarding the extent of ecosystem damage in this region, such as whether quarries dug during highway construction may be restored and how recovery varies with increasing altitude. We conducted a transect survey of 39 quarry-restored grasslands reseeded with Elymus nutans and adjacent paired natural grasslands on the Gongyu and S308 highways on the Qinghai-Tibet Plateau (2800-5100 m above sea level). By comparing the community height, coverage, and species composition between restored and natural grasslands, we evaluated the degree of recovery and altitudinal gradient pattern of the grassland community and explained the limiting factors that led to the differences in recovery. After two-year restoration, quarry coverage and species composition were not restored to the degree of natural grassland. As altitude increased, the coverage recovery degree of quarry recovery community continued to decline, and the dissimilarity of community composition between the restored and natural grasslands was rising. We showed a breaking point at ca. 4100 m where recovery degree dropped dramatically, indicating a threshold for highway construction. The soil nutrients in quarry decreased significantly with increasing altitude. Structural equation modelling indicated that increasing altitude decreased the coverage of quarry by reducing temperature and soil nutrients. Due to the limitations of single species sowing, low temperature and nutrient availability, the effect of recovery decreased with increasing altitude. Therefore, we propose that during highway construction in the permafrost of the Qinghai-Tibet Plateau, quarries should avoid operation at high altitudes. When the altitude exceeds a certain height (e.g., 4100 m in this study), we do not recommend excavating quarries to destroy alpine grasslands.