Climate change-induced shrub encroachment changes soil hydraulic properties and inhibits herbaceous growth in alpine meadows

Alpine meadows have been experiencing rapid shrub encroachment, which can have profound effects on the hydrological processes and functions of the Qinghai-Tibetan Plateau, the headwater region of many of Asia's largest rivers. However, little is known about how increasing degrees of encroachment affect the herbaceous growth and the soil hydraulic properties of these ecosystems. To fill this knowledge gap, we examined various plant and soil characteristics across a gradient of shrub encroachment at four sites. These sites encompassed an undisturbed alpine meadow as well as alpine meadows experiencing varying degrees of shrub encroachment, ranging from light to moderate and severe. Our results showed that herbaceous cover, density, above- and belowground biomass, and species richness of all shrub-encroached meadows were significantly lower than those of the unaltered alpine meadow. Conversely, soil field water capacity, total porosity, and noncapillary porosity were significantly higher at the encroached sites. Importantly, as the percent shrub cover increased, soil volumetric water content decreased at shrub understories. In general, plant and soil characteristics were significantly more altered at shrub understories than at interspaces, and the magnitude of these modifications was highly correlated with the percent shrub cover. Our study demonstrates that shrub encroachment in the Qinghai-Tibetan Plateau improves soil hydraulic properties, but at the cost of alpine meadow herbaceous growth, and accompanied by a decrease in surface soil water content. Hence, we recommend that to effectively address shrub encroachment in these vital ecosystems, sustainable management strategies should prioritize the maintenance of grasslands and the protection of water resources.