Shrub encroachment increases soil erosion risk in hillside alpine meadows of the Qinghai-Tibetan Plateau, NW China

Shrub encroachment is advancing in alpine meadow ecosystems under climate warming, which can alter meadow community and soil properties. Given the key role of alpine meadows in soil and water conservation, meadow changes that could lead to higher erosion are particularly worrisome. However, little is known about plant and soil characteristics related to erosion resistance change with increasing levels of shrub encroachment. To address this issue, we evaluated multiple plant and soil variables closely related to erosion resistance along a shrub encroachment gradient in the Qinghai-Tibetan Plateau. Plant variables were herbaceous density, richness, root biomass while soil variables included bulk density, moisture, temperature, compressive strength, and shear strength. These were measured at multiple plots at four treatments, including non-encroached hillside alpine meadow (control) and hillside alpine meadow with low (<30% shrub cover), moderate (30%-60% shrub cover), and high (>60% shrub cover) levels of shrub encroachment. Our results showed that shrub encroachment significantly decreased herbaceous plant density and herbaceous species richness of meadow communities and that this effect strengthened along the shrub encroachment gradient. Encroachment also reduced herbaceous root biomass by 40.6%, 59.4%, and 73.3% in low, moderate, and high levels of encroachment at the 0-10 cm soil depth. We also found that shrub encroachment decreased soil bulk density by 10.9%-21.8% and soil moisture by 3.7-13.5% at 0-10 cm soil. Most important, both compressive and shear strength of topsoil gradually decreased with increasing levels of encroachment, and were approximately twice as low on high encroached plots than nonencroached plots. Overall, our results indicate that shrub encroachment leads to sparse community density, fewer herbaceous roots, and topsoil looseness, ultimately decreasing the strength of hillside alpine meadows' soils. These findings emphasize the role of shrub encroachment in driving the increase of soil erosion risk in hillside alpine meadows under climate changes.