Top-down degradation of alpine meadow in the Qinghai-Tibetan Plateau: Gravelization initiate hillside surface aridity and meadow community disappearance

Alpine gravelization by glacier melting may subsequently cause degradation of downstream vegetation. The thinning rates of glaciers outside ice sheet peripheries in some regions has doubled over the past 20 years. However, no study is available about the effects of alpine gravelization on alpine meadow ecosystem. Here, by investigating and measuring the community characteristics, soil moisture, and soil temperature of alpine meadows under three different gravel coverage levels (No gravel covered meadow, half gravel covered meadow, and fully gravel covered meadow), we examine the response of surface microhabitats and community to gravel cover on a typical hillside alpine meadow. Results showed that gravel cover significantly decreased soil/gravel ratio and topsoil moisture. Topsoil water content of the half-covered and the fully-covered gravel meadow was outstandingly low and reached<8% and 5%, respectively. Meanwhile, the larger decrease rates for coverage (-72.53%), above-and below-ground biomass (-76.61% and-94.09%), community density (-81.76%), species richness (-77.75%) and diversity (-55.98%) were found after gravel cover. Gravel cover induced the changes of the dominant species composition from Kobresia pygmaea to Setaria viridis, and ultimately to Trigonotis peduncularis and Ranunculus pedatifidus. Cyperaceae functional group species almost disappeared with increasing gravel coverage. In general, the increase in gravel coverage caused a decrease in soil water content, coverage, above-ground and under-ground biomass, community density, species richness, and the disappearance of Cyperaceae functional group species. Our results provide a new insight into the effect of alpine gravelization on soil water environment and alpine meadow degradation.