Alpine meadows at different stages of degradation in the Beiluhe Basin of the Qinghai-Tibet Plateau: Productivity and community structure characteristics

In recent years, alpine grassland has become severely degraded in parts of the Qinghai-Tibet Plateau (QTP). This has had a negative effect on livestock production in the region. At some point in the process of the degradation of the grassland, there is a point beyond which it is extremely difficult to restore the grassland. Determining this point in the grassland degradation is an important question for restoration of the grassland. We selected five areas along a gradient from intact to extremely degraded alpine meadows in the permafrost region of the Beiluhe Basin on the QTP to explore the effects of degradation on the above-ground and below-ground biomass of vegetation, and the relative coverage of different plant functional groups (grasses, sedges and forbs). The results showed: 1) as the severity of grassland degradation increased, the relative coverage of grasses and forbs increased while the relative coverage of sedges decreased; 2) there was no significant change in aboveground biomass in the slightly degraded area, and it began to decrease in the moderately degraded area; 3) both belowground biomass and belowground net primary productivity (BNPP) decreased significantly from the moderate degradation stage, and the BNPP moved to deeper layers as the severity of degradation increased; and 4) the relative coverage of sedges and the proportion of BNPP at 0–10 cm soil depth showed a positive correlation with degree of degradation, no significant correlation at 10–20 cm soil depth, and a negative correlation at 20–30 cm and 30–50 cm soil depth. While the relative coverage of forbs at different degrees of degradation and the proportion of BNPP at each soil depth showed an opposite trend to that of sedges, the moderate degradation stage is the critical stage of grassland degradation.