Change in the trade-off between aboveground and belowground biomass of alpine grassland: Implications for the land degradation process

Alpine grassland on the Qinghai-Tibet Plateau has been degrading in recent decades. A better understanding of biomass trade-off (biomass allocation between aboveground [AGB] and belowground [BGB]) change with alpine grassland degradation could uncover the degradation processes and its underlying mechanisms. We surveyed the plant and soil characteristics of alpine meadow (AM) and alpine steppe (AS) at varying degradation levels on the plateau. In the AM, the AGB was only reduced when very severely degraded, but the BGB declined at any level of degradation. In the AS, the AGB showed no significant difference in the slightly, moderately, and severely degraded levels, and the AGB only significantly declined in the very severely degraded, whereas the BGB was nonsignificantly higher in the slightly degraded and gradually declined when the levels changed from slightly to very severely degraded. In the undegraded grasslands, the biomass trade-off was towards AGB and BGB in AS and AM, respectively. With degradation, the biomass trade-off shifted from AGB to BGB at the slightly degraded levels and then gradually reached zero trade-off at the very severely degraded level in the AS. In the AM, the trade-off gradually reached zero as moderately degraded progressed to very severely degraded level. The change in plant community composition and bulk density affected the biomass trade-off change independently with AM degradation, whereas only soil moisture, soil nitrogen, and soil pH interactively influenced the biomass trade-off change with AS degradation. Our results suggest different AGB and BGB change processes and mechanisms with AS and AM degradation.