Shifts in functional compositions predict desired multifunctionality along fragmentation intensities in an alpine grassland

Abstract

Abstract Grassland ecosystems are experiencing increasing perturbations from climate change and anthropogenic activities, which often cause vegetation fragmentation and grassland degradation. Although individual ecosystem functions have been extensively investigated in the vegetation fragmentation and grassland degradation processes, the multiple ecosystem functions are rarely evaluated. Moreover, little is known on how species richness and functional compositions are associated with individual functions, and how such functions contribute to multifunctionality during vegetation fragmentation. In this study, 14 interconnected functions were derived from the seven fragmentation intensities of grassland on the Tibetan Plateau. According to vegetation continuity, three communities (an ungrazed grasses and Kobresia humilis community, a lightly-grazed Kobresia humilis and K. pygmaea community, and a medium-grazed K. pygmaea community) were treated as the pre-patchiness stage, and four communities (a heavily-grazed cracked K. pygmaea community, an eroded K. pygmaea community, a severely eroded K. pygmaea community, and a bare black-soil crust) were treated as the vegetation patchiness stage. Multivariate diversity interaction framework was used to test the effects of species richness and functional composition on multiple ecosystem functions. Desired multifunctionality was assessed based on three relatively independent functions: aboveground biomass, belowground biomass, and soil organic carbon (SOC) storage. The threshold was identified to signify the rapid decline of the multifunctionality. Our results showed that positive effects of species richness were observed among most of the functions in pre-patchiness communities. Such effects disappeared in vegetation patchiness communities. Shifts in functional composition explained the large variations in most of the functions in the two stages. Furthermore, a trade-off in abundance between grasses and sedges contributed to the negative correlation between the aboveground and belowground biomass in the pre-patchiness stage. Consistent decreases in the abundance of functional groups led to a rapid decline of the individual functions, and, as a consequence, an abrupt decline in the multifunctionality. Our results demonstrated that managing grassland for ecosystem multifunctionality and species conservation requires a detailed understanding of the effects of the various drivers, including species richness, functional composition, grazing intensity, and climate conditions.