High grazing pressure accelerates changes in community assembly over time in a long-term grazing experiment in the desert steppe of northern China.

Abstract

Although numerous studies have shown that grazing gives rise to community succession from the communities or even species perspective, there is a lack of discussion about how grazing drives community assembly based on plant functional traits in a long-term experiment. We find different grazing intensities lead to temporal effects on trait-mediated multidimensional community assembly processes, including community-weighted trait mean (CWM), trait filtering, and trait distribution (divergence/convergence). CWM, trait filtering, and trait distribution of different traits transformed over the 16-year grazing experiment. Major findings include the following: (1) CWM changed rapidly under higher grazing intensity, and the removal of unsuitable traits from communities over time was accelerated with higher grazing intensity, such as higher specific leaf area (SLA), rich epidermal appendages (PAP), deep root system (RD), and growth form (shrub and subshrub) and dispersal mode (DM, e.g., insect spread) with higher scores. (2) Patterns of trait filtering strongly depended on grazing intensity and trait types, most traits, such as SLA, DM, PAP, RD, and onset of flowering (OFL), were filtered at high grazing intensity area, and effects of trait filtering in the community assembly process strengthened with grazing time. (3) Traits related to the cycling of biological matter, such as leaf area (LA), SLA, reproductive height (RH), photosynthetic (PHO), and GF more frequently diverged after long-term grazing, especially in higher grazing areas. Community assembly in intensely grazed ecosystems takes over a decade to support fundamental functions, highlighting the need for grazing intensity thresholds for sustainable grassland use.