Grassland soil seed bank responses to grazing exclusion: a global synthesis

Soil seed bank plays a vital role in restoring degraded ecosystems, and its compositional similarity with aboveground vegetation serves as an early warning signal of ecosystems approaching critical thresholds toward irreversible degradation. Grazing exclusion has been widely implemented to restore degraded grasslands. However, the global pattern of soil seed bank response to grazing exclusion and the underlying mechanisms remain ambiguous. To address this, we conducted a comprehensive meta-analysis using 1011 paired observations from 76 publications to assess the effects of grazing exclusion on soil seed bank characteristics (i.e., density and richness) and the similarity between soil seed bank and aboveground vegetation in grasslands worldwide. Our results showed that grazing exclusion increased soil seed density and richness by 54 % and 13 %, respectively, while it did not alter the similarity between seed bank and aboveground vegetation. Additionally, grazing exclusion increased the soil seed density of grasses, forbs, perennials and annuals, albeit to varying degrees. The positive effects of grazing exclusion on soil seed bank were more pronounced in regions with lower precipitation, in topsoil layer relative to deeper layers, in heavily degraded grasslands relative to light and moderate degraded grasslands, and in steppe relative to other grassland types. Moreover, the positive effects of grazing exclusion on soil seed banks strengthened with its duration, whereas its benefits for aboveground vegetation diminished with longer duration. Finally, structural equation modelling demonstrated that variations in soil seed banks under grazing exclusion were jointly explained by changes in aboveground biomass, aboveground species richness, and the soil seed density of different plant functional groups. This study advances our understanding of soil seed bank dynamics under grazing exclusion and offers clear practical applications for the management and restoration of degraded grasslands.