Abrupt changes in grassland productivity induced by shrub encroachment

Grasslands are important ecosystems that sustain productive pastoral enterprises and support multiple ecosystem functions. Yet grasslands are threatened by shrub encroachment, the transformation of grass‐dominant states to those dominated by woody plants. The extent to which encroachment initiates abrupt changes in ecosystem functions, and the major drivers of these changes, remain elusive.

We quantified five ecosystem functions (i.e. plant productivity, soil stability, soil carbon storage, soil decomposition capacity and soil nutrient cycling), and calculated trade‐offs among them across 102 sites along a gradient in shrub cover. We assessed the response of ecosystem functions to increasing encroachment and explored the relative importance of biotic and abiotic factors on ecosystem functions in encroached grasslands.

Plant productivity exhibited a non‐linear, bimodal response to shrub encroachment with two encroachment thresholds. Productivity declined sharply from 0% to 3% shrub cover (grass‐dominant stage), stabilized at 3%–25% cover (grass‐shrub transitional stage) and decreased substantially beyond 25% shrub cover (shrub‐dominant stage). Trade‐offs among productivity and ecological functions were stage dependent, being intense in the grass‐dominant stage but shifting towards synergy in the transitional and shrub‐dominant stages. Intensified encroachment decoupled the effect of above‐ and below‐ground species richness on functions. Productivity was positively related to below‐ground invertebrate richness under grass dominance, but became driven by above‐ground plant richness and soil bulk density in the grass‐shrub transitional stage as encroachment intensified.

Synthesis and applications . Our results demonstrate the non‐linear response of productivity to shrub encroachment and highlight the importance of incorporating encroachment thresholds into grassland monitoring and management. Our study shows that trade‐offs among productivity and ecological functions are strongest during the grass‐dominant stage, suggesting that management aimed at maintaining multiple ecosystem functions should be prioritized during the early stages of encroachment. Moreover, intensified encroachment decoupled the effects of above‐ and below‐ground species richness, indicating that maintaining below‐ground species richness is critical in the grass‐dominant stage when shrubs are sparse. The results highlight the importance of initiating brush management interventions at the early stages of encroachment (~3% shrub cover) and provide a scientific basis for guiding the timing and optimization of shrub removal strategies.