Impact of shrub encroachment on ecosystem functions

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 110 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. Productivity exhibited a nonlinear response to shrub encroachment with two encroachment thresholds. Productivity declined sharply from 0 to 3% shrub cover (grass-dominant stage), stabilized at 3–23% cover (shrub–grass transitional stage), and collapsed beyond 23% 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. Ecosystem functions in the grass-dominant stage were mainly associated with soil water content and aboveground diversity, whereas belowground diversity was important during the transitional stage. Such an effect was modulated mainly by variation in climate (aridity and rainfall variability) via changes in soil physical properties and biodiversity.