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Shrub encroachment is a critical environmental issue in arid and semi-arid landscapes. However, due to the complicated effects of shrub characteristics and environmental variables, ecological consequences of shrub encroachment significantly differ in different regions. As a result, although the phenomenon of shrub encroachment is widespread in the grasslands of northern China, its ecological consequences and associated drivers have not been well documented at the regional scale. We explored the effects of shrub encroachment on ecosystem structures (herb richness and abundance) and functioning (above- and belowground biomass, and soil carbon and nitrogen content) and evaluated the relative importance of climatic variables (temperature and precipitation) and shrub characteristics (cover, area, and height) as drivers of these effects. Based on field data from 53 sites that stretch across approximately 3000 km in northern China, we conducted the relative interaction intensity (RII) of the response variables between shrub patches and grassy matrix, and then used the structural equation modeling (SEM) to assess the relative importance of chosen drivers on the ecological consequences. Shrub encroachment increased the landscape heterogeneity by decreasing the richness, abundance and aboveground biomass of herbs, while by increasing the soil carbon and nitrogen content. The SEM explained 25–43% of the variation in shrub characteristics and 5–34% of the variation in the effects of shrub encroachment. Shrub height had greater effects on the RII of herb richness, soil carbon and soil nitrogen content, and the standardized total effects were 0.48, 0.25 and 0.18, respectively. Precipitation and temperature positively affected the impacts of shrub encroachment on herb richness, abundance, biomass, and multi-indexes. However, the direct effects of climate could be partially mediated by indirect effects via shrub characteristics. The height, size and cover of shrub patches significantly influenced the community structures and functioning of grassland ecosystems. The increasing aridity could potentially exacerbate the negative consequences of shrub encroachment in arid regions. Our results also demonstrated that the effects of climate variables on grassland ecosystems could be mediated by shrub management.

Shrub encroachment in grasslands will lead to a mosaic landscape with shrub patches interspaced with grass patches and thus alters local and regional water and heat balance. Syntheses on effects of the climate and native grassland community on the shrub-encroached grassland (SEG) are very limited at a large scale. In this study we examined the variation of their community characteristics in the Inner Mongolian grasslands, using 192 SEG plots data from field survey during 2012–2013. Our results indicated that shrub cover and patch size exhibit a significant increase with longitude, but showed a hump-shaped latitudinal pattern. Shrub patch density and herb abundance were negative associated with latitude and longitude, whereas plant height showed an opposite trend. Such geographical patterns were likely co-determined by climate and native vegetation. Precipitation was the major controller for shrub cover and patch size, and temperature was the primary factor influencing shrub height and patch density. Consequently, shrub cover and patch size were greater at dry and warm habitats than at moist and cool sites. In addtion, native vegetation also significantly influenced the variations in that of shrub patches. Diverse habitats with tall vegetation were likely more resistant to shrub encroachment. Furthermore, although we did not fully evaluate the effects of soils on SEGs, the discussion on the soil-SEG relationship suggests an important role of soils in characterizing SEG structures. Our study provides the first regional assessment on the variations of SEG structures, and has an implication in the regional shrub expansion management and control.