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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 has taken place in many of China’s northern grasslands. This study attempts to answer the following questions: which plant communities are present in these shrub-encroached grasslands (SEGs)? What are the species richness and composition of these communities? Which environmental factors determine the spatial distribution thereof? We investigated the community characteristics of 255 SEG plots with a size of 20 m × 20 m at 69 locations across grasslands in northern China. In each plot, paired 1-m² quadrats were established within shrub patches and the neighbouring grassy matrix to record herb species composition. The quantitative characteristics (abundance, coverage and height) of herbs were measured in 0.25-m² subquadrats, and soil samples were collected in the quadrats. Two-way indicator species analysis (TWINSPAN), detrended canonical analysis (DCA) and canonical correspondence analysis (CCA) were used for community clustering and ordination. The DCA and TWINSPAN results suggested that the SEGs in northern China can be divided into six community types: Potentilla fruticosa + Carex atrofusca, Spiraea hypericifolia + Festuca ovina, Caragana acanthophylla + Stipa sareptana, Caragana microphylla + Leymus chinensis, Caragana microphylla + Stipa klemenzii and Caragana tibetica + Cleistogenes songorica. At a regional scale, climate and soil nutrients controlled the spatial patterns of species richness and community composition of the SEGs in northern China. Temperature exerted a negative impact, whereas precipitation and nutrients had positive effects on species richness. Among the environmental factors used, climate was the major controller of the variations in community structure. These results provide new insights into the community composition of SEGs in China and enrich the global dataset of SEGs.

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.

The grasslands of northern China store a large amount of soil organic carbon (SOC), and the small changes in SOC stock could significantly affect the regional C cycle. However, recent estimates of SOC changes in this region are highly controversial. In this study, we examined the changes in the SOC density (SOCD) in the upper 30 cm of the grasslands of northern China between the 1980s and 2000s, using an improved approach that integrates field-based measurements into machine learning algorithms (artificial neural network and random forest). The random forest-generated SOCD averaged 5.55 kg C m−2 in the 1980s and 5.53 kg C m−2 in the 2000s. The change ranged between −0.17 and 0.22 kg C m−2 at the 95 % confidence level, suggesting that the overall SOCD did not change significantly during the study period. However, the change in SOCD exhibited large regional variability. The topsoil of the Inner Mongolian grasslands experienced a significant C loss (4.86 vs. 4.33 kg C m-2), whereas that of the Xinjiang grasslands exhibited an accumulation of C (5.55 vs. 6.46 kg C m−2). In addition, the topsoil C in the Tibetan alpine grasslands remained relatively stable (6.12 vs. 6.06 kg C m−2). A comparison of different grassland types indicated that SOCD exhibited significant decreases in typical steppe, whereas showed increases in mountain meadow, and were stable in the remaining grasslands (alpine meadow, alpine steppe, mountain steppe and desert steppe). Climate change could partly explain these changes in the SOCD of the different grassland types. Increases in precipitation could lead to SOC increase in temperate grasslands and SOC loss in alpine grasslands, while climate warming is likely to cause SOC loss in temperate grasslands. Overall, our study shows that northern grasslands in China remained a neutral SOC sink between the 1980s and 2000s.

This study aimed to evaluate the effect of shrub encroachment on soil organic carbon (SOC) content at broad scales and its controls. We conducted a meta-analysis using paired control data of shrub-encroached grassland (SEG) vs. non-SEG collected from 142 studies worldwide. SOC contents (0–50 cm) were altered by shrub encroachment, with changes ranging from −50% to + 300%, with an effect size of 0.15 (p