2.1 Study region and experiment design
The present study was conducted in the restoration area of C. microphylla shrub encroachment (43°33′-43°38′ N, 116°40′-116°44′E) in the middle of the Inner Mongolia Steppe, China. The climate in this region is typical semi-arid continental climate with a mean annual temperature of -0.4℃, ranging from -22.3℃ (minimum monthly temperature, January) to 18.8℃ (maximum monthly temperature, July), and mean annual precipitation ~348mm. The typical vegetation type of this region belongs typical steppe, and the perennial rhizome grass Leymus chinensis (Trin.) Tzvel. and the perennial bunchgrass Stipa g randis P. Smirn. dominate these communities (Bai et al., 2004). The major soil types of this area are calcic chestnuts and calcic chernozems (Schönbach et al., 2011). Due to the irrational utilization, most of this area has degraded with the deceasing plant diversity and vegetation cover. In the seriously degraded area, C. microphylla shrub expanded rapidly and showed a gradient of cover along the degradation degree (Peng et al., 2013).
Three sites that had been fenced since 1979, 1983 and 2003 were selected, where C. microphylla shrub encroachment had emerged before fencing. These sites are characterized by an average altitude of 1,182 m, a mild slope less than 5%, and shared the similar climatic conditions (because the geographical distance between pairwise sites was less than 5000 m). Within each site, there was a shrub cover gradient along the disturbance intensity.
In August 2018, we selected three plots of 100 m × 100 m, representing a gradient in shrub cover and density from low (<10%) to medium (10%-30%) to high (>30%) within each site. We quantified shrubs cover and density using the average value method in each plot within three 20 × 20 m sub-plots (established randomly). Within each plot, we sampled five 0.5 × 0.5m quadrats placed entirely beneath the canopy of shrubs and the same number of quadrats in open areas. In each quadrat, each vascular plant species and its density (cloned plants were calculated by tiller number) were recorded.