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.