4 Discussion
4.1 Proliferation conditions
for shrubs in desert steppe
Grazing is one of the main factors influencing the dynamics of grassland
shrubs (Van Auken, 2000; 2009). The overgrazing hypothesis suggests that
chronic overgrazing reduces the biomass of herbaceous plants, which
leads to increased resource availability for shrubs, resulting in shrubs
becoming the dominant vegetation (Archer et al., 1995; Van De Koppelet
al., 2002; Tefera et al., 2007). Our findings showed that when the sheep
grazing is introduced or the stocking rate increased, Stipa
breviflora desert steppe will not be encroached by shrubs. This finding
does not support the hypothesis that chronic overgrazing is a driver of
grassland shrub proliferation in the desert steppe. Zhang et al. (2006)
studied the responses of Caragana microphylla morphological
characteristics to different stocking rates in a typical steppe in Inner
Mongolia, and found that the height of Caragana microphylladecreased significantly with increasing stocking rate, which suggested
that shrubby plants may resist herbivore feeding via individual
miniaturization (Wang et al., 2010). The results of these studies are
consistent with the results of our experiment, and support the
hypothesis that grazing inhibited encroachment of desert steppe by
shrubs (Zhang et al., 2014). Therefore, sheep introduction can prevent
the proliferation of shrubs in the desert steppe, while removal of sheep
grazing can release shrubs from top-down controls. Sheep grazing is one
of barriers limiting shrub encroachment in desert steppe.