1 Introduction
Herbivore grazing is an ecological and economic use of grasslands
worldwide and is a primary disturbance affecting the plant community
(Asner et al., 2004). Although maintenance of a desirable mixture of
shrubs and herbaceous plants is beneficial to the functions and services
of the grassland ecosystem (Barbosa da Silva et al., 2016), grazing
management can intentionally or unintentionally change the abundance and
distribution of shrubs and herbaceous plants in the community (Madany et
al., 1983; Brown et al., 1999). Many studies have found that long-term
grazing can reduce grass cover, and aboveground and belowground biomass,
which not only increases resource availability for the recruitment of
shrubs and facilitates shrub regeneration, but also reduces surface
combustibles, which may reduce fire frequency (Archer, 1994; Lin et al.,
2010; Lohmann et al., 2014). These effects change grassland vegetation
structure from herbaceous- to shrub-dominated communities and can
accelerate the process of shrub encroachment (Case and Staver, 2017;
Devine et al., 2017). For example, studies in savanna grassland suggest
that overgrazing reduces grass cover and shifts dominant herbaceous
plants from perennial to annual species, which have been linked with
reductions in competition between shrubs and grasses, ultimately
facilitating shrub development (Goheen et al., 2004; Belsky and
Blumenthal, 2010; Graz, 2010; February et al., 2013). Studies in the
western United States have shown that increasing density of grassland
shrubs is related to the introduction of cattle grazing, because
livestock can be used as a carrier for shrub seed diffusion while
feeding on grassland plants (Painter et al., 1993; Tews et al., 2004).
In contrast, other findings in semiarid north Chinese steppes have shown
that grazing is not a direct cause of encroachment (Zhang et al., 2014)
and some studies have even shown that grazing delays or limits shrub
encroachment in a subhumid south American grassland (Altesor et al.,
2006). In the latter case, grazing by domestic animals kept the shrubs
at a smaller size and a low or static density, while shrub size and
density increased with removal of the grazers (Friedel, 1985; Van Auken,
2009). Similarly, grazing exclusion in the temperate steppe in northern
China has led to Caragana micropylla invasion, because of
preferential utilization of palatable shrubs by grazers, such that after
grazing exclusion vegetation species composition shifts from more to
less palatable species due to the release of herbivore feeding pressure
on grasses (Zhang et al., 2014). It has also been suggested that
livestock consumption of seedlings and young branches of shrubs delays
shrub development, suppressing their ability to compete with the
dominant herbaceous plants (de Dios et al., 2014), and even causing
shrub frequency to decrease or disappear.
There are often different shrub species on the grassland that form the
‘background plant’ of the plant community, giving the vegetation
structure a clear mosaic pattern (Wei et al., 2013). Although there are
many shrub species in grasslands, only a few species have actually
become aggressive encroachers in regional flora (Stockes et al., 2010;
Eldridge et al., 2011). For example, in a Sonoran Desert grassland, the
coverage of Prosopis velutina increased markedly from the 1950s
to the 1990s, whereas other shrubs have maintained low coverage (Brikes,
2017). Therefore, shrub proliferation depends not only on external
driving factors, but also on the characteristics of the shrub itself
(Van Auken, 2009). From the perspective of community theory, population
distribution and species richness are the result of a regional species
pool passing through a series of abiotic environment and biotic
community processes, such that species with unsuitable environmental
traits are excluded from the community (Keddy, 1992). Invasive shrubs
must have a strong response to at least one encroachment driver, but
must also overcome all other environmental factors in order to survive.
Non-invasive shrubs may not respond to this driving factor, or may be
limited by other environmental factors.
In summary, the direction of succession in grassland plant communities
under grazing disturbance is the result of the interaction of grazing
livestock and shrub characteristics. When shrubs are consumed
selectively by grazing livestock, shrubs in the plant community will be
at a disadvantage, and when grazing animals mainly feed on herbaceous
plants, the herbaceous plants will be at a disadvantage (Van Auken and
Smeins, 2008). Therefore, the succession of grassland vegetation should
be comprehensively judged according to the regional climate, dominant
plant species and grazing animals.
The Stipa breviflora desert steppe is located in the transition
zone of the Eurasian steppe from the typical steppe zone to the desert
zone. The vegetation in this transition zone is shaped by the influence
of a harsh climatic environment and scarce soil resources, and the
ecosystem is sensitive to interference. More than 10 years of monitoring
has shown that there are more than 60 plant species in the community,
including shrubs and semi-shrubs such as Artemisia frigida ,Kochia prostrata , Caragana microphylla andCeratoides latens . Long-term research was carried out in this
region to answer the following three questions. First, does shrub
encroachment in Stipa breviflora desert steppe increase with the
introduction of sheep grazing? Second, how do shrub species respond to
grazing under different stocking rates? And third, can biotic or abiotic
factors predict the direction of succession in the desert steppe under
different stocking rates? Elucidating these questions can not only
deepen our understanding of shrub-herbaceous plant interactions and
succession patterns in desert steppe vegetation, but also provide a
theoretical basis and experimental evidence to guide grazing management
in desert grasslands to improve the ecological environment.