3 Discussion
3.1
Alpine meadow multi-steady-stage coexistence is a response and
adaptation to grazing disturbance under harsh environmental conditions
and is mainly controlled by grazing intensity
3.1.1 Alpine meadows are
characterized by their long grazing history and extremely fragile
environments
Kobresia evolves approximately 240 million years
(Shen
et al., 2008; Herzschuh, 2006) and alpine meadow soil began forming on
the Qinghai-Tibet Plateau 4 ka BP as aeolian dust rapidly accumulated in
the Late Holocene and alluvial sediment parent material formed during
the middle Holocene (~7–5 ka BP)
(Zhang,
2019). Livestock grazing has been performed for approximately 7,300
years
(Simons
& Schön, 1998;Simons et al., 1994; Georg et al., 2009) and large-scale
livestock husbandry for approximately 3,600 years
(Qiang
et al., 2016). Consequently, alpine Kobresia meadow has a long
history of livestock grazing, during which both the climate and
population densities have fluctuated (e.g., there was a large population
during the 6 AD) (Liao & Sun, 2003). Thus, alpine meadows on the
Qinghai-Tibet Plateau have the ability to remain stable and recover
after repeated disturbances.
3.1.2
Multi-steady stage coexistence is a strategy to resist the adverse
effects of disturbance in a harsh environment
Many studies have shown that the degradation succession process in
alpine meadows is a typical jump succession process
(e.g.,
Liu et al., 1999). Variations in livestock grazing intensities,
particularly those resulting from the household contract system, can
change the characteristics (i.e., coverage, height, leave area index) of
plant communities and the strategy or nature of plant community
organization in response to disturbances
(Zhang,
1990).
Most alpine meadow plants (e.g.,K.pygmaea or K. humilis ) propagate asexually
(Zhou
et al., 1986). In the current study, it was found that the density of
tillers, leaves, and young shoots increased with a light grazing
intensity, this is because the colonization of the asexual branches
begins from the outermost leaf layer and the spacing between
the
propagule shortened with increasing grazing intensity, and the root-soil
ratio (R/S) decreases rapidly, accelerating the imbalance of limiting
nutrient in soil supplying and the plant growth demanding, especially in
its asexual branches core area (Cao et al., 2010), which leads those
branches dead and to emerge bare mini-patches in the centers ofK. pygmaea patches in heavy grazing intensity K.pygmaea or K. humilis meadows (Fig. 5).