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).