FIGURE 2. Population density (A) and productivity (B) of A. polytrichoides cushions at indicated study sites.Populations are presented along with increasing elevation as PJ1 (4384 m), PJ2 (4505 m), PJ3 (4727 m) and PJ4 (4920 m) in the Pujing pasture, and CM1 (4560 m) and CM2 (4656 m) in the Core mine mountain range, respectively, both on the Baima snow mountains; only one population was selected at the Puyong pass on the Daxueshan snow mountains (PY, 4595 m) and Yulong snow mountain ridge (YL, 4193 m), respectively. n = 25 (2 × 2 m quadrat) for PJ1, PJ3, PJ4, CM2 and PY populations, n = 24 for CM2 and YL population, and n = 22 for PJ2 populations, respectively. Different letters indicate significant differences at the α < 0.05 level (one-way ANOVA with Tukey HSD tests).
Beneficiary plants sequentially intrude into cushion surfaces, culminating in the exclusion of individual cushion plants (Box 1). Specifically, in the early successional stage (represented by ‘primary community’; e.g. , PJ3), Kobresia vidua is the most abundant beneficiary species, followed by Saussurea leontodontoides, Sibbaldia purpurea and Polygonum macrophyllumand various others. However, when individual cushions have been completely excluded (leaving only dead traces of cushions), only the abundance of Kobresia vidua is higher and the abundances of all other species is lower (Figure 3). Subsequently, at an intermediate successional stage (represented by ‘balanced communities’; e.g. , PJ2), there is a different set of abundant beneficiary species, withS. leontodontoides, K. pygmaea , and Cyananthus macrocalyxoccupying most of the living cushion surfaces. At this stage, when individual cushions have been excluded, only S. leontodontoidesis more abundant, while the abundance of the other beneficiary species remains very similar (Figure 3). Finally, when the succession reaches the ‘climax community’ stage (relatively stable community; e.g. , PJ1), most of the vegetation area (including living cushion surfaces and surrounding habitats) is occupied by K. pygmaea followed by Hedysarum tanguticum andPotentilla saundersiana (Figure 3). The abundance of other species, such as P. saundersiana and P. macrophyllum , is only slightly lower following the exclusion of individual cushions. In summary, from primary to climax stages, the sequence of intruding beneficiary plants and the interactions between them and cushions both change. Kobresia vidua , S. leontodontoides, P. macrophyllum and S. purpurea may intrude first, followed by K. pygmaea, C. macrocalyx , P. saundersiana and then others. Finally, K. pygmaea will overwhelmingly dominate the communities once cushion plants completely disappear (Box 1; Figure 3).