Indicator species response to plant interactions in the condition of stress
This study shows that co-occurring plant species under the shrub canopy may exhibit convergence in CSR plant life strategies in the conditions of similar levels of stress, while different stress levels lead to functional divergence. For instance, in the arid region, the dominant strategy of indicator species under A. kopetdaghensisconverged to S-R in the HG site. At the same time, species under the shrub’s canopy exhibited transition from S-R to R-selected in low grazing intensity. This is likely because important drivers of vegetation structure, such as disturbance (grazing) and stress (aridity), cause the loss of biomass and redistribution of resources (Caccianiga, Luzzaro, Pierce, Ceriani & Cerabolini, 2006), S-selected species prevail under the canopy of Artemisia . However, on the LG sites in the arid region, the canopy protects the surrounding plants from aridity only, so the stress is less intensive than on the HG site. Therefore, most of the indicator species under the shrub’s canopy were annual forbs and grasses with R strategy on the LG site. Plants with R‐selected strategy absorb nutrients more rapidly, grow faster, have shorter growth periods and invest more into reproduction (Grime, 1977; Caccianiga et al., 2006; Wang et al., 2018). Stress intolerant species were better candidates for facilitation than stress-tolerant species (Graff & Aguiar 2011). For instance, in the arid conditions of Mediterranean shrublands, the stress avoidant species, with high specific leaf area and rapid growth, coexist with species featured by very low specific leaf area (Gross et al., 2013).
In the semi-arid region, A. kopetdaghensis canopy showed a facilitative effect on the HG sites, supporting the establishment of species with C-selected strategy, like Elymus hispidus orLactuca orientalis , which have larger leaves and are generally more palatable to livestock. This is mainly because A. kopetdaghensis is an unpalatable, stress-tolerant shrub, and its canopy creates microsites, protecting other species against grazing by large herbivores. In contrast, A. kopetdaghensis canopy has a competitive or neutral effect on the perennial stress-tolerant species on the LG sites in the semi-arid region. A. kopetdaghensis shrubs have a neutral effect or even compete with the under-storey species for light and nutrients. Therefore, the dominant strategy of indicator species under A. kopetdaghensis in HG site shifted from C- to S-selected in LG site.
On the contrary, we found only a few indicator species in the open plots in both the arid and semi-arid regions. Poa bulbusa was present in the HG sites of the arid region, Aegilops triuncialis andAchillea biberesteinii were on the LG sites of the semi-arid region (Table 3). As suggested by Grime (1977), when the disturbance is relatively low, species with S-strategy can maintain their dominance in a community by occupying aboveground and belowground space rather than by competing for resources.