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.