Sampling design
The two studied regions were 1600 ha and 1035 ha in size for the arid
and semi-arid regions, respectively. The HG and LG sites were of similar
size in both climatic regions. The distance between individual sampling
areas within each climatic region was less than one kilometer, which is
the least distance where we could identify plots with similar
characteristics. The HG and pairwise LG sites were relatively homogenous
in terms of topography, land use, and vegetation. The LG sites were
located within fences that have prevented grazing for around 35 years,
whereas HG sites were open and therefore have suffered long-term
overgrazing. Each plot was characterized by geographic coordinates and
altitude. In 2017, the number of individuals and percentage cover of all
vascular plant species was recorded between April and June, when the
growing season peaks in this region.
The decision about the grazing status of the sites (high grazing
intensity vs. occasional/low intensity grazing) was based on the median
number of dung droppings: 55.3 dung droppings per square meter in the HG
and 6.2 in the LG sites, and also on the width of the microterrace
livestock paths in a horizontal way (0.27±0.09 m for the HG site and
0.04±0.03 m for the LG site (see more information on the grazing history
in Table 1).
The sampling design was arranged in a hierarchical way: In each of the
two climatic regions (arid and semi-arid), we selected six sampling
areas, with a high grazed and a low grazed site in each sampling area,
arranged in a pairwise way (hereafter referred to as HG and LG sites).
Then,
we
sampled three plots under the A. kopetdaghensis shrubs and three
adjacent plots outside the canopy of A. kopetdaghensis (hereafter
referred to as under-canopy and open plots) in each HG as well as LG
site. Altogether, 144 plots were sampled: 2 climatic regions, 6 sampling
areas in each climatic region, one pairwise HG and one LG site in each
sampling area and 6 plots (3 under-canopy and 3 open) in each HG or LG
sites (see Figure 1). We recorded the numbers of individuals of all
vascular plant species and their percentage covers (as a proxy for
biomass) and then calculated the Shannon index of species diversity (H =
-∑si=1 pi ln pi) for each plot (Shannon, 1948); pi is the proportion
(n/N) of individuals of one particular species (n) divided by the total
number of individuals (N), and s is the number of species.
To obtain comparable samples for assessing species richness in the
surrounding ‘open’ plots (outside the canopy of A.
kopetdaghensis ), matching the size of each sampled A.
kopetdaghensis canopy, we sampled at haphazardly selected paired
points, located ~1 m away from the canopy edge of each
sampled A. kopetdaghensis shrub. When the size of A.
kopetdaghensis was not measured, a wire loop was shaped to match the
size of the sampled A. kopetdaghensis canopy plot and then used
to define the size of the patch sampled in the ‘open’ plot (Farzam &
Ejtehadi, 2017). Again, all established plant abundance present on these
open plots were recorded and identified to the species level.