Discussion
Shrub
canopy mediated abiotic and biotic stresses
Changes
in relative interaction intensity (RII) indicate changes in interaction
type along a combined gradient of biotic and abiotic stress. For all of
the three RII indices (cover, richness, and Shannon diversity), there
was a decreasing trend from the highest towards the lowest stress levels
(Figure 2).
The
shrub (A. kopetdaghensis ) showed facilitative effects, preserving
species diversity and richness as well as the total cover of species
under its canopy. However, the facilitative effect was significantly
stronger in the drier climatic region. Previous researchers (Bertness &
Callaway, 1994; Brooker & Callaghan, 1998; Butterfield, Bradford,
Armas, Prieto & Pugnaire, 2016) have documented increases in the
facilitation effect of the shrub by moderating the aridity stress. In
arid environments, facilitation usually involves increasing the water
and nutrient availability (Claus Holzapfel & Mahall, 1999). Besides
that, the shade from the shrub reduces extreme temperatures and
decreases evaporation from the soil, which may further facilitate the
germination of seeds and growth of seedlings. Therefore, this may
explain why the shrub shows higher facilitation in the arid than in the
semi-arid regions (Smit et al., 2007; Tirado et al., 2015; Farzam &
Ejtehadi,
2017).
The
effect of A. kopetdaghensis canopy was consistently facilitative
under intensive grazing. As A. kopetdaghensis is unpalatable, it
is not usually grazed by livestock during the growing season. Therefore,
it provides mechanical refugee for palatable grasses and forbs (reviewed
by Milchunas & Noy-Meir 2002; Baraza et al., 2006; Graff et al., 2007;
Holthuijzen & Veblen,
2015).
This result is consistent with the “repellent plant hypothesis”,
suggesting that grazing intolerant plants are protected by the
surrounding grazing tolerant plants (Milchunas and Noy-Meir 2002).