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).