Relative interaction index along the stress gradients
Changes in type and/or intensity of plant-plant interaction along the stress gradients have been one of the most discussed issues in plant ecology in recent decades (Maestre, Callaway, Valladares, & Lortie, 2009). There are contrasting reports on this in literature, as some researchers conclude that the amelioration of abiotic stress was more important than protection from grazing (Howard et al., 2012; Arroyo et al., 2015). Other studies demonstrated that grazing was a more important driver of the plant-plant interactions than abiotic stress in the African savanna (Louthan et al., 2014; Filazzola et al., 2017).
In the arid region, strong facilitation was observed in both grazing intensities, which suggests that the protection against aridity is more important than protection from intensive grazing (Maestre et al., 2005; Soliveres et al., 2011). Accordingly, a theory by Smit et al. (2009) predicts relatively low importance of protection from grazing in water-limited environments. In arid climates, herbivores are sparsely distributed, and the availability of water or nutrients is more critical for vegetation than protection from grazing (Ellis & Swift, 1988). In water-limited environments, the shrubs usually improve soil fertility and microclimate under their canopies (Cortina & Maestre, 2005, Maestre et al., 2009). Also, shade from shrubs’ and trees’ canopy can retain soil moisture at the soil surface and facilitate neighbours with shallower roots (Maestre, Bautista & Cortina, 2003). Therefore, the dominant shrub may promote species richness and productivity by providing safe microsites for species growing in harsh conditions (Bruno et al., 2003).
On the other hand, in the semi-arid region, where plants presumably grow in higher water availability, livestock grazing played a critical role in determining the type and relative intensity of the shrub’s interaction with under-canopy species. The effect of the shrub’s canopy (RII) was positive on the HG sites, but turned into neutral or even negative with lower livestock grazing intensity. A negative RII means that herbs prefer to grow in the open areas rather than under the canopy of shrubs, where they need to compete for light, nutrients and water (Graff et al., 2007; Le Bagousse-Pinguet et al., 2012).