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).