Enhanced facilitation at the extreme end of the aridity gradient in the Atacama Desert: a community‐level approach

Abstract

Plant facilitation is now recognized as an important process in severe environments. However, there is still no agreement on how facilitation changes as conditions become increasingly severe. The classic stress gradient hypothesis (SGH) predicts a monotonic increase in facilitation, which rises in frequency as conditions approach the extreme end of the environmental gradient. However, few studies have evaluated the validity of the SGH at the community level, the level at which it was formulated. Moreover, few studies have tested the SGH at either extreme of the gradient, and very few have excluded the effect of livestock on community response to stress. In line with the SGH, we hypothesized that several spatial pattern summary statistics would change monotonically from the least to the most arid sites, indicating increasingly aggregated patterns. In this study, we performed an evaluation of the SGH both within communities of shrub species and across a large portion of the Atacama Desert, and we isolated the abiotic component of the SGH. Our environmental gradient covered an extreme aridity gradient (< 20-130 mm annual precipitation). To perform point pattern analysis, we established 13 sites with environmental conditions representing four distinct levels of this gradient. Further, we conducted species co-occurrence analyses at 19 sites along the gradient. Both sets of analyses showed stronger positive spatial associations among plants at the most extreme end of the gradient. This was true regardless of whether we included all individuals, only small individuals located around large ones, or individuals in species pairs. Moreover, species tended to show greater co-occurrence as environmental severity increased. This increase in aggregation in the plant community seems to correlate with an increase in the strength of positive interspecific interactions, rather than greater clustering within each species. These monotonic increases in species co-occurrence and spatial association in more severe environments are consistent with some of the predictions of SGH, and collectively these results suggest that as the climate becomes more arid, positive species pairs interactions tend to be prevalent in the community.

Functionally or phylogenetically distinct neighbours turn antagonism among decomposing litter species into synergy

“…Low WHC may be representative of a resource‐conservative strategy as we find it in stressful environments (Wright et al., ). It has been suggested that facilitation is particularly important in such environments (López, Squeo, Armas, Kelt, & Gutiérrez, ; Maestre, Callaway, Valladares, & Lortie, ). The observed synergy in decomposition might be another form of facilitation, which has not been recognized so far: synergistic decomposition might indeed increase nutrient availability and benefit to both species providing the mixed litter.…”

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

“…As suggested by a variety of ecologists (Butterfield 2009, Armas et al 2011, He et al 2013, Michalet et al 2014, Soliveres et al 2015, Liancourt et al 2017, the apparently opposing views of the original and recent versions of the SGH may stem from inter-study differences in the nature of stress gradients (resource or non-resource stress), life-history of response species (stress tolerant or competitively inferior), scale of study focus (community or species level), component of stress factors (single or multiple stresses), or whether the observed gradient is complete. Even though a large number of studies from various ecosystems have explored the SGH, empirical tests of the hypothesis in extreme environments remains insufficient (but see Armas et al 2011, de Bello et al 2011, Castanho et al 2015, Lopez et al 2016, Liancourt et al 2017.…”

Non‐hierarchical competition among co‐occurring woody seedlings in a resource‐limited environment

“…) may alter the frequency and strength of facilitation among plants (Malkinson and Jeltsch , López et al. ). This is important because we have observed (1) size‐specific differences in drought tolerance, for Ambrosia in particular (Miriti et al.…”