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

Ramiro Pablo López, Francisco A. Squeo, Cristina Armas, Douglas A. Kelt, Julio R. Gutiérrez

Research output: Contribution to journalArticlepeer-review

29 Scopus citations

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.

Original languageEnglish
Pages (from-to)1593-1604
Number of pages12
JournalEcology
Volume97
Issue number6
DOIs
StatePublished - 1 Jun 2016
Externally publishedYes

Bibliographical note

Funding Information:
The Instituto de Ecologia y Biodiversidad (IEB), CONICYT (Consejo Nacional de Ciencia y Tecnologia)/Beca Doctorado 2013/Folio 63130048 and FONDECYT 1110228 funded the study.

Publisher Copyright:
© 2016 by the Ecological Society of America.

Keywords

  • Aridity gradient
  • C-score
  • Deserts
  • Facilitation
  • Plant-plant interactions
  • Spatial patterns
  • Stress gradient hypothesis

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