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York University (2016)

Direct and Indirect Consequences of Dominant Plants in Arid Environments.

Sotomayor Melo, Diego Alejandro

Titre : Direct and Indirect Consequences of Dominant Plants in Arid Environments.

Auteur : Sotomayor Melo, Diego Alejandro

Université de soutenance : York University

Grade : Doctor of Philosophy (PhD) Geography 2016

Résumé
In arid environments, dominant woody plants such as shrubs or trees, usually facilitate a high density of species in their understories. This phenomemon is composed by a series of direct and indirect effects from the dominant plant to the understory species, and among understory species. The aim of this project was to determine these direct and indirect consequences of dominant plant-plant facilitation in a collection of field sites along the coastal Atacama Desert. The following objectives and hypotheses were examined in this project : (1) to summarize and contextualize the breadth of research on indirect interactions in terrestrial plant communities ; (2) that the positive effects of dominant plants on understory communities are spatiotemporally scale dependent, from micro- to broad-scale spatial effects, and from within-seasonal to among-year temporal effects ; (3) that dominant plants via their different traits determine the outcome of plant-plant interactions ; (4) that dominant plants determine the outcome of interactions amongst understory species and that their responses are species-specific ; and (5) that facilitation by dominant plants generates sufficiently different micro-environmental conditions that lead to consistent differences in seeds traits of understory plants. Overall, we found that multiple factors determine the outcome of plant-plant interactions along the field sites studied in this project. These factors impact both the direct and indirect effects of dominant woody plants on their understory communities and include species-specific traits of both the dominant and understory species, and the spatial and temporal environmental gradients that manifest their effects at different scales. Dominant plants usually facilitate increased species richness and density of plants in their understory, that in turn mediates effects amongst these species. However, these direct effects seem to have a limit given that at extremely stressful environmental conditions they tend to change to neutral and even competitive effects of canopies on their understories. This provides evidence that positive effects of dominant plants collapse under extreme spatiotemporal stress. Although we did not find evidence of evolutionary effects of top-down facilitation, the methodology proposed here represents a contribution to test the conditions under which these results hold. Overall, this project illustrates the importance of understanding the multiple drivers that determine the outcome of biotic interactions.

Mots Clés : Annual plants ; Atacama Desert ; Coexistence ; Community ecology ; Ecotypic differentiation ; Facilitation ; Nurse plants ; Ecological gradients ; Indirect interactions ; Stress gradient hypothesis ; Species specific ; Spatial gradients ; Temporal gradients

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