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Variation de l’intensité de la facilitation avec la salinité et l’aridité dans les dépressions salines continentales de Tunisie.

Abstract : We aim to clarify the debate of the community ecology literature on changes in plant-plant interaction along water stress gradients. We focused on the importance of the type of stress (resources vs. non-resources) and the method used (observational vs. experimental) to quantify biotic interactions. The continental saline depressions (Sebkhas) of Tunisia, characterized by the existence of a strong gradient of salinity in contrasting climatic conditions, is an excellent model system to clarify this debate, in particular by separating the effects of salinity from water stress.In a first step, we carried out an observational study on the scale of the Tunisian arid zone in order to identify the consequences of the possible interactions existing between topography and climate and determining variations in salinity and water stress and ultimately the structure, composition of plant communities and ecosystem functioning. We set up a field design combining a topographic position treatment (with four habitats) and a water stress treatment (with two climate conditions: the wet arid climate and the dry arid climate) with four replicates at the regional scale. We also carried out two experiments, one on the effect of neighbors using the observational method with transplants of three dominant species in all treatments and a second on the effect of the method (observational vs. experimental) with transplantation of three Poaceae in the two least saline habitats.The observational study clearly showed that salinity was the major direct factor determining the composition, diversity and structure of communities, as well as the productivity of ecosystems and their fertility. Aridity has less, more subtle effects, which can only be spatially compared in the least saline habitat between the low and high arid climates.The first experimental study showed that salinity stress (a direct, non-resource factor) induced a facilitation collapse that increased with increasing aridity, both temporally and spatially. The switch to competition in extreme conditions of aridity predicted in the literature was not found. This is very likely due to the weakly competitive strategy of Chamaephytes dominating the most saline habitats. The second experimental study showed that, before the dry summer season, the short-term interactions measured by the experimental method (removal method) were negative whereas the long-term interactions, measured by the combination of the observational and experimental methods were positive, thus, supporting our hypotheses and the literature. However, after the dry summer season, the short-term interactions turned to positive, whereas the long-term interactions collapsed due to vanishing the buffering effect of the vegetated soil on humidity.These results are crucial because they showed that in this arid and saline system, the collapse of facilitation is the major process explaining the structure, diversity and functioning of ecosystems, both along salinity gradients and when water stress increases. On the other hand, the combination of the observational and experimental methods was crucial to show that the increase in facilitation predicted by SGH with increasing water stress is only a response of plants to the increasing negative effect of abiotic environment (environmental-severity effect). This contrasts with the increased competition in unstressed environments and increased facilitation from highly stressed to intermediate environments, both being neighbor-trait effects.
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https://tel.archives-ouvertes.fr/tel-03153406
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Submitted on : Friday, February 26, 2021 - 12:28:08 PM
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Ghassen Chaieb. Variation de l’intensité de la facilitation avec la salinité et l’aridité dans les dépressions salines continentales de Tunisie.. Ecosystèmes. Université de Bordeaux; Faculté des sciences de Bizerte (Tunisie), 2020. Français. ⟨NNT : 2020BORD0284⟩. ⟨tel-03153406⟩

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