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Ben-Gurion University of the Negev (2011)

Positive feedback interactions between annual plants and geomorphic processes in northern Negev semi-arid shrubland

Hoffman, Oren

Titre : Positive feedback interactions between annual plants and geomorphic processes in northern Negev semi-arid shrubland

Auteur : Hoffman, Oren

Université de soutenance : Ben-Gurion University of the Negev,

Grade : Master of Science (2011)

Interactions between vegetation and soil are key elements to ecosystem function in drylands, with roles in both spatial pattern formation and plant interspecific interactions. This research focuses on the relations between annual plant communities and shrub-dominated mounds of a shrubland in the semi-arid northern Negev. Ecosystem engineering through landscape modulation has been studies extensively in this area, focusing mainly on animals and woody plants, while regarding annual plant communities and productivity as response variables. Through the combination of several analytical methods, I evaluated the control strength of mound development and shrub state over annual plant community dynamics. A multiscale observation approach was used in an attempt to relate mechanisms acting at patch-scale and smaller (< 1m2 ) to patterns observable at the meso-scale of slopes (  100m2 ). Positive correlation between mound size and annual plant density and cover alludes to a positive feedback interaction between mound development and annual vegetation growth. A micro-scale (25 cm2 ) field experiment of sediment dispersal following natural rain events affirmed that the run-off reducing effect of surface cover by annual plants and litter was enhanced underneath a shrub canopy, due to the reduction of direct raindrop impact. By integrating the results obtained from this experiment with measurements of shrub-mound morphology, and comparing them to predictions derived from a computer simulation, I propose that annual plants are involved I a positive feedback loop between run-off and capture, and plant density growth. I further propose that the land modulation by annual plants affects mound morphology, an effect that has profound impacts on the rate and pattern of run-off in the landscape, and therefore also on landscape evolution and susceptibility to erosion. Recent changes in rainfall amount and pattern have given rise to a significant loss of shrub cover over most of the landscape, with high mortality rates of N. mucronata dwarf-shrubs. Due to the interactions between shrubs and annuals, and their combined geomorphic effect, such phenomena can have significant consequences for community and ecosystem processes. A combination of amongpatches community similarity analysis and within-patch incidence-abundance analysis has highlighted the effect of the recent changes on spatial community dynamics. At the patch-scale, shrub canopy loss acts as a disturbance, leading to community reorganization. At the watershed-scale, the combination of environmental stress and patch-scale disturbances has led to an abundance reduction or loss of several annual plant species. Results indicate a possible ecosystem state-shift, which might lead to a reduction in resources, productivity, and biodiversity, due to positive feedback processes.

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