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Hétérogénéité de la végétation et du pâturage : conséquences fonctionnelles en praireis naturelle

Abstract : Grazing influence the primary productivity and the quality of the vegetation and can influence the fluxes of elements and the biogeochemical cycles. The consequences of this interaction between herbivores, plants and soil on the functioning of the ecosystems could vary because the impact of grazing on the functions of the vegetation is depending on the plant communities and the grazing intensity. The objective of this thesis is to analyze the impact of a grazing intensity gradient on the functioning of a humid grassland. The biological model of this study is a grassland of the Marais Poitevin (France) which is grazed since the 10th century. Three plant communities occur in this grassland and the spatial variations of grazing intensity led to the creation of a patchwork of plant patches within each plant community. Within each plant community, the grazing intensity gradient led to a spatial heterogeneity of the functional processes of the grassland linked to the heterogeneity of the vegetation. Along the grazing gradient, primary productivity decreases while the quality of the vegetation increases and the net N mineralisation rate increases. Within each plant community, the range of the nitrogen cycle variations appears to be linked to the range of the variations of the vegetation functions. The impact of grazing on the vegetation functions was driven by two main mechanisms. The long-term effect of grazing influences the floristic composition of the plant communities and thus influences the vegetation functions. The grazing gradient led to a wide modification of the floristic composition of the plant communities. The effect of this grazing gradient on the vegetation functions was more important when the dominant plant functional types in ungrazed conditions, perennial monocots, was replaced by annual monocots and dicots. In grazed situation, the functions of the vegetation were influenced by the plants' responses to the defoliation. Defoliation stimulated the growth rate of the vegetation. The intensity of this stimulation was strongest with the low frequency of defoliation and allowed a compensatory growth which differ amongst the patches. This response was characteristic of each plant patch. The interaction between defoliation frequency and floristic composition led, along the grazing gradient, to a more important decrease of primary productivity within the meso-hygrophilous plant community than within the mesophilous one. The impact of the grazing gradient on the functions of the vegetation drives the quantity and the quality of the inputs of organic matter through the soil and leads to an effect on the net N mineralisation rate. The quantity of carbon inputs appeared to control the soil microbial growth and to stimulate the nitrogen microbial immobilisation which led to a decrease of the mineral nitrogen pool available for the plants. Consequently, the reduction of plant litter inputs along the grazing gradient led to a stimulation of the net N mineralisation rate. The stability or the change of the functions of the vegetation, in terms of primary productivity and vegetation quality, along the grazing gradient appears to drive the impact of grazing on the biogeochemical cycles.
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Submitted on : Friday, September 24, 2010 - 1:55:01 PM
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  • HAL Id : tel-00520859, version 1

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Nicolas Rossignol. Hétérogénéité de la végétation et du pâturage : conséquences fonctionnelles en praireis naturelle. Ecologie, Environnement. Université Rennes 1, 2006. Français. ⟨NNT : ⟩. ⟨tel-00520859⟩

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