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Régimes de pâturage et hétérogénéité de la structure et du fonctionnement de la végétation prairiale (Marais Poitevin)

Abstract : Spatial heterogeneity of the vegetation is one of the major feature in understanding ecosystems functions. This work deals with the role of grazing by livestock on creating vegetation heterogeneity and the mechanisms leading to plant mosaics. Consequences on some functional edaphic parameters are also studied. The “common grasslands” in the Marais Poitevin (France) are the biological model of this study; mixed grazing (by bovids and equids) traditionally occurred since the 10th century on these commons, where three plant communities co-occurred on a topographical gradient of inundation and salinity. Selective grazing by the two herbivore species, cows and horses, was found at two levels : (1) at the level of the three communities by preferential selection of some, preferences whether or not the same; (2) at the community level. Selective grazing leads to patchy vegetation, especially of different grazing lawns and taller vegetation patches. Tall patches with horse and mixed grazing are' horse latrine areas'. Number, composition and surface area of patches was found to be controlled of herbivore species (cows, horses or both mixed) and stocking rate. Horse and mixed grazing create the most heterogeneous grasslands. Each vegetation patch is defined by its structure (canopy height, bare soil cover), floristic composition and plant diversity. With horses alone, the less diversified patches are the latrine areas, dominated by the perennials Elymus repens or Agrostis stolonifera depend on the community. With mixed grazing, dominance of these two species is limited by heifers that grazed on horse latrine areas, thus the two herbivores show compensatory effects. On the grazing lawns, the two herbivores show additive effects. At the level of enclosure, the most heterogeneous grazing treatments lead to the highest plant diversity. Herbivores created also a functional heterogeneity, studied here for some parameters : aboveground plant biomass, thickness of the soil organic layer, soil organic content, soil organic and mineral nitrogen content and electrical conductivity. The soil of the community situated on intermediate slopes, where a sub-halophytic community is developed, shows the most important heterogeneity of soil parameters : different grazing intensity leads to large differences in soil mineral nitrogen contents and electrical conductivity. The formation of the plant mosaics in sub-halophytic community depends on the respective effects of the different components of grazing (defoliation, trampling, bare soil creation) experimentally tested. Vegetation response shows different floristic compositions which are similar to those observed in grazed conditions. Some of the sub-halophytic vegetation patches were temporary excluded of grazing and showed contrasted dynamics of reversibility. Maintenance of plant diversity associated to the persistence of these patches seems to be grandly dependant of the permanence of grazing.
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Submitted on : Friday, September 24, 2010 - 1:39:11 PM
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  • HAL Id : tel-00520850, version 1

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Gregory Loucougaray. Régimes de pâturage et hétérogénéité de la structure et du fonctionnement de la végétation prairiale (Marais Poitevin). Ecologie, Environnement. Université Rennes 1, 2003. Français. ⟨tel-00520850⟩

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