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Prédiction de propriétés agroécologiques de prairies permanentes et de leurs compromis : l’exemple du massif vosgien

Abstract : Grasslands play a key role in global agrosystems. In France, permanent grasslands are associated with agronomic and ecological characteristics: they provide half of overall forage, shelter vegetal and animal species, and store carbon. Increasing our understanding of agroecological characteristic determinants, and the synergies and trade-offs between characteristics, could help farmers and advisors to promote high-diversity grasslands, but also a diversity of grasslands. The objectives of this thesis are 1) to predict grassland characteristics using environmental, agricultural practices and vegetation criteria, 2) to predict grassland characteristics using vegetation classifications without information about environment and agricultural practices, 3) to study and predict trade-offs between characteristics at grassland scale, 4) to query knowledge transfer between researchers, farmers and farmer advisors. For this purpose, I run statistical analyses on a database of almost 800 permanent grasslands from previous studies. I then selected a representative sample of 59 grasslands for this database over which I conducted field and lab analyses of botanical compositions, yields, forage qualities, antioxidant contents and soil properties. I also collected information about agricultural practices, climate and topography for each of these grasslands. My results show that botanical compositions are mainly influenced by agricultural intensification, soil and elevation gradients, and that knowledge about agricultural practices are dispensable because already correlated to other criteria. However, despite I used several criteria, the quality of botanical composition prediction remains weak. Prediction of agroecological characteristics show wide variabilities: some agronomicaland ecological- characteristics are predicted well by soil, climate, landscape and botanical composition (species presence and abundance, vegetation classes) criteria. The agronomic characteristics that are predicted well comprise pastoral value, metabolisable energy, and assimilable proteins; the ecological characteristics predicted are Shannon index, richness in oligotrophic species, nectar production index and soil carbon stock. However, using only vegetation classifications could not reliably predict ecological characteristics, despite the improvement of prediction quality when combining classifications. Study of synergies and trade-offs highlighted the impossibility to combine all the agroecological characteristics for one grassland. However, I observed synergies between yield and botanical diversity, between the different indices of nutritive value, and between patrimonial species and flexibility of management. Finally, all studied tools can be used to transfer scientific knowledge to farmers and their advisors, but need to be carefully chosen to match their individual expectations. In particular, the definition of forage quality greatly varies between farmers (i.e., it can be based on quantity or quality), and a difficult equilibrium between tools accuracy and ease to use have to be found. This thesis work brings new insights in our understanding of permanent grassland agroecological characteristics and their trade-offs, thanks to the inclusion of many predictive criteria related to environment, agricultural practices and vegetation, but also thanks to the prediction of unknown characteristics. Finally, this thesis addresses the issue of developing polyvalent tools that can be used to predict grassland agroecological characteristics.
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Geoffrey Mesbahi. Prédiction de propriétés agroécologiques de prairies permanentes et de leurs compromis : l’exemple du massif vosgien. Agronomie. Université de Lorraine, 2020. Français. ⟨NNT : 2020LORR0086⟩. ⟨tel-02958612⟩

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