In intensive agricultural landscapes, plant species previously relying on semi-natural habitats may persist as metapopulations within landscape linear elements. Maintenance of populations' connectivity through pollen and seed dispersal is a key factor in species persistence in the face of substantial habitat loss. The goals of this study were to investigate the potential corridor role of ditches and to identify the landscape components that significantly impact patterns of gene flow among remnant populations. Using microsatellite loci, we explored the spatial genetic structure of two hydrochorous wetland plants exhibiting contrasting local abundance and different habitat requirements: the rare and regionally protected Oenanthe aquatica and the more commonly distributed Lycopus europaeus, in an 83 km 2 agricultural lowland located in northern France. Both species exhibited a significant spatial genetic structure, along with substantial levels of genetic differentiation, especially for L. europaeus, which also expressed high levels of inbreeding. Isolation-by-distance analysis revealed enhanced gene flow along ditches, indicating their key role in effective seed and pollen dispersal. Our data also suggested that the configuration of the ditch network and the landscape elements significantly affected population genetic structure, with (i) species-specific scale effects on the genetic neighborhood and (ii) detrimental impact of human ditch management on genetic diversity, especially for O. aquatica. Altogether, these findings highlighted the key role of ditches in the maintenance of plant biodiversity in intensive agricultural landscapes with few remnant wetland habitats.

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International audienceContext Studying communities using a trait-based approach has contributed to major advances in the understanding of community assembly mechanisms, but research has primarily focused on the effect of local biotic and abiotic processes on plant assemblages. Objectives At the landscape level, we expect that the diversity of trait values (i.e. functional diversity) in plant assemblages is not random because there might be strong environmental constraints occurring as a filter of plant species at this level. We expect that fragmentation, through connectivity loss, may reduce functional diversity by filtering plant dispersal traits within the global pool of species toward those that present the trait values that are the most adapted. We tested this hypothesis at the metacommunity level and focused on ditch plant dispersal traits. Methods We characterised the functional diversity of 27 ditch bank metacommunities for three seed traits encompassing the whole dispersal process: production, buoyancy and mass. We computed four connectivity variables based on the ditch networks surrounding each metacommunity. Null models were designed to quantify functional convergence (i.e. convergence of the trait values around a mean) or divergence for each trait (i.e. dissimilarity in plant trait values). Results At the metacommunity level, we demonstrate that (1) connectivity loss reduces the functional diversity of dispersal traits (seed production and seed buoyancy), leading to the convergence of trait values, whereas more connected landscapes promote randomness, or even divergence (seed mass) in trait values; (2) these changes are due to the modification in the number of occurrence of plant species over the nine local communities sampled, rather than in species identity. Conclusions Our results indicated that, at the metacommunity level, reduced connectivity acts as a filter on the dispersal traits of plant species, promoting the broad distribution of species with efficient dispersal abilities in such landscapes. Thus, functional diversity helps us to understand the mechanisms underlying the effects of fragmentation on biodiversity