French landscapes are submitted to increasing anthropogenic pressures. These pressures can affect plant species diversity by modifying the quality and spatial configuration of habitats, including forest habitats. This project addressed the role of the composition and configuration of the forest and non forest landscape mosaic on forest plant diversity. It was based on 1932 plots from the National Forest Inventory located in 19 "départements" of Northern France. Landscape indices such as fragmentation were measured or calculated from aerial photographs and existing national forest maps. A specific field study was devoted to analyse the depth of the effect of a particular linear landscape feature: the forest road. Landscape patterns had a significant impact in addition to local habitat quality. Plant composition was influenced by three main landscape gradients: (1) forest or non-forest habitat, (2) presence of fellings and young stands and (3) species composition (broadleaves or coniferous). The openings in the surrounding landscape (agricultural lands, forest felling and young stands) had an effect on plants through their quantity, number of patches and edge length. This effect was positive for most species, which confirms the role of the openings in maintaining plant diversity. The species traits discriminated were epi-zoochory and anemochory, mass x length of seeds and some Grime adaptive strategies. This work also underlined the importance to analyse landscape pattern effect within a buffer zone of more than 1 km around the plots. In contrast, we found that forest roads had a short depth of edge influence on plants (< 5 m) and road effect was more often positive than negative. The mechanisms underlying the influence of landscape mosaic are discussed in light of a temporal effect on plants. The implications for habitat and forest management to protect biodiversity are presented.
