Local effects of poplar plantations on ground beetle diversity (Coleoptera: Carabidae) in the Seine, Aube and Marne floodplains (France)

Hybrid poplar plantations are becoming an important component of forestry and agricultural systems, especially in alluvial floodplains, since poplars are easy to cultivate and are among the fastest-growing trees in the temperate region. However, these intensively managed woodland habitats have been accused of impoverishing biodiversity, including ground beetle diversity, as compared to other traditional land uses, such as grasslands and sub-natural forests. To assess the impact of poplar plantations on carabid diversity, we compared the assemblages found in seeded grasslands, poplar plantations and sub-natural forests in Champagne-Ardenne floodplains in North-eastern France. We hypothesized that poplar plantations could represent surrogate habitats for most of the carabid species encountered either in grasslands or forests. We expected that the role of surrogate habitat could depend on plantation age: young plantations hosting the species found in grasslands, while adult plantations sheltering those found in sub-natural forests. In the Champagne-Ardenne region, the three valleys of the Seine, Aube, Marne rivers were investigated using pitfall traps on a sampling design of 63 plots, stratified according to land use type and plantation age (12 seeded grasslands, 20 young and 20 adult poplar plantations, 11 adult forests), during three sampling periods. To test the above hypotheses, carabid data were analysed both on the assemblage level (species richness and composition) and on the individual species level (presence/absence). The cumulated species richness of ground beetles, rarefied on the number of sampling plots, did not vary significantly among habitat types. In contrast, the composition of carabid assemblages did show some differences among habitat types. The ordination by non-metric multidimensional scaling with the Bray-Curtis dissimilarity index revealed a gradient of canopy-closure, from seeded grasslands towards adult forests, young and adult plantations ranking successively at an intermediate position. The significance of the ANOSIM test on the dissimilarity matrix confirmed the effect of habitat type on the species composition of ground-beetle assemblages. Thus, to better understand the distribution patterns of each species, we identified which model best explained species occurrences, among the six alternative models: null model, habitat-type model, landuse-type model, soil-humidity model, tree-basal area model, eutrophisation model and landuse-history model. For each species, we ranked these six models according to the Akaike Information Criterion, modified for small sample sizes (AICc). Depending on the species, either tree basal area, habitat type or soil humidity proved to give the most parsimonious explanation of species occurrences. We then focused on the species responding in priority to land use type or to some indirect effects of land use (habitat type, canopy-closure, eutrophisation). To test the "surrogate habitat" hypothesis, we especially tested (i) whether some species were restricted to the reference habitats (either the seeded grasslands or the adult forests) as compared to the poplar plantations; (ii) whether some species were equally frequent in any kind of open habitat (seeded grasslands and young plantations), respectively closed habitats (adult plantations or forests) and (iii) whether some species were more frequent in poplar plantations. Such analyses are under finalization. Depending on the confirmation of the above expected results, we might conclude that poplar plantations could contribute to the conservation of ground-beetle biodiversity.