In the large European river floodplains, closely related species segregate between habitats within numerous forest communities whose structure and dynamics are shaped by the river and its tributaries. Factors of segregation are mainly geochemistry (lime content and texture), water stress and fluvial dynamics. When sympatric, the closely related species coexist by niche differentiation (life-form, place in the successional stages, phenology and abundance).
We have made a comparative study of the niches of 70 species belonging to 24 genera in the alluvial forest communities of the upper Rhine valley (France), using a descriptive and qualitative approach. Results were compared with the literature on European biogeography. Particular attention was paid to seven woody genera (Populus, Salix, Ulmus, AInus, Acer, Crataegus and Primus) and to the behaviour of hybrids along the environmental gradients of the study area.