Comparison of the Regeneration Patterns of Woody Species between Norway Spruce Plantations and Deciduous Forests on Alluvial Soils

— Restoration of floodplain forests has become a primary aim of sustainable forestry in Europe. Although several studies have investigated the conversion of coniferous stands outside of their biogeographical area and on unsuitable sites, none focused specifically on alluvial areas. We surveyed 208 riverine stands, distributed homogeneously throughout the Grand-duché de Luxembourg (western-central Europe) for the abundance of woody forest species (trees, saplings, seedlings and shrubs). The influence of both the canopy nature (Norway spruce or deciduous) and the biogeographical region (Oesling or Gutland) on the understorey regeneration were tested. The mean species richness was 10.5 woody plants per stand. Correspondence analyses showed that canopy tree regeneration was not different between the two forest types. Following an analysis of variance, we found that functional species group distribution (regarding mode of dispersal, type of seed bank and establishment strategy) was more determined by the biogeographical variable than by the canopy variable. Most woody species were significantly more abundant in deciduous forests and in the Gutland area. Only Rubus idaeus, Sambucus racemosa and Sorbus aucuparia were strongly associated with Norway spruce stands or the Oesling region. The differences between deciduous forests and coniferous stands are mainly due to shrub abundance and not to the canopy tree regeneration. In conclusion, Norway spruce plantations act as nurseries for late-successional riverine species like Fraxinus excelsior, Acer pseudoplatanus and Quercus robur.