Effects of tree species composition on within-forest distribution of understorey species

Question: Do tree species, with different litter qualities, affect the within-forest distribution of forest understorey species on intermediate to base-rich soils? Since habitat loss and fragmentation have caused ancient forest species to decline, those species are the main focus of this study. Location: Three ancient forests, along a soil gradient from acidification-sensitive to base-rich, were studied: Limbrichterbosch and Savelsbos in The Netherlands and Holtkrat in Denmark. Methods: Canopy and soil surveys along transects generated data for Redundancy Analysis on tree - humus relationships. We analysed the distribution of forest plant species with Canonical Correspondence Analysis. The explanatory factors were soil characteristics (pH, organic matter, loam content and thickness of the humus layers), external crown projection, groundwater and canopy data. We further analysed the relationship between forest species and humus characteristics with Spearman correlations. Results: Tree species have a significant impact on humus characteristics through the nature of their litter. Humus characteristics significantly explain the distribution of forest understorey species. The pH of the first 25 cm mineral soil and the thickness of the F- (fermentation) layer are the primary factors affecting the distribution of ancient forest species. Conclusion: This study indicates that the species composition of the forest canopy affects the distribution of forest understorey species. Ancient forest species are more abundant and frequent underneath trees with base-rich litter. On acidification-sensitive soils these relationships were stronger than on more base-rich, loamy soils.