Tree species diversity and soil patchiness in a temperate broad-leaved forest with limited rooting space

Abstract

Summary In natural forest communities of Central Europe, Fagus sylvatica predominates in the tree layer under a wide range of site conditions due to a remarkable competitive ability. An exception are skeleton-rich soils such as screes where several other broad-leaved tree species co-exist with, or even replace, F. sylvatica . Little is known about the mechanisms that lead to the stable co-existence of 4–6 or more tree species on these sites. In a Tilia-Fagus-Fraxinus-Acer-Ulmus forest on a scree (stone content 57 vol. %), we analysed the above- and below-ground biomass structure, and the horizontal distribution of the tree fine root systems. Fine roots were identified on the species level by means of morphological attributes. In the highly patchy soil we assessed the evidence for below-ground habitat partitioning which might promote species co-existence. Tilia platyphyllos accounted for 46% of the stems and 40% of the total leaf area, whereas F. sylvatica reached 42% of the leaf area with a much lower stem density (24% of the stand total). T. platyphyllos was the dominant species in the rhizosphere with 56% of the total fine root biomass in the topsoil (0–10 cm depth). F. sylvatica contributed only 19% to the fine root biomass despite its extensive leaf area. The other 4 species held further 18% of the leaf area and 25% of the fine root biomass. The fine root systems of the trees showed stem-centred distribution patterns with low root densities at distances > 5.5 m, and a remarkably small overlap among the co-existing species. In 22% of all soil samples we found no tree roots at all; roots of only one species were present in 62% of the samples, roots of two or more species were confined to only 16% of the collective. Compared to other Central European mono-specific F. sylvatica forests with a less restricted soil volume, the scree forest was characterised by a low average fine root density, a high proportion of root-free soil patches and very high local fine root densities. Our results indicate a spatial segregation of the fine root systems of neighbouring trees due to a high stone content which may reduce root competition in this mixed stand. We suggest that partitioning of below-ground rooting space due to physical barriers is a mechanism that promotes tree species diversity in Central European forests on skeleton-rich and patchy soils.