Influences of Vegetation on Soils

Influences of vegetation on the fertility and water relations of soils belong to the most important biotic-abiotic interactions in ecosystems. Earlier discussion of primary successions emphasised the contribution of pioneer plants in providing the first organic litter in the formation of soils and, thus, in preparing conditions at a site for more demanding plants. The quality and amount of organic litter are very important for pedogenic processes and the characteristics of soils. This litter determines the C/N ratio of soils (especially the A-horizons) as well as possibly starts or increases soil acidification.

Plants also influence the input of nitrogen and other nutrients (as well as pollutants) either directly through N-fixation (Leguminosae) or indirectly through their effects on atmospheric nutrient deposition. Rode et al. (1996) showed how diverse these inputs can be depending on the species of trees and shrubs at a given site. Input is high in birch-pine forests, mainly owing to the roughness of these stands and the presence of needles on pines in winter. Interception is three to ten times higher than for oak and beech forests. That individual tree species are able to determine the chemical characteristics of soils was proved by Reich et al. (2005) using 30-year old plantations of 14 different tree species. Especially those species that have high calcium concentrations in their litter increase soil pH, exchangeable calcium, per cent base saturation and forest floor turnover rate. In addition, such species also favour earthworm diversity and abundance.

One result of current experimental studies on biodiversity shows that increasing plant species diversity increases availability of N in the soil (Dybzinski et al. 2008). An important reason for this higher N (and C) accumulation is most probably the complementarity of species belonging to different plant functional groups such as legumes and C4 grasses (Fornara and Tilman 2008) by increasing the supply of nutrients via both greater inputs and greater retention (Sect. 20.4). Not only the content of certain nutrients but particularly their availability are closely linked to the organic material in soil. However, the accumulation of allelochemicals and organic substances that are difficult to degrade may render a site unsuitable for particular plant species.

Vegetation affects not only the chemistry of soils but also their physical characteristics. Depending on the rooting systems of plant species and on the type of litter, soil structures are formed that allow aeration of the soil and contribute to the transformation of an abiotic substrate into a habitat for a very wide range of organisms in the soil (edaphon).

The structural characteristics of soils and their content of organic substances brought about by vegetation are closely linked to the water relations of the soil. A dense plant cover not only protects against erosion but may also increase water retention, contributing to more balanced run-off, and may limit flooding over large areas.