Plants and their communities not only make demands on the habitat, but also change it. Through qualitative and quantitative changes in nutrient availability, and modification of the meso- and micro-climate, species alter conditions, in some cases making them more favourable to themselves, but in others making them less favourable, and providing the environment required for other species to establish.
The climate within the community is particularly important (eco-climate). In well-structured communities, i.e. forests, the environment within the crown, between the tree trunks, and close to the soil surface can be very different. Even amounts of precipitation are modified via the surface roughness and interception in comparison with the condi tions outside the forest. Also, the influence of the wind can be reduced, or it creates adapted plant types in small-scale vegetation patterns that can be seen in dune complexes, for example (see Fig. 4.3.5). Even the variations in river valleys and river beds are not caused just by the physical runoff, the vegetation plays a part. Plants assist in biological weathering as well as in weathering of stony material, aiding soil formation, and are responsible for the formation of surface crusts especially in dry areas. Furthermore, some plant species are able to influence the site by producing organic substances which are difficult to degrade and allelochemicals; others (e.g. Legu-minosae) influence soil conditions, just as nitrogen fixation changes habitats by enrichment with nitrogen.
upper soil decreased, as did the soil pH, with increasing distance from the base of the stem. Detailed explanations of the effects of vegetation on soils, outlined here with a few examples, are given in Chapter 2.3.
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