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Influences of Vegetation on Weathering and Formation of Relief

Even though climatic and tectonic factors are most important for geomorphological processes, influences of organisms on the formation of relief and thus for changing areas must not be overlooked. Viles (1988) distinguishes, in his volume on biogeomorphology, between active and passive relations between organisms and the site factors which depend on relief. The former are biogenic weathering, formation of biogenic sediments (e.g. lime deposits), bioturbation (e.g. burrowing animals) and also bioerosion (e.g. from grazing animals). Passive relations, from interrelations between components of the vegetation, can be seen in dune formation and in accumulation of organic matter (e.g. in bogs or as a consequence of damming rivers). These interactions will be explained with some examples (see also Chap. 2.3).

Weathering or formation of soils was generally interpreted as an exogenous, climatic and physicochemically regulated process. Today, organisms are regarded as playing an important role in weathering in all climatic zones of the earth. For example, cyanobacteria participate in the formation of desert varnish, a covering of iron and manganese compounds - thus securing their own habitat with a protective crust. The metabolic processes of lichens, blue-green algae and fungi result in release of acids and chelating compounds which, under humid conditions, dissolve some components in rocks either completely (as with limestone) or selectively dissolve and remove the mineral components. Lichens, in particular those living within rocks (endolithic: types such as chasmoendoliths, euendoliths and cryptoendoliths) and also those living on the surface (epilithic), influence weathering in all climatic conditions (Belnap and Lange 2001). Under moist conditions, crusted rocky surfaces

Potential dune forest

White dunes Primary dunes intertidai zone

Sandy beach

Grey dunes

Fig. 4.3.5. Sequence of habitats in coastal regions with dune formation. (After Ellenberg 1996)

Brown dunes Grey dunes carbonate-rich sand ___ Fresh ground water pockets

Brackish groundwater

Brown dunes Grey dunes carbonate-rich sand ___ Fresh ground water pockets

White dunes Primary dunes intertidai zone

Sandy beach

Grey dunes

Brackish groundwater are conserved by lichen cover, but in arid regions lichens can be very destructive. Danin (1986) recognised in the weathering forms observed in the Negev a dynamic equilibrium between surface destruction and crust formation. Formation of particular types of crusts and patterns of dissolution caused by different moisture conditions allow conclusions to be drawn about palaeoclimatic conditions.

Bioturbation and its various forms (e.g. by earthworms in temperate habitats, ground-living small mammals in tropical high mountains) and consequences for habitats are only mentioned here, as is bioerosion with its often catastrophic consequences in overgrazed mountain regions (see Fig. 4.1.30D and Chap. 4.1). An example of the passive role of vegetation in forming the relief is the living submerged macrophytic vegetation. Dynamics of river beds and river valleys is particularly regulated by vegetation. Plants occupying river banks, or submerged, influence water flow and thus affect erosion and deposition of materials transported in the river. In non-canalised rivers in Europe, for example the lower Loire, islands covered with canary grass (Phalaris canariensis) develop in the river bed. The German tamarisk (Myricaria germanica), now very endangered, not only withstands the mechanical strain of floods, but also contributes considerably to the stabilisation of river banks in Alpine low lands.

In other climatic zones there are similar, typical influences of vegetation on the formation of areas along rivers and their banks. For example, in the low land tropics, vegetation near river banks acts as zones trapping sediments and floating materials, thus slowing the flow of the river and providing protection from erosion by damming the river. In near-natural river valleys small mosaics of different plant communities frequently occur. These communities provide information on the water supply and the mechanical strain caused by water, as well as about the modifying influences of plants. Such mosaics can only be understood if the interactions between vegetation and site are considered (see also Fig. 4.1.38 and Chap. 4.1).

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