Influences of Vegetation on Weathering and Topography

Even though climatic and tectonic factors are most important for geomorphological processes, the influences of organisms on the formation of relief and, thus, for changing site conditions must not be overlooked. Viles (1988) distinguishes between active and passive relations between organisms and the site factors that depend on relief. The former are biogenic weathering, formation of biogenic sediments (e.g. lime deposits), bioturbation (e.g. burrowing animals) and bioerosion (e.g. from grazing animals). Passive relations, arising from the mere presence of the vegetation, can be seen in dune formation and in the accumulation of organic matter (e.g. in bogs or as a consequence of damming rivers).

Weathering or soil formation 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. For example, cyanobacteria participate in the formation of desert varnish, a covering of iron and manganese compounds, thereby securing their own habitat with a protective crust. The metabolic processes of lichens, cyanobacteria and fungi result in the release of acids and chelating compounds that, under humid conditions, dissolve some components in rocks either completely (as with limestone) or selectively and remove the mineral components. Endolithic lichens, living within rocks (e.g. from the genera Caloplaca and Buellia), and the many different species living on the surface of rocks (epilithic) contribute to biophysical and biochemical weathering in all climatic conditions (Belnap and Lange 2001).

Under moist conditions, crusted rocky surfaces 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. The formation of particular types of crusts and patterns of dissolution caused by different moisture conditions also allow conclusions to be drawn about more humid palaeoclimatic conditions.

Bioturbation and its various forms (e.g. by earthworms in temperate habitats, grounddwelling 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 (Fig. 17.17d, Chap. 17). An example of the passive role of vegetation in forming reliefs is living submerged macrophytic vegetation. The dynamics of river beds and river valleys are particularly regulated by vegetation.

Plants occupying river banks or that are submerged influence water flow and, thus, affect erosion and the deposition of materials transported in rivers. In non-canalised rivers in Europe, islands covered with canary grass (Phalaris canariensis) develop in the river bed. The German tamarisk (Myricaria germanica) not only withstands the mechanical strain of floods but also contributes considerably to the stabilisation of river banks in the valleys of the alpine lowlands.

Other climatic zones see similar, typical influences of vegetation on the topography along rivers and their banks. For example, in the lowland tropics, vegetation near river banks acts as zones trapping sediment and floating materials, thereby 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 (Fig. 17.25, Chap. 17).