rig. 4.3.4. Influence of a shelter belt on the microclimate. (After Reichelt and Wilmanns 1973)

is higher and more balanced than on open spaces.

Various forms of wind shear show impressively the influence of wind on vegetation. It may be concluded that plant stands also weaken wind, for hedges provide protective wind barriers (Fig. 4.3.4). There is a dependence on height, width and permeability of the vegetation and the reduction in wind speed and the development of turbulence, in turn, cause large differences in conditions within small patches of vegetation. Also the transport of energy and materials and how they are affected and absorbed by vegetation must be evaluated.

Dune systems along coastlines worldwide characterise the littoral zones. The sequence of different sites along the coast into the hinterland is shown in Fig, 4.3.5. Plants are able to stabilise the mobile sand, which is transported by wind, and influences transitions from the coast to the hinterland enormously. These plants are able to withstand the mechanical effects of wind and the salt carried by it. Following from the sea towards the land, communities are sea rocket (Ca-kile maritima), lyme grass (Elymus arenarius) and marram grass (Ammophila arenaria). The latter is the most important primary dune spe

Potential dune forest

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

cies and is not only able to stabilise dunes, but also to tolerate being covered by sand because it forms layers of rhizomes and survives the cover of primary dunes (white dunes) which are often metres high. If the marram grass is not covered by sand, secondary and tertiary dunes (grey and brown dunes) develop from the gradual accumulation of humus from the decaying plant material. Tertiary dunes are recognised by an increasingly closed cover of dune grasses and shrub communities. Without plant species adapted in such a way, this formation of relief and zonation would not be possible. At the moment, dune complexes disturbed by tourism are successfully protected by planting with the salttolerant sand dune couch grass (Agropyron jun-ceum) and marram grass (Ammophila arenaria) which are tolerant to being covered by sand.

Plants also play a role in triggering formation of inland dunes. Long (1954) traced the origin of the Nebket (Arabian term for dune systems) in North Africa to single, particularly resistant, dwarf shrubs. Sand blown in by the wind is deposited on the leeward and windward sides of these shrubs. With time the accumulation of sand improves nutrient and water supply of the 'initial shrub' which is able to grow better and thus offers possibilities for other plants to establish and finally for animals to settle. Thus, particularly at the edges of deserts, island-like dune systems develop with communities surprisingly rich in species in an otherwise hostile environment.

This shows that vegetation, with its floristic composition and most of all with its structural characteristics, greatly influences the variability of environmental conditions in the stand. Many vegetation patterns in small spaces can only be understood if the influences of vegetation which cause the differences in the local climate are known.

0 0

Post a comment