Ecosystem, shrub-steppe

We will consider both Steppe and Desert ecosystems as deficient in atmospheric humidity (evapotranspiration exceeds precipitation). The Sub-Boreal, Semi-arid and Arid Steppe and Desert ecosystems occupy a significant part of the global area. This territory includes Sub-Boreal zones (Steppe, Arid Steppe, and Desert Steppe ecosystems) with total area of 9.23 x 10 km, as well as subtropical zones (Shrub Steppe and Desert Steppe ecosystems) with total area of 7.04 x 10 km. These areas do not include Subtropical and Tropical Sandy Desert ecosystems (5.77 x 10 km-) and Stony Desert ecosystems (8.96 x 10 km ). Thus the extra-tropical arid area takes about 20% of the World s terrestrial ecosystems. Most of this area goes to the inter-continental regions of Eurasia and, partly, of North and South America. The biogeochemistry of semi-arid and arid ecosystems shows distinctive parameters, which allow us to consider the quantitative features of element turnover and dynamics in natural fluxes. 

With aridity increasing, various plant species of forage crops become gradually less numerous to finally disappear. In Dry Steppe ecosystems xerophylic half-shrubs and salt-tolerant plants replace the grasses. However, the ash content is higher in these species. This is attributed not only to a higher concentration of major ash elements in the plant tissue, but also to the exposure to finely dispersed dust adhered to the plants exterior (Table 9). 

Grasses, small shrubs and shrubs, whose number of xerophytic and ephemeral forms increases with aridity, are the chief representatives of Arid ecosystems. At the north and south peripheries of the arid zone, herbaceous ecosystems are predominant. In regions with a well balanced atmospheric humidity prior to human activities, of wide occurrence were Meadow Steppe ecosystems intermixed with Broad-leaved Forests. With an increase in climate continental properties, the Meadow Steppe ecosystems of Eurasia grade into Forb-Fescue-Stipa and Fescue-Stipa Steppe ecosystems. In turn, the natural vegetation has been destroyed by humans and replaced by pastures and... 

The smaller Cw values are also connected with water deficiency in Steppe and Desert ecosystems. However, the concentration of various chemical species in rainwater of background regions is higher that that in the Forest ecosystem belt. The major reason is the wind deflation of the soil s surface owing to lack of tree species and only a lean protective layer of grasses and half-shrubs. A large mass of soil particles becomes entrained into the air migration. The most characteristic example is connected with the Yellow sand phenomenon (see Box 7). 

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