Invasive Successions as the Indicator of Desertification of Dry Steppe by Way of Example of Central Mongolia

The studies of the steppe ecosystems in Central Mongolia have showed that the simplification of steppe communities has taken place over recent decades. This occurred by reduction of species diversity and abundance of indigenous dominants—tussock grasses—as a result of sharp rise in pasture loads and a long dry period. We have identified two types of introduction of invasive species from different landscapes: (a) extraand intrazonal and (b) zonal. The first type of succession is characterized by focal distribution of Ephedra sinica from ecosystems of low mountains to the surrounding mountain plains. The second type of succession in dry steppes is associated with the expansion of Allium polyrrhizum, whose distribution has largely a frontal character and is caused by weakening of the competitiveness of indigenous species of grass communities because of their significant digression. A further factor in ensuring the conditions for invasion is the aeolian alkalization of the upper horizons of zonal chestnut soils. The ecological and biological features of these two species, widespread in the desert-steppe and desert landscapes and penetrating into the steppe ecosystems, make it possible to speak about biological desertification. The wide area of Ephedra sinica and Allium polyrrhizum indicates a progressive character of the studied types of succession, and as a result of this, the borders of these areas have reached at present the southern periphery of the Baikal Lake basin. The paper describes the processes which lead to the reduction of the fodder value of pastures and jeopardize the maintenance of cattle breeding in Central Mongolia.