SPATIAL AND TEMPORAL VARIABILITY OF DRY STEPPES OF EASTERN MONGOLIA

Spatial-temporal structure and coenotic diversity of dry steppes of Eastern Mongolia was identified by analyzing characteristics of naturally occurring vegetation connection to the regional landscape structure. Different types of combinations of plant communities (phytocoenochores) were determined in the vegetation structure of the Eastern Steppe Station Tumén-Tsogt (in Sukhebator district). Temporal dynamics of steppe ecosystems was defined from the studies of steppe cover fluctuations in 2008. The coenotic role of eight annual plant species that form synusiae in the steppe communities was shown through analysis of species constancy, projective cover, and activity. Knowledge about the trend of successions and the manifestation of fluctuations in vegetation cover is necessary for the development of science-based system of management options to maintain the number and abundance of different plant groups in plant communities. Monitoring the state of natural ecosystems has a major scientific and practical importance, since steppe ecosystems are the basic component of the pasture’s resources of the country.


INTRODUCTION
Studying the fundamental properties of vegetation and, above all, its spatial-temporal structure has always been in the spotlight and one of the urgent tasks of biogeography. A variety of natural plant complexes, their distribution patterns (spatial differentiation of steppe communities in relation to the gradients of environmental factors), and temporal dynamics of communities caused by various environmental changes represent a present-day ecological potential of the territory. Steppe ecosystems exist in constant dynamics related to changes of environmental parameters caused by natural and anthropogenic impact. Fluctuations and successional changes are regarded as certain stages of steppe communities in a series of dynamic manifestations. However, the natural invariants of steppe ecosystems have certain average characteristics that determine their primary structure and relative stability to the impact factors. Indigenous communities and their varying components represent a single dynamic system. Sustainable development of steppe ecosystems is associated with preservation or restoring their natural structure. Vegetation cover is one of the most important indicators of ecological condition of the whole region and serves as an explicit measure of its well-being or as an indicator of existing environmental problems. To solve the pressing problems of balanced (sustainable) development of steppe regions, a reliable information on the current state of natural ecosystems and their base constituent, i.e., biodiversity, is necessary.
In 2008, in practically all communities of the dry-steppe of the Eastern Mongolian steppe biome, significant changes in their organization have been studied. These changes were manifested by strong development of synusiae of annual species and changes in the composition of dominants, structure, products, and physiognomy of the steppe communities. Such stages of steppe ecosystem development represent cyclical changes and relate to the fluctuations caused by changes in climatic parameters (volume and regime of rainfall) in the current and previous years.
A comparative analysis of changes occurring in steppe ecosystem and the modern trends of their further development are of interest to the general scientific studies of steppes and are necessary for developing practical recommendations for their use and conservation.

OBJECTS OF STUDY AND METHODS
The Eastern Steppe Station Tumén-Tsogt (in Sukhebator district) is located in the Herlen River basin and practically coincides with the area of a similarly named Soum of Sukhebator district. The steppes of this territory pertain to the Eastern Mongolian sub-province of the Eurasian steppes according to botanicgeographic demarcation [Lavrenko, 1970]. Studies of the arid steppes of the Eastern Mongolian biome at the Tumén-Tsogt Station have a long history [Volkova, 1988;Khramtsov, Dmitriev, 1995].
The specific feature of the Eastern Mongolian steppes is that they combine features of mountain and plain steppes characteristic of the landscape structure of the territory. The vegetation consists of herb-bunchgrass and dry bunchgrass steppes on chestnut soils whose distribution corresponds well to the geomorphologic structure and the soil cover of the territory. The Station area is about 2000 km 2 .
The relief at the territory consists of a hummock denudation plain with a system of uneven-aged hills and of intermontane depressions between the hills. Apical surfaces of the hills vary in morphology depending on lithology of the composing rocks. In the areas with the dominance of weak sandstones, shale, and effusive rocks, there are smooth dome-shaped or flattened tops. Rocky peaks and ridges are formed by outcroppings of granite, quartzite, jasper and limestone (Fig. 1).
The climate of this part of Eastern Mongolia is continental: summers are short (the average July temperature is +20°C), winters are long and almost snow-free (the average January temperature is -20°C). The average annual rainfall is 200-240 mm, reaching 360-460 mm in some years. According to the Underhan

Spatial organization and coenotic diversity of the Eastern Mongolian steppes
Vegetation structure that evolves in the process of its development determines stability and spatial distribution of communities according to natural conditions and geomorphological composition of the territory [Sochava, 1978]. In phytocoenology, there is a notion of the types of territorial units as regular combinations of communities of a certain hierarchical level that compose the optimum spatial vegetation system contributing to its highest stability.  [Glazovskaya, 1988] and slope development [Voskresenskyi, 1971;Ogureeva, 1994]. Within phytocatanae, the integration of elementary ecosystems of the eluvium, trans-eluvium (transit), and accumulative positions into linked series takes place, defining spatial differentiation of vegetation on the slopes. Functioning together, they are the product of the close interaction between all the components and with the environment. During the survey of the steppes at the Station in 1994, the average year in terms of moisture indicators (see Fig. 2), it was noted that in the steppes, annual species (Chenopodium viride, C. acuminatum, C. aristatum, Salsola monophera, S. collina, and Artemisia palustris ) were present infrequently and in small numbers. However, their role increased locally with the impact of grazing, rodents, and on the fallow lands (Miklyaeva and Lysak, 1996). The 2010 summer sampling survey (also the average year in terms of precipitation during the growing season) of the steppe communities showed that coenotic diversity of the steppes remained the same as in 2008, however the fluctuations of species composition in the form of synusiae of annual species were not manifested. Some of the annual species were present in the communities, but did not play any meaningful coenotic role. Comparison of these materials suggests that temporal conditions (fluctuations) of the steppes occupy a certain place in the multiyear natural dynamics of steppe ecosystems. The fluctuations as cyclic changes in ecosystems represent an integral part of their natural dynamics. Selfregulation of steppe ecosystems restores the original structure of the communities. In the multiyear development cycle of the steppes, their composition is marked with changes at the population level of speciesdominants and species-determinants that remain in the communities at all variations of the hydrothermal regime.
Floristic richness of the steppe communities was markedly reduced: in the dry years, from 40-45 species/100m 2 to 12-15 and, rarely, to 7-8 species. As a result of mass development of annual plants, the total projective cover of grass steppes rose to 50-80%, but the true coverage (swarding) was low, not greater than 10-20%. This is typical for the well-defined stages in a series of fluctuating alternations; annual species are the fluctuation dominants in the steppe communities [Rabotnov, 1978] and define their appearance. The abundance of plant species was assessed according to their projective cover as a measure of their coenotic role in the structure of the steppe communities at the Station (see Table 1). Two species, i.e., Salsola collina and Chenopodium viride, contributed the most to the projective cover (21-40%) of the communities of different altitudinal levels. The highest abundance indices were characteristic of Salsola collina; its projective cover in shrub steppes (Caragana microphylla, C. stenophylla) and three-stipa (Stipa grandis, S. krylovii, S. sibirica) steppes on the slopes of the intermediate hummocks reached 60-70% (Fig. 3). The projective cover   The coenotic role of the annual species in communities of the same high-altitudinal level varied considerably (see Table 1). Chenopodium aristatum had the greatest coenotic role (projective cover 21-40%) in the communities of the plains in (Fig. 4), while Salsola monopthera and Artemisia palustrisin the depressions. Salsola monopthera had its maximum projective cover (40%) in the stipa (Stipa krylovii) steppes on the plains adjacent to the saline depressions (Fig. 5). The high projective cover (40%) was also observed for Salsola monopthera in the communities of the fallow land. The coenotic role of other annual species was negligible. Low and very low coenotic importance was found for the species whose projective cover never exceeds 10% -Axyris amarantoides, Chenopodium acuminatum, and Artemisia scoparia (see Table 1).
Annual plants' activity represents an integral indicator of their role in the steppe communities (Yurtsev, 1968). Activity is calculated as follows: where A -activity of a specie, F -average projective cover (%), D -constancy (%).
Activity of annual and biennial species varied in different types of steppe mountain complexes (Fig. 6). In the high hummocks, all annual species were inactive; two species -Artemisia scoparia and A. palustris do not even occur in the steppes at this level. In the intermediate hummocks, there were two species (Chenopodium viride, C. aristatum) that

CONCLUSION
The research on spatial-temporal structure of steppes is of special scientific and practical importance and is the basis for studying ecosystems at the current level of their protection and restoration. In extreme years, temporary synusiae appear as a result of varying volume and regime of precipitation. Annual species become fluctuating dominants during this period. [Rabotnov, 1978].
Analysis of the 2008 studies of the current state of the steppe ecosystems showed fluctuations in the sequence of their temporary states with clearly expressed annual species synusiae. Each annual species in a synusia is characterized by a specific ecological range and the coenotic optimum. These species had different ecological amplitude and played different coenotic role in the steppe communities. The species with broader ecological amplitude (Chenopodium viride, Salsola collina) in the steppes of Eastern Mongolia played a more significant role in the steppe communities, whereas species with narrower ecological amplitude (Salsola monophera) had lesser coenotic significance.
Sustainable use of pasture ecosystem resources depends on obtaining the most complete information about steppe vegetation. Pasture regime should be based on the landscape ecological approach with consideration given to pastures' feeding values and positions in the altitudinal zonal structure.
In order to preserve the invariant steppe communities and to restore steppe ecosystems, measures for improving of the steppe ecosystems should be developed. These measures should be also based on ecological principles that consider the ecosystems' position in vegetation structure of their altitudinal zones (mountains, hummocks, valleys, depressions) and in specific types of vegetation cover.
In this regard, published literature explores different measures aimed at improving floristic composition and structure of steppe ecosystems ensuring their sustainable development and functioning [Ecosystems…, 1995]. Among the proposed measures, in our opinion, the following activities will be most beneficial.
Management of pasture use regime (schedule and duration of grazing and rotation); Regulation of grazing load (modeling of optimal livestock numbers and herd composition depending on the pasture type); Restoration of steppe ecosystems (incorporating "rest" of the steppe communities, improving feeding value of pasture by reseeding forage plants); Creation of grasslands to establish reserve stocks of feed; Organization of a monitoring system of steppes (multiyear observations of steppe plant communities and identification of fluctuations and succession stages); Identification and preservation of reserve areas of steppe ecosystems of different altitudinal zones to maintain the gene pool and replenishment of the seed pool of main species-forming steppe communities.
It is also necessary to continue work on developing the ecological framework of Eastern Mongolia for preservation of steppe ecosystems. A network of protected areas, including sanctuaries and a system of standard natural sites, should be developed. The network would link Mongolian steppes and the integrated system of protected areas of the Central Asian steppes, and the centers for biodiversity.