Estimation of the distribution of eastern and western marginal localities of plant species in the Southern Tuva Mountain Chain

The increase of number of the western marginal pla nt localities is revealed in the western part of the Southern Tuv a Mountain Chain, from the Mongun-Taiga Massif to the Sengilen Mt. Range, i n comparison with the eastern and central parts of this mountain chai n. The increased number of the eastern marginal plant localities is reveale d in the eastern part of the mountain chain in comparison of its central part. S uch distribution of marginal localities suggests the Southern Tuva Moun tain Chain to be a corridor for the westward and eastward migration of plant species.


Introduction
The Tyva Republic is limited from the south by the mountain chain which is the world watershed among the Arctic Ocean basin and the drainless area of the Central Asia. This mountain chain includes, from the west to the east, Mongun-Taiga Massif, Tsagan-Shibetu, Western Tannu-Ola, Eastern Tannu-Ola, Horumnug-Taiga and Sengilen Mt. Ranges. It acts as an outpost of the southward distribution of the Siberian mountain taiga landscapes and of the northward distribution of the Mongolian desert steppe landscapes [1]. The Tannu-Ola Mt. Ranges, which are parts of the Southern Tuva Mountain Chain, also represent a botanical-geographical boundary separating the depressions of Central Tuva and depressions of Southern Tuva and Mongolia. These ranges are an obstacle for the southward spread of some steppe species and act as a barrier for humid air masses. As a result, conditions are created for penetration of certain forest and meadow species into the Tuvinian Depression from the mountain framing of this depression [2]. At the same time, it is of interest whether the Southern Tuva Mountain Chain form a structure that not only prevent latitudinal migration, but also promote the longitudinal migration of plant species. The article is applied to estimate westward and eastward migration of plant species in the Southern Tuva Mountain Chain.

Material and methods
The numbers of the western and eastern marginal localities of plant species are used for estimation of the level of species migration in western and eastern directions. If the number of western marginal localities is more in the western part of sublatitudinal mountain chain in comparison of the number of marginal localities in the central and eastern parts of the chain, then author assumes that species during migration along the mountain chain had not any substantional obstaсles and the chain presented a corridor for plant species migration. The same assumption was accepted for estimation of eastward species migration.
The published dot distribution maps of 2569 vascular plant species and subspecies of Siberian flora [3] were used for revealing western and eastern marginal localities. The localities in dot distribution maps were geocoded by means of Geographic Information System ArcView GIS 3.2a (registration number is 843181116338) and the module ArcView Spatial Analyst 2.0a (registration number is 849601104159). The procedure and results of geocoding were published in Global Biodiversity Information Facility (GBIF) [4].
The coordinates of geocoded localities and the corresponding names of taxa were entered into the Localities Database in MS Access. Then, for every species or subspecies presented by not less than two dots, western and eastern marginal localities were entered in Marginal Localities Database. The selection of marginal localities were made by means of a program writen by author in Visual Basic for Applications.
The marginal localities, embraced by the frame with sides N 50°-51.5° and E 89°-98° where the Southern Tuva Mountain Chain are situated, were selected from Marginal Localities Database. Some territories besides the Southern Tuva Mountain Chain were partially embraced by the frame as well: the Tuvinian and Ubsunur Depressions, Alash and Kaа-Khem Highlands. Localities from these territories were excluded from consideration. Many species from the Southern Tuva have rather vast ranges, which extend beyond the boundaries of Siberia. Such species, which extend westward of the western marginal localities in Siberia, for example in the Middle Asia [3], were excluded from the consideration as well. Also the species which occurs eastward of the eastern marginal localities in Siberia were excluded from the consideration as well. They are species which have been registered for botanical-geographical districts of the Central and Eastern Mongolia: Prikhubsugulskiy, Hangaiskiy and others [5]. Several species were not taken into account because the results of geocoding of their dot distribudion maps [3] were not in accordance with the species distribution by Tuvinian nature districts [6]. They are for example lowland species of the Erzinskiy steppe nature district [6]

Results and discussion
The