Syntaxonomy of steppe vegetation of the Republic of Buryatia

Abstract

Transbaikalia is the vast region in Southern Siberia withnumerous mountain ridges within the upper basins of the three big rivers, the Amur (Pacific Ocean basin), the Yenisei and the Lena (Arctic Ocean basin). The most part of Transbaikalia territory is covered by woodlands, however, the steppes are an inherent component in the southern part of the region. Steppe communities occur throughout the broad range of habitats varying in humidity, edaphic conditions, and anthropogenic disturbance. The main aims of presented study are: (1) to develop a phytosociological classification of transbaikalian steppes and to characterize the syntaxa distinguished, (2) to reveal principal ecological factors that have an influence on regional syntaxonomical patterns of the steppe vegetation. From 2005 to 2013, the field studies were carried out in the territory of the Republic of Buryatia. During this period we collected the data set of 829 steppe relevйs. Supplementary characteristics were followed by every relevй: geographical coordinates (measured by GPS); altitude; inclination and exposition; rock outcrops, large stone and debris cover; sand content in the soil. For each relevй climatic parameters from the WorldClim web-database were determined. Plant species indicator values for southern Siberia have been applied (Korolyuk, 2006); the average indicator value for moisture at every relevй was calculated. The data set was stored and managed in the IBIS 6.2 software. To analyze the data structure, a canonical correspondence analysis (CCA) was used. As a result 11 associations were distinguished, all of them belong to the class Cleistogenetea squarrosae Mirkin et al. ex Korotkov et al. 1991. The class represents main dry grassland diversity of the East Siberian–Central Asian sector of Palaearctic (Korolyuk, 2002). The continuous range coincides with the steppe and forest-steppe zones and mountain belts, encompassing Southern Siberia in Russia, as well as the significant part of People’s Republic of Mongolia and Inner Mongolia province in China. Under conditions of the ultracontinental climate, the steppe communities penetrate far to the North in North-Eastern Asia. This class consists of two orders: Stipetalia krylovii (typical bunchgrass steppes of the steppe zone and mountain steppe belt) and Helictotrichetalia sсhelliani (meadow steppes and rich-in-species bunchgrass steppes of mountain forest-steppe). The order Stipetalia krylovii Kononov, Gogoleva et Mironova 1985 represents the central order of the class. It comprises typical steppes predominated by bunch grasses as usual. The order is diagnosed by the same species like those of the class. N. Ermakov (2012) attempted to narrow this order. In his understanding (see below underlined), the order combines typical steppes of Yakutia and is characterized by following features. Communities are extremely poor — 4–9 species per 100 м2. I think that this statement is incorrect. In original publication (Mirkin et al., 1985), wh ere the new association Stipetum krylovii has been described, authors plotted the releve at the size of 1 м2 only. Therefore I can assume that species richness of these steppe communities has been wrongly estimated in regard of 100 м2 plots. According to my own data from the Lena River valley in vicinities of the city of Yakutsk the richness of 20 species (in average) per 100 м2 plot is typical for xerophytic steppes. Yakutian steppes of Stipetalia krylovii are differentiated by meadow steppe species (mesoxerophytes): Artemisia commutata, Festuca lenensis, Poa transbaicalica with a high constancy and many species with a low constancy. From my data, in Transbaikalia such species as Artemisia commutata and Festuca lenensis are common in petrophytic steppes both typical and meadow. In Yakutia, Poa botryoides is typical (d. s. Stipetalia krylovii) instead of Poa transbaicalica. Participation of numerous mesoxerophytes with the low constancy is common for typical steppes from different regions of Southern Siberia and Mongolia (Hilbig, 1995; Namzalov, 2015; Makunina, 2016). Yakutian steppes of Stipetalia krylovii are differentiated by halotolerant plants, namely Saussurea amara and Puccinellia tenuiflora. According to results of my studies as well as literature data, presence of the both species is characteristic for steppe communities in the alases only. Alases are formed as shallow depressions among the taiga landscapes in Central Yakutia, maded by thermokarst, and covered by the meadow or steppe vegetation (Troeva et al., 2010). In such landscapes steppes are spatially patterned into combination with halophytic communities. In well-drained landscapes halotolerant plants are absent. In the definition of Stipetalia krylovii N. Ermakov (2012) based on publications devoted to alases, when steppes from drained terrains were omitted. Thus, none of the three criteria suggested by N. Ermakov (2012) and mentioned above is not acceptable for distinguishing typical steppes of Yakutia from analogue communities of Southern Siberia, including Transbaikalia. This means that the attempt to validate the order Cleistogenetalia squarrosae Mirkin et al. ex Ermakov 2012 was superfluous (Art. 29c, ICPN). Consequently, this order is a synonym of Stipetalia krylovii. Also, the alliance Poo attenuatae–Cleistogenion squarrosae Mirkin et al. ex Ermakov 2012 is to be re­cognized as a synonym of the alliance Stipion krylovii. The alliance Stipion krylovii Kononov, Gogoleva et Mironova 1985 is the central alliance of the order Stipetalia krylovii. It includes non-petrophytic steppes which are common mainly in the mountain steppe belt and the steppe zone. In Western Transbaikalia the alliance is represented by two associations: Cymbario dahuricae–Stipetum krylovii Hilbig (1987) 1990 (the central association of the alliance) and Cleistogeno squarrosae–Festucetum dahuricae Dulepova et Korolyuk 2015 (sandy steppes). Alliance Thymion gobici Mirkin et al. ex Hilbig 2000 comprises xerophytic petrophyte steppes of Transbaikalia, Central and Eastern Mongolia, the Inner Mongola province (China). In Buryatia it is represented by three associations. Ass. Eremogone capillaris–Festucetum lenensis Mirkin et al. 1988 — the central association of the alliance. Petrophytic communities are common both on convex windswept slopes and ridge tops in the forest-steppe and steppe belts. Physiognomy of communities is determined by small cushion-forming species and dwarf semishrubs. Communities of the ass. Selaginelletum rupestris–sanguinolentae ass. nov. are often confined to ridge-top parts of north-facing slopes with outcrops or screes in the Selenga river basin. Community physiognomy is determined by predomination of the low-growing trailing perennial Selaginella sanguinolenta which makes communities noticeably darker as compared to the surrounding steppes. Ass. Selaginello rupestris–Artemisietum subviscosae ass. nov. — the endemic association found in the Barguzin depression. The communities occur on fine gravel soils on river terraces, deluvial foreslopes and convex slopes in the mountain lower belt. Order Helictotrichetalia schelliani Hilbig 2000 comprises meso-xerophytic grasslands, mainly meadow steppes of East Siberian–Central Asian sector of Palaearctic. It unites the most widespread non-forest plant communities of forest-steppe landscapes. The area of the order extends considerably to the north in comparison with the order Stipetalia krylovii. In Transbaikalia, the meso-xerophytic grasslands predominate in the mountain forest-steppe belt up to the heights of 900–1000 m a. s. l. Alliance Helictotrichion schelliani Hilbig 2000 — the central alliance distributed in Altai-Sayan mountains, Western Transbaikalia, northern mountains of Central and Western Mongolia. In the study area we distinguished two suballiances. Suballiance Stipenion baicalensis suball. nov. — the central suballiance representing meadow steppes on fine soils. Communities are found at gentle straight slopes, sometimes occupying slightly concave sites of mountain ridge slopes. The suballiance consists of three associations. The central association is Veronico incanae–Stipetum baicalensis Peterson ex Hilbig 2000. It occurs in the steppe and forest-steppe belts, occupying gentle slopes and piedmont foreslopes. Ass. Carici pediformis–Spiraeetum aquilegifoliae ass. nov. represents meadow steppes with well developed shrub layer. The communities usually occupy steep south-exposed slopes with large stones or outcrops, often of concave shape. By their physiognomy and species composition, the described communities look like the association Amygdalo pedunculatae–Spiraeetum aquilegifoliae, which substituted by them in more humid forest-steppe landscapes. Ass. Thalictro foetidi–Koelerietum cristatae ass. nov. unites the more moist type of the suballiance. The communities are found in the forest-steppe landscapes, usually along gentle straight or concave slopes. Often they occupy steeper parts of slopes along the edges of larch and birch forests. Suballiance Thymenion baicalensis represents petrophyte meadow steppes of Western Transbaikalia. The communities are common in the forest-steppe and steppe landscapes on stony, often convex slopes, ridge tops, sometimes on screes. Ass. Potentillo acaulis–Thymetum baicalensis Chytry et al. 1993 — one of the most widespread steppe plant communities. Ass. Sela­ginello rupestris–Artemisietum monostachyae ass. nov. comprises rich meadow petrophytic steppes. The communities are found in forest-steppe landscapes in the Uda and the Barguzin rivers basins. Ass. Sedo aizoi–Festucetum lenensis ass. nov. includes the more moist variants of meadow petrophytic steppes. The communities are rich and polydominant, mainly with predomination of mesoxerophytic meadow steppe species. They are confined to stony slopes and ridge tops, usually at the forest margins in the Selenga river basin. Strong ecological gradients have been revealed by the numerical analysis of associations. The first axis of CCA-ordination is correlated with humidity which is determined by precipitation, and the soil moisture that depends on landform features as well. The second axis represents edaphic conditions — stones and sand content in the soil. The third axis is the most likely of complex character, however it is especially related with temperature and solar radiation. Due to interlacement of various environmental gradients a high variability of habitats and consequent complicated vegetation patterns within the studied region occured. Further development of the dry grassland syntaxonomy of Inner Asia could be advanced by the involvement of the data from the territories of Eastern Transbaikalia, People’s Republic of Mongolia (Outer Mongolia) and the province of Inner Mongolia in China. This will allow to delineate the distribution of syntaxa, as well as to make a general revision of the class Cleistogenetea squarrosae all over its area of distribution.