The Distribution Characteristics of Vegetation in the Subrange of the Altai Mountains, Xinjiang

The Altai Mountains are an important center of biodiversity and are a major habitat for threatened and endemic species in Asia. Moreover, the Altai Mountains are a valuable site for the study of the evolution of central Asian vegetation. The Xinjiang Altai subrange represents the largest part of the southern Altai Mountains and has many unique plant communities. After conducting a thorough literature review and field investigation, we utilized the Chinese vegetation categorization system to identify the dominant plant communities in the Xinjiang Altai subrange and report their composition and distribution characteristics. Our results show that (1) the natural plant communities present in the Xinjiang Altai subrange can be divided into eight vegetation types, eighteen vegetation subtypes, and 50 communities. Among these, two communities—Form. Calligonum rubicundum and Form. Seriphidium borotalense-Festuca valesiaca—are present only in the Xinjiang Altai subrange. (2) The Xinjiang Altai subrange is located at the junction of three major biomes containing unique vegetation types (coniferous forest, temperate broadleaf forest, and desert). Thus, the Xinjiang Altai subrange is distinct in its staggered transition from mountainous boreal taiga to temperate desert. This research provides textual data to contextualize the cultural heritage of the Xinjiang Altai subrange and also provides a scientific basis for the protection and sustainable management of natural resources found in the Xinjiang Altai.


Introduction
Vegetation classification is an important part of biogeographical research and also comprises one of the research's most complex problems [1,2].Vegetation classification can improve our understanding of the structural characteristics of vegetation, along with the species composition and the relationship between different plant species and their environment [3,4].Meanwhile, the research results of vegetation classification can provide a scientific basis for the formulation of and measures within laws and regulations related to the protection and rational utilization of vegetation [5][6][7].
The Altai (synonym: Altay) Mountain Range spans much of Central Asia and crosses Russia, Kazakhstan, China, and Mongolia.It is the largest mountain range in the Western Siberian biogeographic region and is an important center of biodiversity for many species found in ecosystems throughout northern and central Asia [8][9][10][11].The Xinjiang Altai Mountains are located on the southern side of the middle portion of the range.This subrange is among the most abundant in plant diversity and vegetation in the whole Altai range and marks the most southerly distribution of many species of Europe-Siberian flora [12,13].Moreover, the Xinjiang Altai subrange is the most important migration channel for wildlife in the Altai Mountains.In addition to seasonal tourist activities and grazing, there is no interference from industrial projects or agricultural production in the mountains, and the Altai Mountains have preserved a unique natural and original vegetation landscape.From west to east, a series of natural altitudinal zones are present along the southern slope of the Xinjiang Altai subrange.This has created a succession of zonal landscapes from Arctic tundra, alpine meadow, subalpine meadow, Europe-Siberian taiga forest, and the north forest steppe to the Central Asia desert steppe [14,15].These zonal landscapes are typical of the southeastern Altai Mountains, and they exhibit gradual changes in response to different atmospheric conditions [16].The geographical distribution of endemic flora and the presence of special vegetation types, as well as an unbroken vertical zone in the Xinjiang Altai subrange, result in this area being ideal for studying many characteristics of different plant species.In addition to its scientific value, the Xinjiang Altai subrange is also important for the conservation of wild species.
However, since the end of the 20th century, the disorderly development taking place over successive years, grassland overgrazing, grassland degradation, local forest overcutting, aging of forests, and the vigorous development of tourism have all caused serious deterioration in the ecological environment of the Xinjiang Altai subrange [17].Meanwhile, the forest vegetation has degraded, important biological resources have been sharply reduced, and rare species have become endangered [18].The problem of vegetation restoration and sustainable utilization of resources is a prominent one that places serious restrictions on the sustainable development of society.Research on the vegetation of the Xinjiang Altai subrange has mostly focused on small local areas within the mountain system.Some scholars have explored the relationship between pollen assemblage, vegetation, and the climatic environment on a macroscale [19][20][21] and have studied the response of local vegetation cover and major coniferous tree species to climate change [22][23][24].So far, there have been no systematic studies of the biogeographical distribution of the vegetation present in the Xinjiang Altai subrange, which is regarded as a natural geographical unit [25][26][27].Therefore, the characteristics of the vegetation types and the community composition of the Xinjiang Altai subrange were the subject of this research in order to provide a scientific basis for the selection of vegetation restoration species, community construction, protection, and sustainable management of natural resources found in the Xinjiang Altai subrange.At the same time, the third scientific expedition to Xinjiang was launched in 2021, so this research also provided textual data for the third scientific investigation of the Irtysh River basin in Xinjiang.

Study Area
The Xinjiang subrange of the Altai Mountains (approximate coordinates: 45-49 • N, 86-94 • E) is located 750 km long and 60-140 km wide along the northern border of the Xinjiang Uygur Autonomous Region, China.The subrange spans from the China-Kazakhstan border in the west to Mulei County, Xinjiang Province in the east, Russia in the north, and the Gurbantunggut Desert (China) in the south (Figure 1).The average annual temperature is −0.2 • C, and the average annual precipitation is 200-1000 mm.The main soil types in the Xinjiang Altai subrange are brown calcium-rich soil, chestnut soil, mountain grey forest soil, subalpine meadow soil, alpine meadow soil, and mountain tundra soil.The soil type, morphological characteristics, and vertical belt structure of the Xinjiang Altai subrange are typical of mountainous soils and provide a suitable habitat for the occurrence and succession of vegetation in this area and the proliferation of wild plant populations.

Survey Methods
Based on the interpreted remote sensing images of the Altai Mountains and the vegetation map of Xinjiang, seven vertical transects were selected in the survey area from 2012 to 2021 (Figure 2).The typical sample plots were set up in the area with a uniform distribution of dominant species around each sample line [28,29].The sample area was determined as per the minimal area method of phytocoenology [30].The area of the sample plot was 100 × 100 m.Five 25 × 25 m arbor plots, five 10 × 10 shrub plots, and five 1 × 1 herb plots were arranged in the four corners and the center of the sample plot.The types and numbers of trees, shrubs, and herbs in the quadrant were counted, and three to five standard plants were selected for each plant.The plants' height, crown width, and coverage were measured.The longitude, latitude, and altitude were recorded as environmental variables.

Survey Methods
Based on the interpreted remote sensing images of the Altai Mountains and the vegetation map of Xinjiang, seven vertical transects were selected in the survey area from 2012 to 2021 (Figure 2).The typical sample plots were set up in the area with a uniform distribution of dominant species around each sample line [28,29].The sample area was determined as per the minimal area method of phytocoenology [30].The area of the sample plot was 100 × 100 m.Five 25 × 25 m arbor plots, five 10 × 10 shrub plots, and five 1 × 1 herb plots were arranged in the four corners and the center of the sample plot.The types and numbers of trees, shrubs, and herbs in the quadrant were counted, and three to five standard plants were selected for each plant.The plants' height, crown width, and coverage were measured.The longitude, latitude, and altitude were recorded as environmental variables.
The important value (calculated below) was used to evaluate the dominance of the plant population within the community.
The important value of each species was calculated as follows: IV(%) = (Relative height + relative density + relative frequency+ relative coverage)/4.

Vegetation Map Compilation
We drew the vegetation map with 3S technology [31,32].The specific process is a follows: The vegetation map was based on the topographic map of 1:10,000 in the Aleta region of Xinjiang and the Landsat 7 TM image of the Altai region with a resolution of 30 m in June 2021.The topographic map of 1:10,000 in the study area was geometrically cor rected with the software ERDAS IMAGINE (version 9.0), and the satellite imagery wa corrected using the corrected 1:10,000 topographic map.The vegetation distribution data (including natural environment data, field sample field surveys, GPS sample surveys, and The important value (calculated below) was used to evaluate the dominance of the plant population within the community.
The important value of each species was calculated as follows: IV(%) = (Relative height + relative density + relative frequency+ relative coverage)/4.

Vegetation Map Compilation
We drew the vegetation map with 3S technology [31,32].The specific process is as follows: The vegetation map was based on the topographic map of 1:10,000 in the Aletai region of Xinjiang and the Landsat 7 TM image of the Altai region with a resolution of 30 m in June 2021.The topographic map of 1:10,000 in the study area was geometrically corrected with the software ERDAS IMAGINE (version 9.0), and the satellite imagery was corrected using the corrected 1:10,000 topographic map.The vegetation distribution data (including natural environment data, field sample field surveys, GPS sample surveys, and vegetation distribution literature data) were vectorized using the ArcGIS software (version 10.2).The GPS sample, topographic map, and satellite image map were superimposed by ArcGIS software to mark the spot attributes.According to the main basis of ground-landscape type discrimination, the remote sensing satellite imagery was visually interpreted and mapped (Figure 3).Finally, through ground verification, the original image of the visual interpretation was modified several times, and the vegetation map was completed.

Data Processing
Montane vegetation was classified according to species composition as well as the ecological and geographical characteristics of the plant communities.We used the Chinese vegetation classification method, which classifies plant distribution according to a typesubtype-formation/community system [1,33].For instance, a forest type (e.g., deciduous broadleaf forest) can be divided into multiple subtypes (e.g., mountain and river valley deciduous broadleaf forests).We performed plant dominance assessments to identify the dominant species in each plot type [34].We compiled survey data to classify the natural vegetation found in the Xinjiang Altai subrange according to this system.

The Floral Diversity of the Xinjiang Altai Subrange
Our results suggest that the natural vegetation found in the Xinjiang Altai subrange can be divided into eight vegetation types, eighteen vegetation subtypes, and 50 formations or communities (Table 1).

Data Processing
Montane vegetation was classified according to species composition as well as the ecological and geographical characteristics of the plant communities.We used the Chinese vegetation classification method, which classifies plant distribution according to a typesubtype-formation/community system [1,33].For instance, a forest type (e.g., deciduous broadleaf forest) can be divided into multiple subtypes (e.g., mountain and river valley deciduous broadleaf forests).We performed plant dominance assessments to identify the dominant species in each plot type [34].We compiled survey data to classify the natural vegetation found in the Xinjiang Altai subrange according to this system.

The Floral Diversity of the Xinjiang Altai Subrange
Our results suggest that the natural vegetation found in the Xinjiang Altai subrange can be divided into eight vegetation types, eighteen vegetation subtypes, and 50 formations or communities (Table 1).The zonal vegetation of cold-temperate needleleaf forests, mainly distributed on hillsides and river valleys that face wet air currents, accounts for 15.89% of the total area and consists of cold-temperate tree species (Figure 4).Cold-temperate needleleaf forests include two vegetation subtypes: cold-temperate evergreen needleleaf forests and cold-temperate deciduous needleleaf forests.The main species present in cold-temperate needleleaf forests include Pinus sibirica Du Tour, Picea obovata Ledeb., Abies sibirica Ledeb.and Larix sibirica Ledeb.
Deciduous broadleaf forests are located in valleys and on the lower parts of the slopes of older mountains in the Xinjiang Altai subrange; these areas account for 0.30% of the total area of the subrange.Deciduous broadleaf forests contain two vegetation subtypes: mountain and river valley deciduous broadleaf forests.The predominant species present in this vegetation type include Populus tremula L. and Populus nigra L.
The evergreen conifer and deciduous broadleaf shrub vegetation types each included only a single vegetation subtype.These are cold-temperate evergreen coniferous shrubs and temperate deciduous broadleaf shrubs, respectively.Shrub vegetation in the Xinjiang Altai subrange is generally not zonal but is instead widely distributed and accounts for ~0.11% of the total area.The most predominant shrubs present in the Xinjiang Altai subrange include Juniperus sibirica Burgsd., Potentilla fruticosa (L.) Rydb.and Cotoneaster spp.
The desert vegetation type is narrowly distributed in the Xinjiang Altai subrange and is only present along riverside terraces, mountainous alluvial fans, and mountain plains along the Irtysh and Wulungu Rivers at an altitude of 600-800 m.These zones contain shrubby desert, semi-shrubby desert, and dwarf semi-shrubby desert vegetation subtypes.
Steppe vegetation communities in the Xinjiang Altai subrange are zonal and generally consist of perennial herbaceous plants.Steppe vegetation is an important part of the vertical vegetation belt of the Xinjiang Altai subrange, is mainly distributed in the frontal mountain belt at an altitude of 1100-2300 m, and accounts for 30.56% of the total area.Steppe vegetation has four vegetation subtypes and 15 distinct communities.
Meadow vegetation in the Xinjiang Altai subrange is generally not zonally distributed.Meadow vegetation is widely distributed in the mountains, from subalpine to plains areas, which account for 44.24% of the total area surveyed.Four vegetation subtypes were found to be present: alpine swamp meadow vegetation, alpine meadow vegetation, subalpine meadow vegetation, and typical meadow vegetation.These subtypes contain 20 formations, and together they are the most abundant type of vegetation present in the Xinjiang Altai Mountain subrange.Perennial grasses and moss grasses are the most important structural species in meadows.
Alpine vegetation is distributed in the alpine zone at an elevation of 2600-3300 m in screes, alpine rocks, and tundra.This accounts for 6.53% of the total area of the Xinjiang Altai subrange.This vegetation type contains two vegetation subtypes: alpine tundra and alpine sparse vegetation.Alpine vegetation contains various mosses and lichens, as well as a small number of alpine grasses and small shrubs scattered in the alpine tundra.These plant species are typical of the alpine vegetation found throughout Siberia.

Structural Characteristics of the Main Vegetation Communities Found in the Xinjiang Altai Subrange
Affected by climate and habitat environment, the vegetation composition of the Altai Mountains in Xinjiang is dominated by temperate montane plants, and the typical vegetation community structure characteristics are obviously different (Table 2).

Discussion
Significant differences in topography, local microclimate, hydrothermal conditions, and soil content provide suitable conditions for the development and maintenance of multiple vegetation types in the Xinjiang Altai subrange [35].This climatic variability is due to the influences of the cold and humid airflows from the Arctic and Atlantic Oceans, the Siberian-Mongolian high-pressure dry anticyclone system, and the variability in landform types [36,37].The types of vegetation found in the Xinjiang Altai subrange can be divided into eight main types, eighteen subtypes, and 50 formations/communities.Many types of vegetation had obvious zonal characteristics.Cold-temperate coniferous forests constitute the largest part of the total vegetation present; meanwhile, both grassland and meadow vegetation were also widely developed in the Xinjiang Altai subrange.The Calligonum rubicundum, Seriphidium borotalens, and Festuca valesiaca desert steppe communities were present only in the Xinjiang Altai subrange, and they represent a special type of abundant vegetation in the whole Altai Mountains.
The zonal vegetation is cold-temperate coniferous forest, which belongs to the Southern Taiga Forest; the distribution elevation of this vegetation is varied greatly with climatic factors.It is distributed in the northwestern part of the mountain at 1100-2300 m.However, in the southeastern part of the mountain, the distribution altitude has increased, between 1300 and 2600 m, which is influenced by two factors: the low trend of the mountain itself and the arid airflow in Mongolia.From the west to the east of the mountain, the vegetation type not only changes its distribution range but also changes its composition, from Picea obovata Ledeb.and Abies sibirica Ledeb., to Larix sibirica Ledeb., which is the adapted cold and wet climate to the continental climate.The distribution of vegetation communities throughout the Xinjiang Altai subrange is affected by the local microclimate and the arid airflow from Mongolia; here, cold-tolerant mesophytes gradually give way to droughttolerant mesoxerophytes as the latitude increases [38,39].This pattern conforms to the patterns of plant community spatial distribution-i.e., from cold to warm-and dry-adapted plants; this is as predicted by community ecology theory [40,41].The bio-ecological process of plant community type evolution from cold to warm-dry type on the south slope of Altai Mountain was fully demonstrated.
Vertical vegetation zones are an important feature of alpine vegetation distributions [42,43].As the elevation of the mountain rises, the temperature decreases, the sunshine and wind increase, and precipitation increases.In the Xinjiang Altai subrange, the vegetation shows a complete vertical sequence along the southern slope of the Altai Mountains, with noticeable elevation gradients.From top to bottom, the altitudinal zones are as follows: ice/snow, alpine tundra, alpine sparse vegetation, alpine meadow, subalpine meadow, forest steppe, mountain steppe, and desert steppe.The natural altitudinal zones in the Xinjiang Altai subrange and along the north slope of the Altai Mountains constitute a complete and natural vertical spectrum of the Altai Mountains.
The Altai Mountains are oriented from the northwest to the southeast.The higher the temperature, the smaller the precipitation in the southeast.Therefore, the vegetation vertical belt structure in the Xinjiang Altai Mountains also shows a difference.The main performance is as follows: (1) The base belt of the vertical natural zone of the Kanas area is the forest steppe zone, which is located within the more humid western region of the Xinjiang Altai Mountains.Meanwhile, throughout the southeastern Fuyun and Qinghe areas with relatively little precipitation, the vegetation vertical natural zone is developed on the relatively arid desert steppe zone; (2) The vertical boundary of each zone in the vertical zone of the vegetation rises, the mountain steppe zone expands upwards, and the meadow vegetation zone is narrow.(3) The southern Taiga occupies the largest proportion, which is adapted to the cold and humid climate in the western part of the Altai Mountains; in the middle of the mountain, the steppe and semi-desert vegetation are extensively developed, and the forest is dominated by relatively drought-tolerant deciduous coniferous forest; the mountainous vegetation to the southeast is more afflicted by drought; the forest steppe zone in the southeast has replaced the forest meadow zone in the west; and the subalpine vegetation in the southeast shows different degrees of grassing.Therefore, many scholars have summarized this type of vertical belt structure as the vertical belt structure-type group of the Siberian-Mongolian mountain vegetation [44].
The distribution of vegetation is not only affected by environmental factors, which are dominated by heat and moisture, but is also related to plants' individual adaptability and variability to the environment and its location and geological history [45].Therefore, due to the heterogeneity of the environment in different regions, there are also obvious differences in the flora.Due to the complex and ever-changing geological features of the mountainous areas, the altitude changes greatly and the climate is divided, resulting in a large difference in the flora of each mountain system.
In this large-scale pattern, the Altai Mountains developed a variety of vegetation types.The Russian Altai Mountains, located on the northern slopes of the Altai Mountains, are the distribution center of the dark Taiga vegetation type, which is the typical Siberian jungle consisting of Abies sibirica Ledeb., Cedrus deodara (Roxb.)G. Don, and Populus davidiana Dode et al. [29].Due to the influence of the Atlantic airstream, the area is home to a large number of sub-alpine meadows and alpine meadows that are rare in other Siberian mountain regions.The flora in the area is dominated by elements of Siberian flora.The Altai Mountains of Mongolia are located on the western edge of the Mongolian Plateau and on the southeast slope of the Altai Mountains.Due to the mountains that surround the Mongolian Plateau, the entry of humid airflow is blocked.Therefore, due to the local Mongolian high pressure, the main vegetation types on this slope are desert and desert grasslands [46].The low slopes and valleys of the Altai Mountains of Mongolia are dotted with mountainous sub-desert belts.Due to the combined effects of geographical location and climate, the forest belt has disappeared in the Altai Mountains of Mongolia and is only scattered in the valley.The slope is dominated by drought-tolerant ancient Mediterranean components.The Kazakh Altai subrange in the southwestern slope of the Altai Mountains is located in the European-Central Asian grassland area, which has developed a wide range of temperate broad-leaved deciduous forests and typical grassland vegetation types [47].The forest and grassland vegetation types on this slope are mainly Pinus sylvestris L., Populus tremula Linn., Betula pendula Roth., Stipa spp., Koeleria spp., Festuca spp., etc.The forest steppe is dotted with more wetland marshes.Plants are mainly comprised of species in the frigid zone, subfrigid zone, and grassland area.In the southeastern mountain belt and plains of the Kazakhstan Altai Mountains, a large number of mesophytes and mesoxerophytes desert plants are distributed, and some ephemeral plants have been added.In the middle of the southern slope of the Altai Mountains, the Xinjiang Altai subrange is at an intersection zone of three major vegetation types: taiga forest, temperate broadleaf forest, and desert.Due to the control of the arid continental climate of central Asia and the Siberian-Mongolian anticyclone, the slope has provided a good foundation for the occurrence and development of various vegetation types.Due to the close proximity of Russia to the northwest, the climate is relatively humid; in the Xinjiang Altai subrange, the taiga-type coniferous forest of the northern coniferous forest belt enters Xinjiang and forces hard limits on the distribution area of the Eurasian grassland.It is entirely absent, only in a narrow band at the south of the subrange [46,48].
As the Xinjiang Altai Mountains are located at the intersection of the Siberian Circum-North Taiga, Holarctic coniferous forest, Eurasian mountain broad-leaved forest, Central Asian grassland belt, and Mongolian desert basin, the vegetation of this slope is not as single as the dark taiga of Russia's Altai Mountains, unlike the Altai Mountains in Kazakhstan, which dominate most of the mountains by the real steppe.Unlike the Mongolian Altai Mountains, which have disappeared from the forest belt.This special geographical location makes it possible for the Xinjiang Altai subrange to host a wide variety of plant communities.It has obvious features of interlacing and transitioning the cold temperate zone of the taiga forest in the mountainous area and the middle temperate desert zone, thereby causing a unique and irreplaceable change in the spatial distribution pattern of vegetation in the Altai Mountains.
Throughout the Altai Mountains, the extension of the mountain results in each slope being at a different latitude, which, coupled with the diverse geological and geomorphological forms of the mountain and the high difference between precipitation and local photothermal conditions of the mountain, has led to the diversification of vegetation types in the region.The vegetation from the south to the north is transformed from the humidityand shade-tolerant dark taiga vegetation type to the mesophytes and mesoxerophytes of the steppes and desert steppes; the vegetation from west to east is changed from the dark taiga forest to the typical meadow and then temperate broadleaf forest in the Altai Mountains of Xinjiang, and finally turned into the Gobi grassland in the Altai Mountains of Mongolia.

Conclusions
In our study, we identified eight vegetation types, 18 vegetation subtypes, and 50 formations, demonstrating a diverse range of vegetation types within the Xinjiang Altai subrange.Among them is the Form.Calligonum rubicundum and Form.Seriphidium borotalense.Festuca valesiaca is present only in the Xinjiang Altai subrange, which is unique among the rich vegetation types in the Altai Mountains.The Xinjiang Altai subrange is located at the junction of three major biomes containing unique vegetation types (coniferous forests, temperate broadleaf forests, and deserts).The Xinjiang Altai subrange is distinct in its staggered transition from mountainous boreal taiga to temperate desert.Therefore, it is both unique and irreplaceable in the spatial distribution pattern of vegetation in the Altai Mountains.

Plants 2023 , 19 Figure 1 .
Figure 1.The location of the study area.

Figure 1 . 1 Figure 2 .
Figure 1.The location of the study area.

Figure 3 .
Figure 3.The specific process of Vegetation map compilation.

Figure 3 .
Figure 3.The specific process of Vegetation map compilation.

Table 1 .
The vegetation types of Altai Mountain, Xinjiang.

Table 1 .
The vegetation types of Altai Mountain, Xinjiang.

Table 2 .
Structural characteristics of the typical vegetation communities in the Xinjiang Altai subrange.