Plant Communities of Economically Valuable Forest-Forming Species of the Orenburg Region

The article provides an analysis of their sinusial structure, the species composition of vascular plants of the Ural River floodplain within the steppe zone is studied in detail. For the first time, a classification has been carried out for the study area and a phytocenological characteristic of all tree-shrub vegetation has been compiled; patterns of ecological-dynamic changes in the types of floodplain forests are revealed. It has been established that the Tilia cordata and Quercus robur formations are quite rare for the territory of the Orenburg region. The growth class of oak forests is assessed in grades II – IV, and linden forests in grades III. Two groups of types of oak forests are characteristic of the floodplain of the study area: blackberry oak and lily of the valley oak. Linden associations are represented by two-tier plantations. Tilia cordata dominates in the first tier, Ulmus laevis and Quercus robur form the second tier. A decrease in the floodplain area under Quercus robur and Tilia cordata on the territory of the Ilek and Tashlinsky forestries was noted. Phytocenoses are transformed by replacing Quercus robur formations with Ulmus laevis formations.


Introduction
The territory of the Orenburg region belongs to sparsely forested areas. Often, the floodplain is almost the only habitat of natural woody vegetation. Therefore, given the important role of forests in protecting the environment and their fulfillment of water protection, anti-erosion, hygienic and other special functions, which are of great importance in the conditions of arid climate, their deep and comprehensive study is necessary. Orenburg Forests are rare plant communities that provide shelter for many rare and endangered plant species such as Quercus robur L. and Tilia cordata Mill [1][2][3].

Problem Statement
An increasing anthropogenic load leads to disruption of the formed natural complexes. Floodplain forests and meadows, as well as the river itself, are increasingly used by man for economic purposes [4,5]. The emerging environmental situation is exacerbated by the powerful industrial potential in the river basin, especially in the chemical, oil and gas industries. In this regard, it is important and necessary to study the preserved sections of forest vegetation, their current status and recovery trends under conditions of the termination of anthropogenic impact [6][7][8].

Experiment
The oak forests of the Ural floodplain are a heterogeneous group of associations differing in the composition and structure of the phytocenoses formed by them, which together make up the floodplain oak formation.
In the ecological ranks of the floodplain forests, Quercus robur is characterized by habitats flooded for a period of about three weeks, and on average for 6-10 days. Oak seedlings and undergrowth tolerate flooding no higher than 50 cm and no more than 10-12 days. Young and middle-aged Quercus robur trees withstand alluvium falling asleep at the bottom of the trunks, while maintaining their viability. In old age, Quercus robur flood resistance decreases. Oak forests occupy predominantly elevated positions (banks of oxbows, manes, elevations) of the central and terrace floodplains, where drainage conditions are better and the duration of flooding is not particularly long.
The reduction of oak forests and the shift of the border of their distribution in the floodplain from 49 ° 40 'N 51 ° 15 'N is not only the result of the deterioration of the forest-growing conditions of the floodplain, but also the prolonged ruthless extermination of this breed by humans, as well as grazing in floodplain forests, leading to the death of undergrowth.
The population of Quercus robur in the floodplain usually occurs under the canopy of elm trees and shrubs, which facilitate the competition of this breed with grassy meadow vegetation and protect Quercus robur from adverse climatic conditions. In the floodplain of the Ural River, oak forests are mainly of overgrown origin; in this regard, they are characterized by worse development and productivity. Seed regeneration of Quercus robur in the floodplain is unsatisfactory. Seed regeneration of Quercus robur in the floodplain is unsatisfactory. The flowers and acorns of this oak often die from late frosts.
The growth class of oak forests is rated in grades II -IV, with a slight improvement in the growth class of stands to grades I а and I. A slight decrease in the productivity of oak forests from north to south is due to a change in climatic conditions in the direction of increasing dryness. In the south, the degree of violation of oak forests by human activities is also increasing. On average, the fullness of oak groves is 0.5-0.7.
Two groups of types of oak forests are characteristic of the floodplain of the study area: blackberry oak and lily of the valley oak. The latter are more common in the Ural floodplain. Floodplain oak forests, in comparison with oak forests on the watershed and in the mountains, are characterized by a somewhat depleted and more uniform composition. Plants characteristic of meadows make up a significant proportion in the grass cover.
Blackberry oak forests (Quercus robur -Rubus caesius) are widespread in high sections of the central floodplain with a short-rate regime on thick flood-plain meadow sandy soils. The forest stand in the oak forest has a crown density of 0.5-0.6, represented by one or two tiers. The second tier, as a rule, is occupied by Ulmus laevis. The forest stand in the oak forest has a crown density of 0.5-0.6, is represented by one or two tiers. The second tier, as a rule, is occupied by Ulmus laevis. Sometimes Acer negundo is mixed with it, forming a third tier. In Blackberry oak forest the growth class of Quercus robur is rated in grade III. Average height 16 m.
The shrub layer is represented by Rosa majalis -sol -sp, Lonicera tatarica -sol, Prunus spinosasp, found on the edge of the forest. Under the forest canopy, usually Frangula alnus -sol.
In the Blackberry oak forest in the first tier, Populus nigra at the age of 60 years and individual trees of the shrinking Quercus robur are noted. This, as well as the absence of Quercus robur undergrowth, indicate that the environmental conditions necessary for the normal growth of this tree species have changed for the worse. At the same time, the presence of good undergrowth of smooth elm (380 ind./ha, aged 10-14 years) indicates a possible change in oak groves by elm trees.
Blackberry oak with viburnum (Quercus robur -Viburnum opulus -Rubus caesius) with a crown density of 0.4-0.5 was noted on the mane of the central floodplain, on floodplain meadow soils. The height of Viburnum opulus reaches 4.5 m, stand density is 0.8. At the edge of the forest is mixed with: Primus spinosa -sp, Rosa majalis -sol, Lonicera tatarica -sol and Ribes nigrum -sol. The absence of Quercus robur undergrowth and the abundant Ulmus laevis undergrowth (up to 1200 ind./ha, with an average height of 7 m, aged 10-15 years) and the blackberry elm forest, located in the immediate vicinity of the oak grove, indicate that in the case of Quercus robur felling he will be replaced by Ulmus laevis.
One of the modifications of blackberry oak forests is nettle oak forest, located in the central flood plain south of Utvinka village on floodplain-alluvial soils enriched with excrement of birds nesting here.
Quercus robur is also missing here. There is undergrowth of smooth elm (139 ind./ha, with an average height of 4-5 m, aged 6-7 years), but it is not as plentiful as in previous cases.
Kirkason oak forest (central floodplain) (Quercus robur -Aristolochia clematitis) grows on the slopes of manes, 4.5-6.5 m high in the central floodplain. Overgrown oaks at the age of 70 years have the lowest growth class -V. The tree stand is a two-tier, as in previous forest types, represented by Quercus robur and Ulmus laevis. The degree of closure of crowns is 0.5. Of all the oak forests, this type of forest is the least productive.
Renewal submitted by Quercus robur. The growth of seed oak is 278 ind./ha, 15 m high at the age of 1-2 years. Occasionally, 10-year-old Ulmus laevis undergrowth is 4 m high, 3-4 cm in diameter.
At higher short-level floodplains, blackberry and kirkazon oak forests are replaced by forest types dominated by lily of the valley.
The most common type in the Quercus robur formation is the lily of the valley oak forest (Quercus robur -Convallaria majalis). Oak forests of this type are confined mainly to higher rarely flooded floodplain areas and are located along the banks of oxbows and their gentle slopes, most often of northern and north-western exposure. The soils occupied by this type of forest are floodplain meadow thick.
Lily of the valley oak forest is represented by seed and overgrown stands. One-tire tree stand, composed of Quercus robur, sometimes mixed with Populus tremula, Populus alba, Ulmus laevis, Acer negundo. One-tire tree stand, composed of Quercus robur, sometimes mixed with Populus tremula, Populus alba, Ulmus laevis, Acer negundo. However, the accompanying Quercus robur tree species do not form a pronounced tier, except for Ulmus laevis and Acer negundo. The age of the oak is 70-80 years, the crown density is 0.6. Growth class with a grade of III-V prevail. Growth class with a grade of III-V prevail. The growth class of preserved sites of seed origin are evaluated I and I а .
Linden associations are represented by two-tier plantations. Tilia cordata dominates in the first tier, Ulmus laevis and Quercus robur form the second tier. Tilia cordata, aged 70, has a growth class of grade III.
Tilia cordata, aged 70, has a growth class of grade III. Individual Quercus robur trees, 10 m high and 45 cm in diameter, have the lowest IVa growth class. The youngest was seed elm at the age of about 40-45 years. Stand density 0,94. Crown density 0,6. Linden is in excellent condition, bears fruit. There is no shrub layer. Occasionally, Prunus spinosa is found at the edge of the forest.

Conclusion
Formations Tilia cordata and Quercus robur are quite rare for the territory of the Orenburg region. In the flora of the studied territory, we noted 8 species included in the list of rare and endangered plant species of the Orenburg region. Most of these species are represented in the Quercus robur formation, in such types as celandine oak, lily of the valley oak, hazel-lily of the valley oak, shrub-lily of the valley oak, shrub-lily of the valley aspen. At the same time, a decrease in the floodplain area under Quercus robur and Tilia cordata on the territory of Ilek and Tashlinsky forestries was noted. Substitution of the Quercus robur formations with the Ulmus laevis formations is noted [20,21].
In order to restore, preserve the genetic fund, increase productivity and enrich the quality composition, linden forests and viburnum-lily of the valley oak forests must be protected and organized protected objects of extractive reserve type; to ensure the restoration and protection of oak forests of hazel-lily of the valley, shrub-lily of the valley, celandine.