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АРИДНЫЕ ЭКОСИСТЕМЫ, 2004, том 10, № 22-23
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УДК 581.52+582.26
ЭКОМОРФОЛОГИЧЕСКИЕ ПРОЦЕССЫ В ПОЙМАХ РЕК: ПРИМЕРЫ ПОБОЧНЕЙ Р. АЛЬЕ (ФРАНЦИЯ) И НИЖНЕЙ ВОЛГИ (РОССИЯ)
© 2004. Т.В. Балюк*, Я.Н. ван ден Берг**
* Институт водных проблем РАН, 119991 Москва, ул. Губкина, 3, Россия 2 **Утрехтский университет, 80115, Утрехт, 3508 ТЦ, Нидерланды
Русловые процессы на реках проявляются в постоянном переформировании пойменной суши. Наиболее интенсивно процесс эрозии и аккумуляции происходит на молодых русловых отложениях - побочнях и осередках. К концу XX века, когда в результате водохозяйственных преобразований русла многих европейских рек были спрямлены и укреплены, полночленные природные пойменные комплексы были нарушены, на фоне климатических изменений возник ряд новых водных экологических проблем. Одна из них - зимне-весенние наводнения. Для решения этих проблем была предложена новая концепция использования ресурсов рек: реставрация гидрографической сети с целью повышения саморегулирующей способности ландшафтов на основе воссоздания их естественной структуры. Таким образом, в настоящее время особую актуальность приобретает изучение структуры и функционирования пойменных природных комплексов в условиях незарегулированного речного стока. С этой целью были проведены исследования, в которых в качестве «эталонных» были выбраны два объекта - на Нижней Волге, в 50 км ниже по течению от Волгоградского гидроузла (побочень Закрутский) и на не зарегулированной реке Алье, берущей начало с Альпийских гор и впадающей в Луару. Исследования были сосредоточены на наиболее активных морфологических единицах пойменного ландшафта - побочнях и осередках. В течение лета 2004 года были изучены флора, растительность и русловые процессы совместно со студентами Нидерландских университетов из Утрехта, Дельфта и Наймегена.
Целью работ было определение закономерностей зарастания свежих русловых отложений (побочней) и установление факторов, играющих важную роль в этом процессе.
Исследования проводились с использованием методов дистанционного дешифрирования аэрофотоснимков на р. Алье за период 1992 - 2002 и космоснимков на р. Волга с 1967 по 2002 гг. и полевых наземных геоботанических и ландшафтных исследований. Были заложены 19 топо-экологических профилей. Для выявления характеристик частоты и продолжительности паводкового заливания анализировались гидрологические данные. На профилях, заложенных при помощи нивелира, делались описания пробных геоботанических площадок размером 100 м2, каждая точка привязывалась с помощью GPS к карте, а также отбирались образцы для определения гранулометрического состава аллювия. Для выявления возрастных характеристик морфологических элементов побочней и осередков был применен метод дендрохроноиндикации. При трансформации пространственных рядов растительности во временные были получены схемы сукцессии растительности на побочнях и возраст отдельных стадий.
Используя возрастные характеристики, были изучены изменения сообществ во времени по основным параметрам: общее проективное покрытие, обилие, количество видов в сообществе и удаленность от основного русла. Для Алье рост всех характеристик наблюдается до 5 лет, т.е., после этого времени пионерные группировки переходят в ранг сообщества, находятся в более мягких условиях аллювиальности начинается формирование молодых аллювиальных почв. После 20 лет, в 200 м от главного русла наступает стадия стабильности, древесно-кустарниковая пойменная растительность приобретает ярусную структуру, деревья имеют меньшую сомкнутость крон по сравнению с 7-10 летними зарослями. На Закрутском формирование сообществ начинается после 3 лет.
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ECOMORPHOLOGICAL PROCESSES IN RIVERS: EXAMPLES OF POINTBARS OF THE ALLIER (FRANCE) AND THE LOWER VOLGA (RUSSIA)
© 2004. T.V. Balyuk*, J.H. van den Berg**
*Water Problems Institute Russian Academy of Sciences, Russia, 119991, Goubkina Str., 3 , e-mail: baluk@aqua.laser.ru 2 Utrecht University, **The Nederlands, PO box 80115, 3508 TC
Utrecht, e-mail: r.vandenberg@geog.uu.nl Introduction
In the condition of river plains is forming current ecosystems: the islands and the point bar system. To understand how it goes very important for managements on riparian areas.
There is a growing demand from society to restore natural characteristics of alluvial rivers. However, in most large alluvial rivers in the western world natural ecological properties have been virtually disappeared, sometimes already for a long period of time. Therefore, there is a need of knowledge from natural ecosystems of comparable rivers elsewhere in order to be able to restore some of the original natural conditions of morphodynamics, biodiversity and vegetation dynamics. In the case of the Rhine plans to increase the discharge capacity of embanked floodplain areas are combined with ecological restoration (Duel, 2001). In this paper results of a comparative study are presented that was executed in pointbars of the the Allier near Moulins, France and the Volga downstream Volgograd, Russia. These rivers are considered as natural counterparts of the Rhine and the Meuse in the Netherlands (Middelkoop et al., 2004).
As compared to the Rhine the Volga is a much larger river. Conversely, the Allier is much smaller. Also, in the sense of climate conditions, bed material, river valley gradient and river parameters that characterize basic morphological properties, such as stream power, the Rhine take an intermediate position. This suggests that morphological processes in the natural Rhine also are intermediate between the Volga and the Allier. The present analysis is restricted to the most active sedimentary unit of the river, the inner bend or pointbar. This implies that vegetation succession is studied until the first stages of softwood forests and that vegetation in more remote overbank areas is not considered.
Description of study areas
Volga. With drainage area of 1360 thousand km2 and a length of 3700 km the Volga is the largest river of Europe. It has a very low river gradient over its total length and starts at a height of only 228 m above sea level. Over most of its length the river has turned in a cascade of 10 reservoirs and dams. Downstream of this dam at present the lower Volga is a large lowland sand-bed river with a natural morphology and extensive floodplains with natural vegetation of shrubs, trees and meadows. It is a 300-km long single to multi channel system. The main channel often shows meander characteristics. Since 1959 the discharge is regulated by a dam at Volgograd. The minimum discharge through the dam is about 5000 m3/s. Every year, a flood with duration of about 10 weeks is allowed with a maximum discharge of about 25.000 m3/s. The annual average of the discharge is 7600 m3s-1.
The studies were conducted on the Zakruktsky pointbar complex. Here, at low discharge the Volga has a width of about 900 m and a valley gradient of about 6 cm/km. The bed material of the Volga at the study area consists of fine to medium sands with a median diameter of 0.15 - 0.50 mm.
Allier. The Allier is often considered as the last "wild" natural river of the Western Europe. The river basin measures almost 13.000 km2. The river originates in the partly volcanic Massif Central, in Southern France, and flows 410 km to the north until it debouches into the Loire River. For a large distance the river follows the Limagne graben, where it is embedded largely in its own alluvial deposits. Within this alluvial reach originally a highly dynamic river pattern was present, with in many areas braided characteristics. Mainly as a result of recent climatic change, and to some extent riverbed incision due to gravel mining during the past century the river pattern has turned or is turning to a meandering pattern (Van den Berg, 1995). Nowadays revetments stabilize most of the bends of the meandering part of the river. The study areas consist of seven pointbars located in unprotected reaches at some distance north and south of Moulins. The minimum discharge is about 12 m3/s. Typical peak discharges amounting 800-1200 m3/s, lasting several days to weeks are produced by autumn to spring rainstorms. The average annual discharge is 140 m3s-1. The studied reach the Allier is a sand-gravel river, with a valley gradient of about 70 cm/km.
ЭКОМОРФОЛОГИЧЕСКИЕ ПРОЦЕССЫ В ПОЙМАХ РЕК 123
Median grain size of the bed material at the surface ranges between coarse sand and medium gravel. The floodplain is 2 - 2.5 km wide, and at low discharge the width of the river is about 90 m. On both sides the present floodplain is bordered by a series of Pleistocene river terraces (Veldkamp, 1991).
Methods
The analysis of this study is based on a combination of spatial data from aerial photos (Allier) and satellite images (Volga) and field observations of morphological features and vegetation in a number of cross-sections over pointbars. As flooding is important for the development of morphology and vegetation in time on pointbars, also hydrologic data were used. As the time scale of vegetation evolution from a bare surface up to soft-wood forest on pointbars and its interaction with morphology is estimated at no more than 10 years, hydrologic data were collected that cover this period.
For analyzing vegetation and morphological processes and patterns in time and space for the Allier study areas aerial photographs were used from 3 -5 years in the period 1992 - 2002. All photographs except those of 1997 were obtained from the Institut Geographique National in France. All photographs were georefereed, using the software program Erdas Imagine Orthobase. In addition from a number of overlapping photographs stereo projections were studied. These stereo images in some cases, enable differentiation between several tree species, especially so for the 1997 photographs. For the Zakrutsky study area (Volga-Akhtuba floodplain) spatial information was obtained using the following sequence of satellite images: 1986, 1991, 1996, 1999, 2000 and 2003. In addition high altitude aerial photo's taken by USA aircrafts in 1962, 1967, 1971 and 1977 (available at the United States Geological Survey website) were used.
In the field cross-sections were measured with a leveler, and located by means of GPS. Along these profiles the occurrence of plants and plant associations were determined and observations were made of bed material. Ages of trees were determinates by counting tree rings in cores made at right angle in the stem. The latter information is especially valuable in the case of Populus nigra, Salix alba, S.viminalis. Populus nigra as these trees begin to grow only on freshly deposited sediment and therefore are a proxy for the age of morphodynamic events.
In the Allier study areas daily measurements of water level and deduced discharges were used provided by the the gauges of Chatel-de-Neuvre and Moulins covering the period 1 January 1993 - 15 August 2003, From this data duration of flooding curves were made. As for the germination of plants and their survival the duration and time of flooding are important flood duration curves were made for all discharge waves surpassing 400 m3s-1. Also, the probability of flooding as percentage of the total period considered was determined.
Morphology, morphodynamics and vegetation
The morphological development of the pointbars in the Allier is quite different from the studied pointbar complex in the Volga. In all pointbars studied in the Allier, a similar division in depositional and erosional morphological units can be made. Accretional units are formed by scroll-bars and chute bars, erosional units are represented by chute channels and erosional banks of outer meander bends. A general picture of these features and their dynamic change in time is given in Fig. 1. Outer bend erosion is followed by the formation of a scroll bar in the downstream part of the inner bend. After a number of years the morphology of a scroll bar is stabilized and a new scroll bar may start to form. During high discharge events part of the flow cross-cuts the pointbar and one or several Fig. 1. Sketch of the morphology and age of a pointbar in the Allier (active bars are indicated). Рис. 1 Схема возраста и расположения морфологических единиц на побочне р. Алье (указаны только ежегодно перемещаемые бары).
The starting point of the Zakrutsky pointbar was an island in a rather straight section of the Volga. In
124 БАЛЮК, ВАН ДЕН БЕРГ
course of time this island became attached to the southern bank of the river. At the same time the northern channel bank was eroded and a meander bend was formed. In Fig.2 the evolution of this bar after 1986 is shown.chute channels may be formed, ending in chute bars. In the Allier, chute bars generally are formed half-way the pointbar. In the studied pointbar complex of the Volga, one big chute channel is cross-cutting the pointbar almost completely, eventually ending in a horse-hoe shaped, large chute bar that partly protrudes into the main channel.
1986 1991
1996 ^ 2000
2ППЛ \Y Legend Water line A >. T —* Flood flow vector * .--- Over bank areas .B-'^ffr Dense vegetation coverage c Light vegetation coverage ** Active front of chute bar or scroll bar *'■">. Inactive from of chute bar or scroll bar
Fig. 2. Evolution of the Zakrutky pointbar after 1986. Рис. 2. Схема развития побочня Закрутский с 1986 г.
The flow vectors in this figure are based on observations of flow direction and velocity deduced from bar alignments visible on photos and satellite images and from indications in the field. When an area becomes heavily vegetated, the sedimentation turns from fine sand to silt and clay, which demonstrates the strong reduction in flow velocity caused by the increased hydraulic resistance. In the case of the Zakrutsky pointbar the blocking of the flow by vegetation in the central part of the bar forced a concentration of the flow in the southern chute channel, which resulted in a rapid growth of a chute bar at the eastern tip of the pointbar after 1996.
Fundamentally, processes of outer bend erosion, inner bend scroll bar accretion and development of chute channel and chute bar morphology in the Volga and Allier study areas are comparable. However, as channel bends of the Volga are one order of magnitude larger, in a relative sense the dymanics of the Zakrutsky pointbar complex are about 10 times more slow. As a result of the much higher dynamics of the Allier study areas, most of the surface of the actual pointbars is produced in the studied period of 10 years, whereas only a small percentage of the surface of the Zakrutsky pointbar was created in this period. Vegetation succession therefore in general can proceed to higher stages in the Zakrutsky area before it is removed by outer bend erosion. Also, because of higher dynamics, the percentage of area without any vegetation or covered by early pioneer stages in the actual situation is larger in the Allier study areas. On
ЭКОМОРФОЛОГИЧЕСКИЕ ПРОЦЕССЫ В ПОЙМАХ РЕК 125
the other hand, with respect to its much lower dynamics, it is remarkable that a rather large part of the Zakrutsky pointbar remained practically uncovered by vegetation for a long period. This is explained by the difference in bed material. Because of the small size of the bed material in the Volga area it is transported as suspended load, whereas in the Allier practically all bed material is transported as bed load. Due to the virtual absence of transport of bed material in suspension in the Allier, morphodynamics are governed by processes of lateral accretion and erosion: after the accretion by a scroll bar front or chute bar front any further deposition of bed material is negligible. Instead, only some clay may deposit from suspension, promoting the germination of pioneer plants. In the case of the Zakrutsky pointbar, however, processes of vertical accretion by deposition of sand from suspension are very important.
Flora and vegetation of Volga and Allier point bars.
The Volga flood plain downstream of Volgograd has a poor flora, as considering its large extension. Due to the long period of about a month of inundation in the spring only species are present that can survive such a long period, followed by a long season of aridity. As a consequence only 319 species from 68 families are present, dominated by representatives of the families Asteraceae, Poaceae, Cyperaceae; these families also demonstrate the general azonal situation. Zonal species of the steppe, that surrounds the floodplain, such as xerophytic grasses and typical Artemisia, Astragalus and Allium species are hardly found in the floodplain. Instead, characteristic species of more northern regions are present, such as Carex, Galium, Thalictrum and Equisetum etc. For the Volga-Akthuba plain typical flora elements are species of the genera Chenopodium, Atriplex, and Kochia. The conditions for the vegetation on the Zakrutsky pointbar are even more demanding due to the strong flood influence. Only 126 species from 38 families were found on this pointbar, dominated by Asteraceae, Poaceae, Cyperaceae, like in the entire floodplain.
For the Zakrutsky area 11 associations were recognized when using the dominant method, and 10 associations when applying the Braun-Blanquet procedure. The following dominant, subdominant and secondary plants were selected for classification of phytocenosis and to characterize associations:
Table 1. Hydrological characteristics of habitats on Zakrutsky area.
Таблица 1. Гидрологические характеристики сообществ и биотопов на побочне Закрутский.
Name of community High, sm Duration, days per year Probability, %
pioneer vegetation 20 30 100
Phalaroides arundinacea+Eleocharis palustris 70 30 100
vegetation of chute channels 81 30-40 100
Bromopsis inermis+Populus nigra, Salix alba 550 25 85
Senecio jacobea+Crypsis alopecuroides 516 20 85
Populus nigra L.+Elytrigia repens 534 25 85
Bolboschoenus maritimus 550 25 85
Stachys palustris L. 520 20 85
Glycyrrhiza glabra 563 15 80
Salix alba L. 459 28 90
Based on the occurrence of the communities with respect to the low water level as recorded in the field and the flooding data from the used gauging stations, some hydrological characteristics where established (see table 1). On Allier's pointbars 306 species of plants from 61 families were found. The dominating families are: Asteraceae, Poaceae, Polygonaceae, and Cyperaceae Dominating plant genera the flora complex of the flood plain can be characterized as spare and mesophytic with indications of azonality.. The plant species appeared to be organized in 20 associations according to the dominant method and 16 associations when applying the West European syntaxonomy method (Braun-Blanquet). Based on the occurrence of the communities with respect to the low water level as recorded in the field and the flooding data from the used gauging stations, some hydrological characteristics where established (see table 2).
126 БАЛЮК, ВАН ДЕН БЕРГ
Table 2. Hydrological characteristics of habitats on the Allier pointbars.
Таблица 2. Гидрологические характеристики сообществ и биотопов на побочнях реки Алье.
Name of community Название сообществ High, sm Высота по Duration, days per year/Продолжительно Probability % Частота
рейке сть паводкового паводкового заливания,
профиля, см заливания (лней в году) %
Corrigiola litoralis 0 50-140 89
Lindernia dubia 0 50-140 89
Eragrostis pilosa+Amaranthus alba 20 40-130 88,5
Salix purpurea+Populus nigra 20 40-130 88,5
Mathricaria maritima+Artemisia 10 30-130 89
vulgaris
Elymus repens 190 12-60 83,5
Sedum album 195 12-60 84
Poa angustifolia 197 10-55 84
Corynephorus canescens 200 10-55 84
Phalaris arundinacea -13 50-150 89,5
Eleocharis palustris -12 50-150 89,5
Lythrum salicaria 0 50-140 89
2. Salix purpurea 210 10-50 84
1. Salix alba 210 10-50 84
3. Populus nigra 210 10-50 84
Lemna minor -12 50-150 89,5
Populus nigra (Salix alba) 250 5-35 82,5
Comparative analysis of dominant families shows the same trend for both areas. T he dominant families
are Asteraceae and Poaceae. This is explained by the fact that these families contain many plants typical for the azonal characteristics of floodplain vegetation indicating a similar character of flora complex and azonal feature of floodplain vegetation.
Communities are more diverse on the point bars of the Allier, because of the larger variation of habitat, caused by the larger sediment variability. Also, the more frequent fluctuation of water level and the absence of long periods of inundation, which is letal for many species in the Zakrutsky area, and the higher habitat variation is in favour of this. A third factor that contributes to a more diverse vegetation in the Allier, is the delivery of seeds from proximal mountainous habitats. Although the Zakrutsky pointbar complex is one order of magnitude larger than the Allier pointbars, it contained only 12 associations, 4 less than on the Allier pointbars. In both areas the similar vegetation of moist habitats is found, consisting of Eleocharis palustris and Phalaroides arundinacea in swamps, and aquatic vegetation: Lemna minor, Potamogeton crispus in ponds and standing water; Also the pioneer community Eragrostis pilosa + Amaranthus alba and The meadows dominated by Elytrigia repens are very similar. Ecological analogs are Sedum album (Allier) and Senecio jacobea + Crypsis alopecuroides (Volga) Populus nigra and Salix alba are dominant trees in floodplain forest of both areas. Habitats of similar humidity in both areas have the same plant community or ecological analogs.
Main factors determining vegetation development
The vegetation on pointbars of the Allier and Volga shows many similarities. In both areas the communities and their dominant species show clearly the mesophytic, azonal character of the vegetation (Fig. 3). However, there are also some clear differences related to: (1) hydrodynamics (frequency and duration of flooding, flow energy), (2) morphodynamics (sedimentation and erosion), and (3) physical conditions (sediment characteristics and climate).
ЭКОМОРФОЛОГИЧЕСКИЕ ПРОЦЕССЫ В ПОЙМАХ РЕК 127
The longer duration of floods in the Volga explains the higher percentage of mesophytic plants on the Zakrutsky point bar. The group of real water plants (helophytes) is smallest, and the group of mesophytes (plants of moist biotopes) is largest - 62 and 51% on Volga and Allier respectively. The percentage xerophytes in Allier is relatively high (38%) because of the real water plants -hydrophytes and helophytes, plants of moist biotopes (mesophytes) and plants adapted for live in arid conditions (xerophytes). shortness flood periods - 2-4 days, whereas in the Volga the duration of a flood can be more than a month. A special adaptation to the long xerophytic period following the flooding is shown by Bolboschoenus maritimus. It forms special "tubers" on its roots, for vegetative reproduction. During the xerophytic period this plant forms "meadows" of completely dry plants.
70
60
50
40
30
20
10
62
SI
□ Zakrutsky
□ Allier
38
28
10
11
mesophytes xerophytes
helophytes
Fig. 3. Comparing of floras by ecological groups (Allier and Volga).
Рис. 3. Соотношение разных экологических групп в локальных флорах побочней р. Алье и Нижней Волги.
In Allier, floods change the phytocenosis of the environment very quickly. Meadows may be transformed to bare surfaces, plants may be taken away with the flow, or buried by a layer of sediment. Tree and shrub vegetation is removed during bank erosion. A sketch of the rejuvenation process in pointbars of the Allier is presented in Figure 4.
Fig. 4. Simplified scheme of floodplain rejuvenation.
Рис. 4. Схема идущих в пойме морфологических процессов.
Every year the flood water brings a number of seeds of Populus nigra, Salix alba, and S. purpurea to the point bar. These seeds are deposited at the water-line and their germination gives rise to rows of trees that follow the original curve of the pointbar. However the seedlings of these trees can only survive if between two high floods enough time is available for the production of leaves and start of photosynthesis. If the time between succeeding floods is too short, no rows of soft wood forest develop and instead a herbaceous pioneer vegetation will develop. On the Zakrutsky point bar the duration of flooding may be a АРИДНЫЕ ЭКОСИСТЕМЫ, 2004, том 10, № 22-23
128 БАЛЮК, ВАН ДЕН БЕРГ
limiting factor for Populus: Probably this explains why this tree is only found here at a minimum height of 3.5 m above the low water level.
The structure of pioneer vegetation on pointbars of the Allier is more complex due to the variable composition of the bed material. The upstream coarse gravel part of the pointbar is generally almost without plant cover, or with some Populus or Salix seedlings. In the central part of the pointbar, with mixture of gravel and sand, communities with Corrigiola litoralis and Portulaca oleracea appear. On patches of gravel covered with a thin layer of clay the community of Lindernia dubia is found. On sandy areas, normally found in the downstream part of the pointbar, communities with Eragrostis pilosa, Xanthium orientale, Datura stramonium and Amaranthus album are present. The pioneer communities exist only for one summer season: during the next flood they are either covered by fresh alluvium, moved away with the flow, or develop into the next stage of succession.
Pointbars in the Allier show a more or less regular shift in the average downstream direction of the river plain. As a result, community age depends on this downstream migration rate.
The most dramatic effect of sediment transport on vegetation is rapid deposition of bed material or suspended material and erosion of river banks. Most herbs and other low plants will die if covered by a layer of several decimeters of sand or gravel. These layers prevent a continued succession of pioneer vegetation, and stop the development of perennial vegetation. Only willows and Populus trees are able to survive, unless they become completely covered by the sediment. In a relative sense, the process of construction and erosion of pointbars in the Volga is going more slowly. Consequently, vegetation succession may proceed to higher levels before it will be reset by outer bank erosion. On the other hand, rapid deposition of sand from suspension on pointbars of the Volga may inhibit any succession for a long period of time. Satellite images of the Zakrutsky area suggest a bare surface of virtually the whole area up until 1970. This history is supported by the age of the oldest trees, which, according to tree rings countings are 35 years at maximum. In contrast to the Zakrutsky pointbar, where rapid deposition can occur over vast areas of the pointbar simultaneously, in the Allier areas of rapid deposition are restricted to relatively narrow zones that are subject to the process of accretion of a scroll bar or the progradation of a chute bar front.
As rapid sedimentation on chute and scroll bars fronts prohibits the development of perennial vegetation, these areas form a specific habitat for one-year pioneer communities, which lodge a limited group of about 20 plants that are adapted to these special conditions:
1. Except for winter conditions, many of these plants are able to germinate and grow quickly directly after the passage of the river flood on the still humid substrate (Corrigiola, Portulaca, Lindernia, Eragrostis).
2. The time for flowering always occurs after high floodwater, at the end of summer.
3. Seeds have special surface for floating in water It is one of the major dispersal mechanisms for plants along rivers. Seeds have various means to float effectively, such as large volume, juicy or fleshy outer layers, and various appendages (Malanson, 1993).
4. During high water leaves get enough light for continuing photosynthesis.
5. The epidermis of the leaves is covered with a special wax, for saving water inside during a dry period.
6. Pollen are transported and delivered to the receiving flowers by water.
7. Due to their morphology Corrigiola, Portulaca, Polygonum are able to reduce evaporation of the soil.
Pioneer trees and shrubs have resting buds on stems, and roots from it start to grow when buried by
sediment.
Plants like Polygonum scabrum and P. minus have flowers on top of stems that remain above the water enabling insects to reach them. In dynamic parts of point bars, gravel shores often show Corrigiola litoralis-Portulaca oleracea communities. Co-dominants of these associations can be Polygonum aveculare and Digitaria ischaemum. In less dynamics places where some clay can settle
the dominant species is Lindernia dubia and co-dominant - Echinochloe crus-galii is found. On sandy bars other dominants are found - Eragrostis pilosa, Xanthium orientale, Datura stramonium, Amaranthus album. These are temporary communities; they exist only one season and in next year, after high water the location of the community will be moved with the morphological unit. Less dynamic parts of a point bar, covered by coarse and medium gravel, are characterised by rough-herb communities with Mathricaria maritima and Artemisia vulgaris, having a low cover, near 5%.
ЭКОМОРФОЛОГИЧЕСКИЕ ПРОЦЕССЫ В ПОЙМАХ РЕК 129
Vegetation successions
Using the age of trees a general remarks of changing characteristic of communities cover, abundance, and number of species over time was made for the Allier and Volga study areas.
For the Allier the increase of all these characteristics starts after 5 years of age, at a distance of about 50 m from the low-water mark in the main channel. At this point the habitat escapes from intensive flooding. Succession enters into a stage of more stability after 20 years, when young soils appear and forest become mature - at a distance of 200 m from the main channel and higher than 2 m above water level. In bars and channel made by chutes the succession is "reset" to pioneer stages, followed by wetland communities with plants like Lythrum salicaria, Filipendula ulmaria, Stachys palustris, Eupatorium cannabinum, Lysimachia vulgaris, Phalaris arundinacea, and Thalictrum flavum. Vegetation succession is determined by a number of factors, of which the presence of water is the most important.
Conclusions
Although the Allier and Volga are located in different climate and vegetation zones, and the sediment characteristics are quite different, the riparian flora, vegetation structure and vegetation succession on the pointbars of these rivers show much similarities. On the same morphological units the same communities are found. Along chutes and left-behind or abandoned channels associations of Phalaris arundinacea + Eleocharis palustris are found. On meadow habitatsPopulus nigra + Elytrigia repens are usually found. In both areas young alluvial forest of Salix alba+Populus nigra is common. On clay deposits communities with similar ecology characteristic are found: in the Allier Sedum album + Mathricaria maritima and on the Zakrutsky bar complex Senecio jacobea + Crypsis alopecuroides. Also, three similar directions of succession are present, starting from a bare surface (1). Herbaceous pioneer vegetation merging into young herbaceous community and ending in Elytrigia or Bromopsis meadows; (2). Development of young alluvial forest and (3). Riparian vegetation of abandoned channels. Differences of vegetation between both areas are mainly caused by the different influx of flora elements from adjacent ecosystems (steppe in case of the Volga), or transported by the river from upstream areas (mountaneous flora elements in the case of the Allier). In the Volga, due to the longer inundation by flooding and the aridity of the climate the species list of flora in the Zakrutsky area is less abundant as compared to the Allier. As a general conclusion it can be stated that many characteristics of riparian vegetation of natural rivers which in morphometric and hydraulic sense are comparable to the original, natural Rhine river, and found at the same latitude, are robust features. This strongly supports the hypothesis that vegetation characteristics that are similar on pointbars of the Volga and the Allier were also present on the pointbars of the former Rhine distributaries in the Netherlands such as the present day Waal river. However, further comparative analysis flora and vegetation characteristics of restored nature areas on the banks of the Waal and other distributaries of the Rhine with the results presented in this report is necessary to provide conclusive evidence for this statement.
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