## GENESIS OF FLORA AND VEGETATION OF THE DANUBE LOWLAND IN RELATION TO THE HYDROELECTRIC POWER STRUCTURES GABCIKOVO-NAGYMAROS

#### Ladislav SOMSAK1, Ferdinand KUBICEK2

1 Department of Pedology, Faculty of Natural Sciences, Comenius University, Mlynska dolina, 84215 Bratislava, SLOVAKIA
2 Institute of Landscape Ecology, Slovak Academy of Sciences, Stefanikova 3, 814 99 Bratislava, SLOVAKIA

CONCLUSIONS This study is directed to analysis of long-term development of flora and vegetation of the floodplain ecosystems in the Danube lowland. This genesis is divided into four periods, connected with the regulation of the Danube river from the long past until present (period of flowing main masses of waters through the line of the Small Danube river up to its turning to the approximate present river-bed, period since the finishing the construction of protective dikes till the end of the fifties, period of a gradual decreasing of the water level in the river-bed up to its damming, and finally, the period since the damming of the Danube river).

Single considerations based on real state prior to and after the construction of the Variant C and the reaction of the vegetation to the changed hydropedological conditions. The given data are related to the Slovak side of the ecosystems, but in the case of realisation of the proposed measures in Hungary (increase of the water supply into the Mosoni Danube, construction of underwater weirs in the old Danube river-bed and others) could be valid after certain corrections also for the right-bank part of the Danube river.

The study do not cover negative consequences of the building of the hydroelectric power structures Gabcikovo-Nagymaros, like removing of the floodplain forest in the extent of 3,267 ha on the Slovak side, synecological changes of the draining part under the Hrusov reservoir and in the narrow shore belt (aggregated rampart) along the set aside the Danube river-bed. But they can be solved by suitable management (plantation of cultivar poplars by depth planting, which was already authorised).

Attention is paid also towards the fact, that the questions of vascular plant biodiversity are being observed only now. But there are no evidences on lowering of phytogenefund from the present experience during the two years since the damming. On the contrary, new biotopes are created by watering of the river arm system in the inundation area (Dobrohost - Palkovicovo) and a huge limozic and littoral zone around the Hrusov reservoir gives the assumptions for the richness of the biodiversity.

Indicated changes as the consequences of historical adaptations of the Danube river can be demonstrated. It is quite problematic to predict the further long-lasting changes, even if they were already stated, however, under different conditions [5]. They have been indicated in the basic features also for the Danube river [11], or also for the planned Wolfstahl project [33]. Basically, it regards on reciprocal equalisation (decrease and on the other hand increase of the groundwater levels), which gives rise to destruction on the one side, and on the second side to rise of the same synecological conditions [15].

In connection with the prediction of the expected changes it is possible to turn to 100 years past of the Danube river. For the period of construction of protective dikes all ecosystems have been adapted, stabilised to the changed conditions. At least, it affirms a majority of authors, introducing the inner delta of the Danube river as a European unique. Thus, it is possible to foresee also the adaptation to these new conditions. Mainly as, that from 80 % simulated condition of the so-called "unique state" (watering of the river arm system, possibility of flood simulation, increase of the groundwater levels underneath Bratislava, periodically flooded margins of the Hrusov reservoir).

Our experience since the end of the fifties is such, that by the decrease of waters after a wide adaptations of the Danube river-bed and after excluding of the sedimentation of deposits by the influence of dams in the Austrian and German stretch of the Danube river, and on the contrary, after the continuation of the erosion trend of the Danube's river-bed, floodplain forests would disappear on the Slovak side of the Danube river. The Hydroelectric power structures Gabcikovo and mainly the Variant C prevented this regression. One huge laboratory has arisen, in which influences can be observed for a long time.

#### INTRODUCTION

Floodplain forests of the Danube lowland are a known object from the botanical point of view. First of all Jurko [9] was deserved about their knowledge. He studied (in 1954-1957) in detail soil-ecological conditions of floodplain forests, water regime of the Danube river in relation to vegetation and their syntaxonomic value. He seized such a way a more-less original state, corresponding to former hydro-pedological regime of the Danube river. Part of these forests under Bratislava (the Vlcie hrdlo - Kalinkovo) was interpreted on detail phytocoenological map in scale 1:10,000 [13]. This map gives at present a valuable basic material for comparison of all changes. After classical Jurko's work [9] followed a study of the other Slovak floodplain forests, mainly along the Morava river [31], in the Tisa river lowland [1], downstream part of the Hron river [35], middle part of the Vah river [16] and the attention was simultaneously paid also to submountain floodplain forests [12, 10, 32, and other]. Detail study of floodplain forests in the northern part of the Danube lowland in relation to soil conditions was made by Dzatko [6]. Vegetation of stagnant waters and dead arms of the Danube river was characterised in detail in [25, 26]. Also phytocoenoses of bank ecotopes of the Slovak rivers floodplains are sufficiently studied [34, 30, and other]. Kubicek et Somsak [18] and Kubicek et al. [17] paid attention to synecological conditions and primary productivity of floodplain forests. At the same time is known reaction of some trees of floodplain ecosystems to changes of groundwater levels by variation of leaf area and leaf litter [24, 25].

Somsak [33] and Jurko [12] have tried to predict changes in floodplain ecosystems in connection with the hydroelectric power structures Gabcikovo-Nagymaros. Complete documentation and inventarisation of the Slovak side flora in the Danube lowland was finished in 1986 by Bertova et al. [2]. All changes of floodplain ecosystem's biota on the Slovak side of the Danube lowland have been monitored since 1991 [19, 20, 21].

So, from given survey it is possible to confirm, that flora of the Slovak floodplain ecosystems, including inundation territory of the Danube river, belongs to the best defined and known types of vegetation. Therefore, it is possible to suppose, that also influence of the hydroelectric power structures Gabcikovo-Nagymaros, including Variant C, can be correctly and objectively judged by the Slovak scientists.

#### FLOODPLAIN FORESTS PRIOR TO CONSTRUCTION OF BARRAGE SYSTEM

Period prior to construction of the Gabcikovo-Nagymaros barrage system can be divided to three stages:

- prior to construction of protective dikes along the Danube river
- period since the finishing of regulation works in 1890-1900 until 1960
- period of decrease of the Danube river waters since 1960 until damming of the Danube river at Gabcikovo (1992).

#### Period until regulation of the Danube river

This period is known from the first maps since the 4. century before our era until 1820 (Jakubec, 1993). It was characterised by flowing down of the main mass of waters in the lines of present Small Danube river and the Black water river and with smaller discharge in right-bank turning from the Danube river, approximately in line of the Rusovecke rameno arm, continuing further to Komarno (comment: it is possible to suppose that it goes more-less in the line of the present flow). These two main streams bordered the large Zitny ostrov island. It is necessary to realise, that left-bank turning represented the main navigation direction and it was regularly adapted.

Already after floods in the 18. century and after long drying up waters had turned aside to present river-bed of the Danube river in the beginning of the 19. century. Information about building up of protecting dikes are considerable confused, but for sure their construction had been started already since 1235 (according to some other data in 1424 [9]) and they obtained a definite appearance already in the beginning of the 20. century. A lot of arms described on maps from that period [8] allow to suppose, that a considerable part of the Zitny ostrov island was regularly flooded. But they existed there numerous elevations, which were not flooded even catastrophic floods. Prevalence of willow-poplar floodplain forests, mostly adapted to recurrent floods, it is possible to estimate by reconstruction of the forest coverage in the area.

#### Period since finishing of regulation works till 1960

This period is characterised by finishing of construction of anti-flood dikes along the both sides of the Danube river, approximately 100 years ago. It represented the first serious interference to the original floodplain ecosystems. Arisen inter-dike space included essential change in hydrological and hydropedological regime of this territory. Floods, which unequally befallen before construction of protective dikes the whole Danube lowland (it means to the Small Danube river), were limited to inter-dike area. But there were more frequent there, several times per year, and their intensity was extremely increased. So, since finishing of the regulation works till 1960 a change of the original conditions, to which biota (in inter-dike space and also in inside of lowland) had to be adapted (adaptive succession), occurred. In summary, manifested changes are as follows:

- loss of communication of open water level in arms inside of lowland with water of main channel
- exclusion of direct surface floods in inside part of the Zitny ostrov island
- exchange of reophile types of arms to dead arms with stagnant water
- destruction of littoral communities of reophile arms inside of lowlands and a rise of stagnant water phytocoenoses (Lemnetea, Potametea, Phragmito-Magnocaricetea and others)
- changes in oscillation of groundwater levels in inside part of the Zitny ostrov island in relation to rhythm of water in the Danube river, eventually in inter-dike space
- floristic, phytocoenological and synecological re-building of azonal floodplain forest in inside of lowland to climazonal ones (Ulmo-Quercetum)
- planar re-building of floodplain forests in inter-dike space in benefit of willow-poplar communities
- increase of number (area) of reophile arms in inter-dike space
- increase of erosion-accumulation activity of the Danube river and its arms in inter-dike space and multiplication of primary succession (recurrent settlement of new deposits).

Hundred years period since construction of protective dikes is a very short period for development of vegetation. But, it was sufficient for floodplain ecosystem to create a certain stability against changing conditions. However, floodplain forests, adapting in that area, cannot be considered for original ones. They are a natural replacement in conditions already twice changed by human. They bring certain traces of natural development and therefore we describe these communities as "natural forests".

According to available sources [9, 13] the classification of these forests is as follows:

- willow-poplar floodplain forests (Salici-Populetum)
- They settled all the lowest situated habitats to groundwater level (far from flow), as well as elevated aggregated ramparts with easy soils. Floods had been at that places several times per year, mainly in June - July.
- ash-poplar floodplain forests (Fraxino angustifoliae-Populetum albae) as transitional type between willow-poplar and ash-elm forests
- Floods influenced habitats of this ecosystem at least once per year.
- These communities are dependent on groundwater level.
- ash-elm floodplain forests (Fraxino-Ulmetum)
- This forests are spread on the higher terraces, rarely flooded. Except groundwater levels is here important already rainfall water.
- elm-oak floodplain forests (Ulmo-Quercetum)
- This is the driest type of forests, developed on gravel or sandy terraces of the Danube river. Groundwater level is here asserted only at catastrophic floods, here is common influence of a condensed moisture and precipitation.
- xerophillous Danube forest steppes (Crataegetum danubiale)
- These communities represent shrub habitats of hawthorns and low oaks with xerothermophillous vegetation. Its occurrence is bound on gravel benches with exceptional cover of sand.
- They are exclusively of climazonal type of vegetation.

Until the end of fifties there had occurred the following secondary forests there:

- poplar monocultures - until the end of fifties they are planted only on clear-cuttings, without surface adaptation of soil.
- They were planted almost to all habitats of natural floodplain forests, but mostly to places after willow-poplar stands. Their extent was in that time about 1,200 ha (8.5 % of the total extent of floodplain forests).
- black locust stands (Chelidonio-Robinietum, Bromo-Robinietum).
- These habitats were mostly scattered as small woods except dike-space in inundation territory only as mixed tree.
- plantations of Junglans regia, rarely Junglans nigra, planted for commercial purposes, mainly in the upper part of the Zitny ostrov island.
- Stands of Ailanthus altissima, of low significance as to their extent, scattered in the whole territory, mainly besides inundation area.

It is necessary to state, that majority of floodplain forests, till the end of fifties did not manifest any signs of structural changes in connection with decrease of groundwater levels, which demonstrably decreased at that time, mainly in the upper stretch of the Danube river. But the whole region was affected by sudden drying of elm (Ulmus minor), which was befallen by spreading of graphiosis.

Characteristic of such state of floodplain ecosystems, even if it was created almost during 100 years, can be considered for as a starting-point conditions at the judging of all other changes.

#### Period of decrease of the Danube river waters since 1960 till damming of the power canal at Gabcikovo in 1992

This stage is characteristic by the decrease of the Danube water levels and also by the drop of the groundwater levels. It is typical for the end of the fifties up to the damming of the Danube river in the end of 1992. The adaptations of the river-bed took the whole century. But they started decisively before the catastrophic floods in 1954 and 1965. Except the anti-flood measures like setting aside of side arms, equalisation of the main channel, decreasing of discharges in the Small Danube river, to the deepening of river-bed contributed by an expressive degree, also the excavation of gravel from the river-bed as well as the Slovak side as on the Hungarian side. Also numerous dams on the Austrian and German stretch of the Danube river contributed to the erosion of the river-bed. These measures beside others had limited the supply of gravel and sand along the river-bed and consequently the loosened energy was shifted to erosion of the river-bed [4]. The decrease of the water levels in the Danube river by about 1.5 to 2 m only for the last 30 years was reflected also in the decrease of the groundwater levels in the same scale[22]. Such a decrease meant "tearing off" the groundwater levels from the soil profile. The root system of the trees and herbs was exclusively dependent on rainfall water in some types of the floodplain forest (Fraxino angustifoliae-Populetum albae, Ulmo-Fraxinetum, the driest types of Salici-Populetum). It is possible to characterise the consequences of such ecological stress during the last 30 years as follows:

- loss of communication of side arms with the open water level in the Danube river in the upper part of the Zitny ostrov island (stretch between Bratislava - Cunovo).
- seasonal limitation of the discharging of the river arm system in the inter-dike area in stretch Hrusov - Palkovicovo (Szap).
- drying out of the dead river arms bed and their gradual settlement by shrub, often to xerothermophillous vegetation (Cornus sanguinea, Crataegus oxyacantha, Salix purpurea, Salix alba). Typical examples are mainly in stretch Hrusov - Bratislava.
- planar destruction of water and marsh vegetation in benefit of littoral vegetation by loss of water communication between the side river arms and the main channel, especially in the second half of the vegetation season. This is very typical for the river arm system of the inter-dike area.
- re-building of soft floodplain forests by means of regression succession from the most humid and humid types (Salici-Populetum myosotidetosum and typicum) to more xerophyllous types (Salici-Populetum variant with Cornus sanguinea - upper part of the territory.
- re-building of elm-ash (Ulmo-Fraxinetum) and ash-poplar (Fraxino angustifoliae-Populetum albae) phytocoenoses to climax types of elm-oak forests (Ulmo-Quercetum), sometimes up to forest steppe shrub communities (Crataegetum danubiale) - the whole territory but mainly the upper part of the Zitny ostrov island.
- drying out of the upper part of the crowns at Populus alba, Fraxinus angustifolia, Quercus robur at elevated places of soft and even hard floodplain forest, gradual destruction of the tree layer and rise of shrub blocking stadia with Cornus sanguinea and Cornus mas - part from Hrusov to Bratislava.
- decreasing of the thickness increment of timber, but mainly at Populus nigra, Populus alba, Salix alba in the upper part of the Zitny ostrov island.
- loss of leaves at willows on aggregated gravel and sandy ramparts along the whole Danube river-bed, finishing by dying off trees.

Decrease of the increment, loss of leaves frequently finishes by the destruction of the willow-poplar forest, forced forest management to widen plantation of cultivar poplar monocultures. Except of the originally planted euro-american clones (Populus robusta, Populus monilifera) also the clone "I 214," which has been shown resistant to changes of the groundwater levels during the last 20-30 years, was started to be planted on places of the inter-dike area (Hrusov - Komarno). It has also decreasing increments on aggradated ramparts with gravel-sand substrate [36]. The extent of such planted lignicultures was about 8,000 ha to the end of 1992, which is approximately 80 % from total extent of floodplain forests in the inundation area [37].

The huge extent of cultivated poplar monocultures in the low part of the floodplain, but also underneath Bratislava, caused an enormous spread of neophyte species, mainly the populations of Aster novi-belgii, Solidago gigantea and in last years also Impatiens glandulifera. In addition there are areas of lignicultures accompanied by expansion of numerous synanthropic species (Cirsium arvense, Arctium lappa, Arctium nemorosus, Calamagrostis epigeios and others).

The decrease of the groundwater levels since the end of the fifties forced also the plantation of monocultures of other trees like Acer pseudoplatanus, Fraxinus excelsior, Tilia cordata, Robinia pseudeacacia, Pinus sylvestris, Pinus nigra, and certainly others too. Similarly to poplar plantations, these trees are strange elements of floodplain areas too and they have considerably changed the natural potential of the ecosystems and species biodiversity.

We suppose that similar consequences were manifested also on the right-side, the Hungarian part of the floodplain forests in the surrounding of Cunovo, Dunakiliti and in the whole Szigetköz. At the same time the groundwater levels were influenced by the water level in the Danube river. The process, which we characterised for the Slovak floodplain ecosystems, is a consequence of a gradual decrease of the water levels. In the case of floodplain forests on the right bank of the Danube, these negative changes will be more extreme, because of a sudden decrease of the water levels by damming of the Danube river in 1992.

#### FOREST ECOSYSTEMS AFTER DAMMING OF THE DANUBE RIVER (1992) - VARIANT C

Damming of the Danube river induced fundamental changes in hydrological and hydropedological regime. It concerns the area between Bratislava and Palkovicovo (Szap). The old Danube channel is connected here with the outlet canal of the hydroelectric power structures Gabcikovo. The whole territory can be, from the standpoint of hydrological changes, but mainly its impact into the vegetation, divided to several stretches as follows (Fig. 1):

- stretch of floodplain ecosystem underneath Bratislava
- floodplain forest south of the Hrusov reservoir
- draining part of floodplain forests
- inundation area (Dobrohost - Palkovicovo)
- floodplain forests not influenced by the Gabcikovo-Nagymaros barrage system (Variant C)
- shore zone in the Hrusov reservoir

#### Stretch of floodplain ecosystems under Bratislava

This area includes the vegetation of Ostrov Kopac island and the residues of forests between the villages Petrzalka and Rusovce. This stretch is temporarily still influenced by the increase of the groundwater level in consequence of the damming. The groundwater levels reach such values as in the end of the fifties [22, 3]. The increase of the groundwater levels was manifested on the vegetation of this area as follows:

- filling of the side river arms by increasing of the groundwater level (they are connected with the main channel of the Danube by seepage canal)
- dying of the shrub formations (mainly Cornus sanguinea) in the dead river arms' floors, which settled there in the previous years with a low groundwater level. This vegetation is not tolerant to the whole-year flooding. A similar situation is also in the river arms between the villages Rusovce and Cunovo.
- beginning of the renovation of stagnant water vegetation (Lemnetea, Potametea, Phragmitii-Magnocaricetea) in the river arms re-filled by water (the Biskupicke rameno, Horne Rusovske rameno, Rusovske rameno arms).
- beginning of the rise (renovation) of the most humid type of willow-poplar forests (Salici-Populetum phragmiti-caricetosum) by snatching of willows in limozic eco-phase (the Biskupicke rameno arm).
- numerous natural reforestation of poplar (Populus alba) from seeds (up to this time rare in this area).
- lowering of leaf losses approximately by about 4 % in some floodplain trees in comparison with 1991, at shrub species Cornus sanguinea up to 50 % and at the neophyte species Negundo aceroides up to 85 % [23, 24].
- mild increase of the thickness increment at poplar (Populus alba) by about 3 mm in 1994 against the previous years [36].

#### Floodplain forests south of village Hrusov

This area represents a polder in the surrounding of the village Cunovo. The vicinity of the water reservoir is reflects also in less fluctuations of its levels, thus the groundwater regime can be defined as mildly fluctuating at least at the upper levels.

As compared to the conditions 40 years ago, and with regard to the damning, the revitalisation of the original vegetation is at present more affected than before. According to a regular biomonitoring [38, 23, 24, 28] this revitalisation is visible mainly in ash-poplar (Fraxino-Populetum), but also elm-ash (Ulmo-Fraxinetum) types of the floodplain forest. The revitalisation of trees, which were being dried gradually since the end of the fifties, will be possible to judge later.

Drying of the tree and shrub vegetation, which has settled to floors of dry river arms in the past, is apparent in the condition of permanent water, also in this stretch.

#### Draining part of floodplain ecosystems

The major part of floodplain ecosystems with an expressive decrease of the groundwater levels arose in the triangle underneath the Hrusov reservoir, between the power canal and the intake structure near Dobrohost. The decrease of the groundwater levels almost about 4 m under the surface sentenced floodplain forest to the dependence on precipitation water.

Similar situation is also on the aggregated rampart along the whole river-bed of the Danube river from the Hrusov reservoir up to the estuary of the outlet canal with the old Danube. A low water level in the old Danube together with penetrated gravel-sands acts as a drainage. It is synecological situation, which in the given situation excludes a natural reforestation of the original floodplain trees. It probably will comply only to cultivar poplars planted by "deep drought" (Varga, personal information). This territory has been monitored since 1991 and from the obtained results a development of regression succession is apparent [38, 23, 24, 28]. This succession tends to a disintegration of the forest ecosystems and their change to shrub formations. It is proved by the following:

- increase of number of individuals regarding to synanthropic species and vice-versa, decrease of number of hydro-hygrophillous species
- damage of tree willow species by loss of leaves up to 80 % and vice-versa, an ideal state of leaves at shrub species Cornus sanguinea
- dying of trees, which were damaged before by extreme defoliation, and this first of all at Salix alba and Salix fragilis.

The whole area is elaborated also dendrochronologically at present. Preliminary results of the balance of thickness increments show, that the process of increment decrease began a longer time ago, and reacted into the decrease of the Danube's waters, approximately since the beginning of the seventies [36].

The drop of the tree increment in Szigetköz is also described. The decrease of the thickness increment at the euro-american poplar "I-214" up to 60 % in 1993 in comparison with 1991. Similarly, attention is paid to dying of the willows along the Danube river. These consequences were discovered undoubtedly after the damming of the Danube river in 1992. But this is in principle an analogy of the consequences of the water decrease, which occurred there also prior to the damming of the power canal, but being more visible and significant. We would like to stress, that this degradation could be eliminated by the increasing of the dotation of water to the Mosoni Danube, and by construction of underwater weirs in the old Danube. On the Slovak side we have the first proofs about that at some trees, namely at Populus alba. This situation is in the stretch underneath Bratislava, where an increase of the groundwater levels occurred by the damming of the Danube river.

#### Inundation area (Dobrohost - Palkovicovo)

The subject of interest is the left-bank river arm system of the Danube river, where by draining of waters from the old river-bed to the power canal significantly decreased the discharges and groundwater levels. The decrease was so expressive, that a total liquidation of floodplain forests could occur. For the elimination of this state the intake structure in the power canal near Dobrohost was built. This structure simulated the former hydropedological regime by watering of 28 m3/s of water to the river arm system. The permeability to the sides of the existing river arms was solved by the adaptation of the existing and filling of new weirs of the river arms. This permanent discharge is conveyed through a suitable number of frame culverts in the interest to enable road communication through obstacles (there are totally 10). Due to the fact that the water level in the old Danube is lower than the watering water in the river arm system, existing connections of arms with the Danube river were dammed. The system of weirs and dotation of the discharge from the power canal provide sufficient depths and discharges of water. It was ascertained [29], that the extrapolated water level from the river arms is on the majority of territory less than 1.5 m under the terrain, thus it is in the sphere of tree roots reach. Such simulation enables to water up to 75 % of the inundation area. It is also possible to achieve surface floods of the almost whole river arm system by a short-term increase of the water dotation to the intake structure to 234 m3/s [29].

Relatively permanent discharges (28 m3/s) are temporarily a disadvantage, that can have negative consequences to the vitality of trees (mainly original), which are ecologically adapted to frequent fluctuations of the groundwater levels. But it is possible to solve this problem technically very easily.

In spite of the fact that seven monitoring areas were established in the inundation area [19], there is only one (No. 10, the Kralovska luka near the village Bodiky) suitable for observation of concrete changes of the intake structure. The other areas are localised in draining part around the old Danube river-bed, or at the back swelling up of the estuary of the outlet canal with the old Danube river-bed (Palkovicovo). It is worth to note, that the monitoring areas were selected with the original intention of monitoring of influences of the hydroelectric power structures Gabcikovo-Nagymaros and not of the Variant C. Except these a larger part of the inundation area is replaced by lignicultures of cultivar poplars, which have a relatively wide valence to hydropedological conditions.

This development can be assumed from up till now obtained reactions of the flora to such situation:

- excellent healthy state of trees in the tree layer without traces of decline
- appearance of natural reforestation of the tree species Fraxinus angustifolia from the seed, which has not been observed till now
- increase of the population number of the protected humid species Leucojum sativum by the influence of the groundwater levels increase
- almost no changes in the thickness increment of the poplar clone "I 214" (the most spread tree of the inundation area).

Regarding to the insufficient number of monitoring areas, reflecting the simulated discharges, supplementary measurements were performed, including dendro-ecological measurements by the Pressler borer in 1994. Preliminary results are at the present of low significance and do not indicate more expressive changes since the damming. Surely, they require longer observations. The floodplain area of the river arm system Dobrohost - Palkovicovo (Szap) requires also the evaluation of the plant biodiversity. Therefore, we point at the qualitative composition of the biodiversity in the floodplain of the Danube river, especially of vascular plants. There are species of the following eco-phases [7]:

- hydro-phase (plants of stagnant or slowly flowing waters - Lemnetea, Potametea)
- littoral phase (plants of shallow waters with oscillating water level - part of the class Phragmito-Magnocaricetea)
- limozic eco-phase (soil surface is saturated by water, often drying off - drier part of the class Phragmito-Magnocaricetea, communities of shore loans and seasonal phytocoenoses - Bidentetea, Plantaginetea majori, Littoreletea, Isoeto - Nanojuncetea)
- terrestrial eco-phase (water as the main factor retreats and it is asserted as soil humidity - Salicetea purpureae, Ulmenion, alluvial meadows and others).

These four eco-phases present in the region Dobrohost Palkovicovo (Szap) approximately 32 % (321 taxons) of the total number of 1,000 vascular plants in the floodplain area (Gabcikovo Nagymaros) [2]. From the above mentioned taxons about 200 are terrestric, 11 are growing in littoral eco-phase, 55 in limozic eco-phase and 52 species in hydro-phase. It means, that shifts, re-colonisation induced by the rise or retreat of habitats in connection with the function of simulated water in the river arms will touch approximately 118 taxons of vascular plants. This number includes also such plants, which very rapidly settle substrates created by water, such as genus Polygonum, Bidens, Phalaris, Phragmites, Caltha, Cyperus, Limosella, Agrostis and more others.

Two-years measurement (1993-1994) of watering to the river arm systems show even an enlargement of area in limozic and littoral eco-phase. Also for plants requiring relatively patient waters, such as Nymphaea, Nuphar, Sagitaria, Butomus, Ceratophyllum, Potemogeton and others new biotopes arose. These arise mainly in side branches of simulated watering arms, where the sedimentation of flooded material prevails.

According to present observation no species of vascular plants are missing, but on the contrary an increase of the population of the protected species Leucojum aestivum, by the influence of raising groundwater levels, was ascertained.

Also in the Hrusov reservoir an increase of limozic and littoral eco-phase biodiversity will occur (it occurs already). It will be really a huge area with alternating shallow water and semi-terrestric conditions at its shallow northern margin.

The river arm system, watered by the intake structure at Dobrohost had no surface floods up to this time. It is not a consequence of inexperienced management of the variant C; this calculates with such situation, at the possibility of a discharge increase in the river arm system to 200 m3/s. The simulation of floods was not planned in 1993 and as an explicit requirement of the forest management (state and private sector) floods did not occur in 1994.

Surface floods have a decisive significance for the recyclisation of nutrients and biomass production. The absence of nutrients in a relatively clean flow of the power canal, which the river arm system is watered from, is considered as a mistake of the Variant C. We paid attention to the fact, that the Danube river already at the mouth to the Slovak territory brings essentially less depositing material. This is a consequence of a number of water structures on the Austrian and German stretch of the Danube river. Kalis et al. [14] compared the amount of deposits in Bratislava since the end of the fifties and for 1970-1987, and they found a difference of up to 60 % (in 1960 = 6.9 mil.t, in 1987 = 2.8 mil.t).

The older data are also interesting, e.g. those which compare the supplies of humus at the flooded and not flooded places of the Danube's floodplain forests [27]. The average content of humus in the surface horizons was in the floodplain (Gabcikovo) 5.15 % and in not flooded, climazonal forests (Podunajske Biskupice) 4.9 %, what is a very little expressive difference. For the plant nutrition of the whole Danube lowland the high content of CaCO3 (18-30 %), MgO (0,80-0,92 %), and phosphorus acid have the first-order significance [27].

#### Shore zone in the Hrusov reservoir

There exists a several tens of meters wide belt of shallow water at the northern margin of the whole Hrusov reservoir. In dependence on the different water level it is, or it will be for different periods submoistened (limozic eco-phase), or only with a shallow layer of water (littoral eco-phase), and at numerous places a longer merging up occurs (terrestric eco-phase). These habitat conditions representing periodical waters are an ideal biotope for the spread of such species as Persicaria amphibia, Equisetum fluviatile, Ranunculus lungua, Ranunculus sceleratus, Ranunculus sardous, species of the genus Batrichium, Persicaria lapathifolia, species of the genus Myriophyllum, Hipuris vulgaris, Nymphoides peltata, Hottonia palustris, Butomus umbellatus, Najas marina, species of the genus Potamogeton, species of the genus Eleochoris, and numerous others. These species were originating from artificial reservoirs (rice fields in the fifties) as well as from natural periodical waters (shallow river arms situated inside the Zitny ostrov island) [7]. Thus, on the Slovak side of the structure an essential increase of biodiversity, or at least its revitalisation will occur. Some of the above mentioned species were already discovered in 1994.

The litter from these highly productive ecosystems reaches up the water of the reservoir about organic materials, which can be transported to the floodplain area of the river arm system (Dobrohost Palkovicovo) at suitable management.

#### Floodplain forests not influenced by the Variant C

Their largest extend is between villages Medvedov and Klucovec. It is possible to deduce to similar trend as for the whole Danube delta, traced out from the evaluation of biota monitoring (two areas) for 1960-1992. It is a mild increase of leaf losses at the tree layer component of original tree species. On the whole it regards on certain stabilisation of the ecosystems to long-lasting changes in conditions [21]. Dendroecological observations, which are performed at present help to express an objective opinion also in this stretch.

#### REFERENCES

  [1] Berta, J., 1970: Waldgesellschaften und Bodenverhaltnisse in der Theisstiefebene. Vegetacia CSSR, Bratislava, 367 pp.

  [2] Bertova, L.et al., 1986: Floristic characteristic from standpoint of genofund rare and threatened species (in Slovak). In: Vegetation bases to assanation and reconstruction of the Hrusov reservoir territory I-II. Mscr. UEBE SAV Bratislava.

  [3] Chalupka, J., 1993: Changes in groundwater levels due to completation of the water structures Gabcikovo. In: Sustava vodnych diel Gabcikovo-Nagymaros, zamery a skutocnost. Proc., Bratislava, p. 327-332.

  [4] Danisovic, P., 1993: Problems solved by the Gabcikovo-Nagymaros project. In: Sustava vodnych diel Gabcikovo-Nagymaros, zamery a skutocnost. Proc., Bratislava, p.27-32.

  [5] Dister, E., 1988: Ökologie der mitteleuropaischen Auenwalder. Vilher-Muner-Stiftung, 19, p. 6-27

  [6] Dzatko, M., 1972: Synökologische Charakteristik des Waldgesselschaften im nordlichen Teil des Donauflachlandes. Biol. Prace, 18, 4, 95 pp.

  [7] Hejny, S., 1960: Ökologische Charakteristik der Wasser- und Sumpflanzen in der Slowakischen Tiefebenen. Donau und Theissgebiete. SAV, Bratislava, 488 pp.

  [8] Jakubec, L., 1993: The reasons for drying off upper Zitny ostrov and the extinction of the branch system of the Danube. In: Sustava vodnych diel Gabcikovo-Nagymaros, zamery a skutocnost. Proc., Bratislava, p.237-244.

  [9] Jurko, A., 1958: Soil-ecological conditions and forest communities of the Danube lowland (in Slovak). SAV Bratislava, 264 pp.

  [10] Jurko, A., 1961: Das Alnetum incanae in der Mittelslowakei (II. Die Auenwalder in der Westkarpaten. Biológia (Bratislava), 16, p. 321-339.

  [11]Jurko, A., 1976: Influence of the Gabcikovo-Nagymaros project to vegetation of the Danube lowlands (in Slovak). Zivot. prostr., 10, 5, p. 230-238.

  [12] Jurko, A., Majovsky, J., 1956: Floodplain forests in the West Carpathians. I. Alnetum incanae in the nothern Orava (in Slovak). Acta Fac. Rer. Natur. Univ.Com. Bot., 8-9, p.363-385.

  [13] Jurko, A., Somsak, L., 1959: Phytocoenological map of floodplain forest at district of Podunajske Biskupice (in Slovak). Mscr. Fac. Sci. Univ. Com., Bratislava.

  [14] Kalis, J. et al., 1993: The sedimentation of the bed and suspended load in the Hrusov reservoir and the possibilities of influencing. In: Sustava vodnych diel Gabcikovo-Nagymaros. zamery a skutocnost. Proc., Bratislava, p. 285-290.

  [15] Kirka, A., 1995: To monitoring of the Danube river biota (in Slovak). Polovnictvo a rybarstvo 47, 3, p. 41, PaRPRESS Bratislava.

  [16] Kontris, J., 1981: Soil-ecological and phytocoenological conditions of the Liptovska kotlina basin floofplain forests. Biol. Prace, 27, 3, p. 1-165.

  [17] Kubicek, F. et al., 1989: Criteria of water management to humid regime of soil in the inundation area from standpoint of proposed structure of forest stands (in Slovak). Final Report, Inst. Wat. Man., Bratislava 53 pp.

  [18] Kubicek, F., Somsak, L., 1985: Production-ecological analysis of the herbaceous layer in floodplain forests (Suballiance Ulmenion Oberd.1953) of western Slovakia. Ecology (CSSR), Bratislava, 4, 4, p. 347-365.

  [19] Lisicky, M. et al., 1991: Report about starting-point state of natural environment of the Gabcikovo-Nagymaros project, degree Gabcikovo from standpoint of biology and landscape ecology(in Slovak). Final Report, SAV Bratislava.

  [20] Lisicky, M. et al., 1993: Report of professional group about monitoring of natural environment influenced by the Gacikovo project in 1992 (in Slovak). Inst. Zool. Ecosoz. SAV, Bratislava.

  [21] Matecny, I. et al., 1994: Monitoring of natural environment of territory influenced by the Gabcikovo-Nagymaros project. Final Report. Faculty of Sciences, Univ. Bratislava.

  [22] Mucha, I., Paulikova, E., Hlavaty, Z., 1993: Impact of Gabcikovo waterwod on ground water level. In: Sustava vodnych diel Gabcikovo-Nagymaros, zamery a skutocnost. Proc., Bratislava, p. 313-318.

  [23] Oszlanyi, J., 1994: Leaf area indices of shrub layer in two floodplain forests. Ecology (Bratislava), 13, p. 361-368.

  [24] Oszlanyi, J., 1994: Losses of leaves of some floodplain trees in the Danube lowlands (in Slovak). In: Matecny et al., 1994.

  [25] Otahelova, H., 1977: Plant communities of the class Lemnetea W.Koch et Tx. in Koch 1954 and Potamogetonetea Tx, et Preis. 1942 in the Danube lowland. Mscr. Charles Univ. Prague.

  [26] Otahelova, H., 1978: Comments to vegetation and ecological conditions of the Danube|s arms in surrounding of Gabcikovo (in Slovak). Biológia (Bratislava), 33, p. 315-320.

  [27] Pelisek, J., Characteristic of floodplain soils of the Slovak Danube delta (in Czech). Proc. of CSAZ, Praha, 15, 6 p. 463-472.

  [28] Pisut, P., 1994: Structure of forest stands on monitoring areas of the Danube lowland (in Slovak). In: Matecny et al., 1994

  [29] Sumdal, J., Sikora, A., 1993: Possibilities for preservation of ecologically acceptable water level regime in left-band Danube river brand system after putting Gabcikovo hydroelectric power into operation. In: Sustava vodnych diel Gabcikovo-Nagymaros, zamery a skutocnost. Proc., Bratislava, p. 261-268.

  [30] Smihulova, A., 1967: Plant communities of litoral in the middle Vah river (in Slovak). Diploma work. Fac., Sci., Univ. Bratislava.

  [31] Somsak, L., 1959: Plant communities of floodplain forest in the Zahorska nizina lowland (in Slovak). Acta Fac. Rer. Nat. Univ. Com. Bot., 3, p. 515-594.

  [32] Somsak, L., 1961: Alder stands of the Spissko-gemerske rudohorie Mountains (in Slovak). Acta Fac. Rer. Nat. Univ. Com. Bot., 24, p. 1-62.

  [33] Somsak, L., 1964: Plant communities of floodplain forest influenced by construction of water structures on the Danube river (in Slovak). Proc. CSAV, Praha, p. 146-164.

  [34] Somsak, L., 1972: Natürliche Phytozönosen des Flusslitorals im unterlauf des Hron - Flusses. Acta Fac. Rer. Nat. Univ. Com. Bot., 20, p. 1-91.

  [35] Somsak, L., 1976: Auenwalder des Unteren Hron - Flusses. Acta Fac. Rer. Nat. Univ. Com. Bot., 24, p. 1-92.

  [36] Somsak, L., Gazdik, M., 1994: Dendrochronology and dendroecology of draining part of floodplain forests in the Danube lowland (in Slovak). Mscr. Dept of Soil, Fac. Sci. Univ. Bratislava.

  [37] Somsak, L. et al., 1993: Floodplain forests influeced by the Gabcikovo project. Data Report for ECE Working Group. Bratislava, 66 pp.

  [38] Uhercikova, E., 1994: Results of plant biomonitoring in floodplain forests influenced by the Gabcikovo project (in Slovak). In: Matecny et al., 1994.