CONCLUSIONS Monitored characteristics and their dynamics over the last four vegetation seasons reveal the following conclusions:

• Changes of monitored parameters (e.g. the structure of tree and shrub layer, the leaf area index) are slow, drab and insignificant at the permanent monitoring plots representing a major part of forest ecosystems of the area affected by the Hydroelectric power structures Gabcikovo. Despite the absence of floods, the ground water level is at a sufficient height to contact rhisosphere of wood plants and the recorded changes are in accord with growth regularities.
• An increase of the ground water level in the upper part of the monitored territory and a partial renaturation of hydropedological conditions led to an improvement of production-ecological parameters of the area. Changes of its structure are of positive tendency, the leaf area index is stabilised at high values and somewhere even increased (in 1994 being by 70 - 80% higher than in 1991).
• Localities with a permanent decrease of the ground water level (band along the old river-bed of the Danube, a dry triangle among the old river-bed of the Danube, the inlet canal and the river arm supplied by the intake structure at Dobrohost and other places) were afflicted by negative changes, locally indicating destruction of tree and shrub layers, with the leaf area index significantly reduced by 20 - 30%.

INTRODUCTION

Before making a comment on results of monitoring of forest ecosystems, it is worth to stress that the forests in the inundation area had been intensively influenced by an anthropogenic activity already before setting the HPSG into the operation. This activity begun in the last century, after the construction of flood protecting dikes. Until then, the flood wave had been spilled over the large territory and thus the flood water had been shallow. There had been outstanding conditions for hard-wood floodplain tree species (oak, ash, elm) at this area. After the construction of dikes, frequency and intensity of floods increased and conditions for existence of original trees of a hard-wood floodplain were deteriorated to such an extent, that these species were almost eliminated from the inter-dike space. In contrast, very good growth conditions arose for fast growing trees of cultivated poplars, that require much water and nutrition.

These novel growth conditions, that were considerably influenced by a man, resulted in remarkable changes of wood plant composition - cultivation of poplar monocultures with a high production of wood and a short rotation period was introduced. Monocultures are very frequently found in the inter-dike space. Their existence is dependent on a systematic anthropogenic intervention and they are not able to regenerate by themselves. From the ecological point of view, the shrub layer consisting mostly of original species is the only stable element of these ecosystems.

Despite a very intensive anthropogenic influence, there occurred a few precious fragments of original ecosystems with high ecological value in the followed area.

MONITORING

Permanent monitoring plots were subjected to our measurements and observations aimed at the evaluation of conditions and possible changes in the structure of the tree and shrub layer as well as in the leaf index area.

The structure of the tree layer and its changes were evaluated on the basis of the following features and parameters:

• composition of species;
• thickness, height and biosociological structure of the main stand;
• tree inventory was evaluated according to the Kraft's tree classes (dominant, codominant, partially codominant, suppressed and overtopping trees). Evaluated were also a length of crown, a width of crown and a degree of utilisation of the production area by crown projections which were analysed at the same monitoring plots;
• crown architecture, canopy cover and a degree of production area exploitation by crown projections were evaluated at some monitoring plots;
• measurements, calculations and evaluations were connected with a certain acreage of the investigated area, that was set, defined and sized within the monitoring plot. All analysed values were calculated to 1 ha.

The analysis of the structure of the shrub layer and its changes was based on the following features and parameters:

• composition of species;
• thickness and height structure;
• dominance and spatial structure of wood-plants.

Measurements, calculations and evaluations were bound to the investigated areas of 2x2 m, or 5x5 m. Values determined at these areas were calculated to 1 ha.

Due to extremely laborious evaluations of the leaf area index, this was monitored at the same permanent monitoring plots only in the interval of two to three years.

The leaf area index was determined on the basis of the tree and shrub layer structure, using destructed samples, e.g. branches divided in to smaller parts and leaves, which were subjected to planimetric analyses and calculations. This parameter represents one-sided leaves area of wood-plants per a number of their pieces at the area of 1 ha. The leaf area index is a very good indicator of production capacity of a plant cover, its vitality and health conditions of a tree and shrub layer.

INTERPRETATION

1. The upper part, e.g. the territory influenced by the Cunovo reservoir, in which an increase of ground water level occurred in 1992. Before putting HPSG into the operation, forest stands of this area had been fully dependent on precipitation water due to a long(term depression of the Danube river-bed. The ground water level was sunk in gravel subbases and out of reach of vegetation. Partial renaturation of the water regime of soils has resulted in the recent improvement of conditions also for wood-plants. It came to their overall revival and clearly positive shift in the vitality of tree and shrub layers in comparison with the situation in the years before filling the reservoir (MB02b, MB3). The monitoring plot MB01 located at the same territory, represents a forest with the structure disintegrated due to hydrological adaptations made longer time ago.
2. The lower part, e.g. a territory between the old river-bed of the Danube and the diversion canal of HPSG between villages Dobrohost and Sap. The ground water level is in contact with production root space of the vegetation also after putting HPSG into the operation. Major part of this territory was not subjected to remarkable changes of shrub and tree layers of forest ecosystems in comparison to their state before running of HPSG. The situation appears to be stable from the production-ecological point of view, since no significant alteration of any single parameter, or at least of majority monitored parameters was recorded (MB10, MB14, MB15).

MONITORING OF PRODUCTION-ECOLOGICAL PARAMETERS AND THEIR EVALUATION

Revival of all parts of the forest ecosystem is apparent at the permanent monitoring plots situated in the upper part, shrub and tree layers being in better productive conditions as compared to a pre-dam situation. Physiological weakness of wood plants that was manifested before by distinct syndromes, is recently not visible. Leaves are rigorous, from more or less adequate to perfect, utilisation of the overground part of production space is favourable and close to optimal.

Some negative trends were observed in the lower part of the territory, at the permanent monitoring plot MB09 (on the shore of the old river-bed of the Danube) and mainly at the plot MB06 (in the dry triangle). Negative shifts in the structure of tree and shrub layers can be qualified here, that are witnessing a partial destruction induced by worsen moisture conditions (decrease of the ground water level to subbase horizons to such an extent that its contact with the root part of production space is prevented). As compared to the year 1992, in 1994 it came to changes in biosociological, thickness and height structure of tree inventory due to dying out of several individuals of suppressed and overtopping trees of white poplar. Moreover, crowns dried out, undergrowth architecture changed and also the canopy cover was altered. A number of individuals of wood plants dominating in the shrub layer was reduced (e.g. Red dog wood, Guelder rose). Lost were those trees that were the thinest individuals with the worst biosociological position in the shrub layer.

MONITORING OF LEAVES AREA INDEX AND ITS EVALUATION

A larger part of monitored territory is represented by monitoring plots at which even repeated measurements of the leaf area index did not disclosed any distinct change of its value after setting HPSG into the operation. Approximately the same values of the leaf area index can be supposed also for other localities in the lower part of the territory (MB10, MB14, MB15). Finally, the leaf area index was remarkably improved at additional monitoring area (followed beside the net of monitoring plots of the BIOTA partial monitoring network) in Tab. 1 designated by "y". At this area, the leaf area index of the year 1994 represents 185% of its value in 1991. Also indices of other permanent monitoring plots (MB02b, MB03) in the upper part of the territory are high in 1993 and 1994 and their prognosis is optimal. On the other hand, the leaf area index is for several years extremely low in the destructed stands of white poplar (MB01) which was already affected by irreversible changes of tree inventory longer time ago.

Compared to 1992, in 1994 the leaf area index considerably decreased at permanent monitoring plot areas MB06 and MB09. These monitoring plots represent " the dry triangle" among the old river-bed of the Danube, the by(pass canal and the river arm supplied by the intake object at Dobrohost and the band along the old river-bed of the Danube.

DISCUSSION

Despite the fact, that an unambiguous and a qualified ecological evaluation of changes requires without any doubt a longer time period of observations (5 - 7 years minimum), from the two year monitoring data it can be concluded on the trends of developments of the forest stand structure and the leaf area index.

Monitored characteristics and their dynamics over the last four vegetation seasons reveal the following conclusions:

• Changes of monitored parameters (e.g. structure of the tree and shrub layer, the leaf area index) are slow, drab and insignificant at the permanent monitoring plots representing a major part of forest ecosystems of the area affected by the hydroelectric power structure Gabcikovo. Despite the absence of floods, the ground water level is at a sufficient height to contact rhisosphere of wood plants and its recorded changes are in accord with growth regularities.
• The increase of the ground water level in the upper part of the monitored territory and partial renaturation of hydropedological conditions led to improvement of production-ecological parameters of the area. Changes of its structure are of positive tendency, the leaf area index is stabilised at high values and somewhere even increased (in 1994 being by 70 - 80% higher than in 1991).
• Localities with a permanent decrease of the ground water level (band along the old river-bed of Danube, a dry triangle among the old river-bed of the Danube, the inlet canal and the river arm supplied by the intake structure at Dobrohost and other places) were afflicted by negative changes, locally indicating destruction of tree and shrub layers, with the leaf area index significantly reduced by 20 - 30%.

A long term observation in the frame of monitoring will make it possible to obtain exhausting and qualified information on persistence of recent favourable conditions of most forest ecosystems at the territory of interest, or on their improvement (in the upper part). Alternatively, it will provide data on required changes in the wood plant composition and the way of management of the smaller part of the territory, at which efforts of hydrologists collaborating with ecologists and foresters will not result in maintaining the conditions of the pre-dam period that are necessary for growth of highly pretentious cultivated poplars.

REFERENCES

[1]] Oszlanyi, J. (1992): Structure, leaves index and loss of leaves in two floodplain forest ecosystems. Institute of Ecobiology, Slovak Academy of Sciences, 23 pp.

[2] Oszlanyi, J. (1993): Completion of informational databases on floodplain forest ecosystems directed to interpretation of their conditions in the area of interest before and after putting HPSG into operation. Institute of Landscape Ecology, Slovak Academy of Sciences, 23 pp.

[3] Oszlanyi, J. (1994): Situation at three permanent monitoring areas of forest ecosystems in the first year after putting HPSG into operation. 34 pp.

[4] Oszlanyi, J. (1994): Leaf index of wood-plants in forest ecosystems influenced by HPSG. Proceedings on Experiences and utilisation of Slovak - Hungarian section of Danube. Bratislava

[5] Oszlanyi, J. (1995): Production-ecological parameters of selected forest characteristics in the second vegetation season after putting HPSG into operation. 28 pp.