CONCLUSIONS Impact of the Danube river damming by means of evaluation of the thickness increment at selected tree species (Populus nigra, P. alba, Salix alba and cultivar poplars) is a little significant. A single increasing of increment on places with increased gound water level in 1993, it means one year after damming, it is possible to consider as temporary. The main mass of side roots react here immediately to improvement of water balance of soil profile. However an increment expressive decreased in next year (1994). Eventual dependence of increment on changes of water regime will be necessary to observe in younger stands (to 10 years) of cultivar poplars. Older trees have probably internal physiological disposition of thickness increment decrease.
Relationship increment of floodplain trees - ground water level - yearly amount of precipitation - total soil humidity the authors of the contribution recommend to observe in longer time intervals. Adaptation to ecological stresses lasts a longer period at tree species.
For ring-year analyses were sampled radial borings by means of Pressler borer. Radial borings were done in 130 cm height from soil surface. At the selection of single tree individuals they were visibly considered these parameters: approximately the same shape of crown, the same diameter in breast height and spatial position in the stand structure. For finding out of increment development were utilised also crossing cuts of cutting stems (at cutting of wood).
Calculation of year-rings from borings was only rarely reliable. The borders of year-rings are at these tree species a little expressive, which is possible to write up to absence of an expressive ending of vegetation period in these conditions. For example still in December it floats from aperture after radial borings a strong stream of solutions moved by transpiration. Neither testing of recommended colouring method of wood [9] as fluoroglucinol with HCL, Vodrazka's diacid, malachite green colour did not express the year-rings. Partial success was reached by slightly concentrated solution of hypermangan as even by wetting of borings in water.
The part of our intention before starting of these observation should be also dendrochronological research with purpose to get in harmony dependencies of thickness increments with various ground water levels and precipitation in single years in last decades. Lower age of cultivated poplars (the oldest cultures had an age about 25 years) allowed to fulfil this intention only partially. In spite of that the development of thickness increment is evaluated under last 15 years.
Curves of thickness increment development were done to relation with curves of ground water level development and curves of annual amount of precipitation, eventually precipitation during vegetation peace (at Populus alba). Judging of correlation degree came out from a percentage of against-running curves.
Tree composition of floodplain forests is relatively in narrow correlation with ground water regime and soil capillary capacity. In consequence of adapting to humid conditions the tree species were formed either to communities dependent only on ground water level (willow-poplar forests, partially also ash-poplar forests), or to phytocoenoses only influenced by ground waters (ash-elm forests), and finally to types dependent on precipitation water and only exceptionally also ground water (elm-oak communities). There are even present on gravel benches climazonal types of vegetation (Danubian forest steppes).
From biological-ecological qualities of single floodplain tree species it is on the first place their ecological plasticity. There are tree species, which have a very wide amplitude of hydropedological conditions. To such trees belong e.g. Salix alba, Populus alba, Populus nigra and partially also Fraxinus angustifolia. Except other it depends also on ability of roots adaptation to changed hydropedological conditions. Some trees, mainly in young age, prove to prolong their roots after decreasing ground water level (Populus nigra, Populus alba). Other ones again react to increasing of ground waters, lasting also for a long time, by creating of secondary (also air) roots. To such trees belong e.g. Salix alba (observation in a new-watering Biskupicke rameno arm [12, 13]). However, rash, long-lasting changes of ground water levels mean in every case an ecological stress. Floodplain trees react differently to such situation. In the case of long-lasting increase of ground water levels the old, main roots putrefied up, as it was observed at Populus alba [4]. It comes to creation of secondary roots, mainly at younger trees, only after certain time. On the other hand, a rush, more permanent decrease of ground water level means a slow adaptation of trees to precipitation conditions. Older trees, which grew in more humid conditions before, such ones gradually died away. If the decrease of water is met with deficit of precipitation, it comes to disintegration of almost all floodplain forests (Uhercikova in [7, 8], [15]).
Tree thickness increments also react to changes of floodplain soil humid regime. But it is a problem to determine, which factor making dependent upon soil humid regime is a limiting one for these changes. There are opinions, that a different ground water level is a main factor responsible for various value of floodplain tree increment. Some authors state, that after turning aside majority of waters from the old Danube river to the power canal thickness increment of cultivated poplars "I 214" on the right bank essentially decreased. And vice versa the increase of increment at Populus alba after increasing of ground water levels after damming of the Danube river under Bratislava is recorded by Somsak [12]. But more complete observations[2, 4] call attention, that there are single knowledge and probably reflect reaction of trees tightly after ecological stress.
Measurements of thickness increments during last years, elaborated to this contribution (dendrochronological and dendroecological view) did not confirm such single view. Results are elaborated from almost 100 individual trees (93) in different conditions of hydropedological regime, which arose by damming of the Danube river in the end of 1992 [14]. Preliminary knowledge forced us to take to consideration not only different ground water levels, but also precipitation situation in single years. At comparison of supposed correlation between precipitation conditions and thickness increment they were the same discovered serious problems. They resulted from this fact, that some authors claim [11], that trees begin to produce a spring wood in the beginning of spring and for production of spring wood they use supplied materials and water accumulated in the period of vegetation peace. This hypothesis is probable also in the region of the Danubian lowland, where regarding to high temperatures about increasing air streaming during vegetation period, there are also high values of evapotranspiration [1]. Except this it is necessary to calculate here also a consumption of water for physiological transpiration [6, 16], which is maximal in vegetation period. Soil in its upper horizons is in consequence of this so dry, that it reaches a percentage of withering coefficient [10]. Against this in time of vegetation peace in consequence of lower air temperatures a majority of moisture remains in soil and creates high supplies of humidity. Dependence of thickness increment on amount of precipitation during a period of vegetation peace, elaborated by against-running curves was confirmed also at dendrochronological observation of Populus alba in the Danubian lowland [4].
It was expressed on more places a significance of soil humidity for increments as summing parameter. Different ground water levels together with various amount of precipitation are the main sources of different soil humidity of floodplain forests. To include this important environmental factor for dendroecology of floodplain forests was impossible. Results of soil humidity measurements were to disposition only since 1989. Except this soil sounds did not cover all synecological stretches of the Danube river, in which thickness increment was measured. Therefore also correlation of thickness increment of observed tree species are a little significant [2, 4].
Total tree increment, it means also thickness increment is also dependent upon position of individual tree in spatial structure of stand. Greatness and shape of crown, its lighting there are transformers of photoassimilation. It was strictly followed to these qualities at the selection of trees. In spite of that, results may be misrepresented mainly at evaluation of year-rings of already cutting trees.