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from phytosociological works, which are based on phytosociological releves made in high oak forest stands. The gradual transition between closed hornbeam-oak stands on drier sites, and more lighted and more wet-demanding oak-ash-elm phytosociological units, described by Ortmann-Ajkai (2002) are well recognisable. The more interesting part of our results is what they say about spontaneous forest dynamics, especially of the regeneration of Quercus robur, as these processes build the base for close-to-nature silviculture (Brang, 2005). According to the gap dynamics theory, regeneration takes place in spontaneous gaps in old stands, due to the more favourable light conditions. In our case, oak regeneration also seems to work better in more grove-forest like stands, i.e. in sites with better light and moisture conditions, supporting results of e.g. Vera (2000), Küssner (2003) in Germany, Čater and Batič (2006) in Slovenia, Ostrogović et al., 2010 in Croatia, Terwei et al (2013) in Italy. The destructive effects of groundwater table drop on floodplain oak forests in Croatia were discussed in details by Prpič (2008). Physiological studies of Škvorc et al. (2012) prove that oaks growing on soils with a higher nutrient content has higher intensity of photosynthesis, consequently better growth. At the same time, Quercus robur seedlings are very scarce both in managed, and also in unmanaged stands (Table 1), supporting findings of many authors (Illés, 1905; Leibundgut, 1993; Prusa, 1985; Vrska, 1997; Janik et al., 2008; Král et al., 2014). A distinct difference of species composition of tree and regeneration layers is demonstrated, parallel to improved light conditions: a strong decrease in Quercus robur and rise of more pioneer species, e.g. Fraxinus angustifolia, and shrub species as Crataegus monogyna and Cornus sanguinea. Some sampling points, characterised by Crataegus monogyna dominance especially in the second canopy, deviate from the high forest structure. In gaps, providing better light conditions, usual mesophilous or wet-demanding species of floodplain hardwood forests, Fraxinus angustifolia, Carpinus betulus, Acer campestre and Ulmus laevis regenerate. Crataegus monogyna also appear in the second canopy, and with a proportion of almost 25%. On the longer run, when old trees of the upper canopy fall, a vegetation type dominated by Crataegus monogyna may appear. Apparently, spontaneous stand development lead from a closed oak-hornbeam-ash high forest towards a gap-fragmented mosaic with dense Crataegus stands. Crataegus shrubs, widespread and well-known elements of vegetation, mentioned only superficially in syntaxonomical works (Ellenberg, 1996; Borhidi, 2012), as secondary, evolving on margins and on unmanaged grasslands, as a stage of spontaneous reforestation (Kevey, 1998). The idea of „cyclic succession mosaic” is thoroughly developed by Vera (2000), who also cites an impressive amount of case studies on closed oak stands opening up, then self-regenerating in a longer run, through a thorny shrub phase, where juvenile oaks found protection against game browsing. Our results raise attention to the fact that different natural forests can be characterized by different types of natural dynamics (Bengtsson et al., 2000). Continuos cover forestry is less conducive to species of high light demand (Gamborg and Larsen, 2003), like Quercus robur. Light-demanding species need larger gaps for their regeneration, created by windstorms or fire, but these factors are not common in our floodplains. Another possibility is that open spaces were created by the hydrogeomorphic dynamics of natural floodplains, but these processes do not work any more in regulated floodplains. The Vera hypothesis offers an alternative. Its key element – protection of oak regeneration by natural thorny shrubs – may be worth to consider to be utilized in close-to-nature forestry practice. CONCLUSIONS Zaključci Our results draw the attention to the necessity of reconsidering the intertwining issues of spontaneous forest dynamic processes, naturalness, conservational values, and close-to-nature silviculture. Main aim of recent forestry is the economic and sustainable management of forests. Close-to-nature forest management creates forests of remarkable conservational values. At the same time, no-intervention conservation allows natural forest dynamics to realize, so it may lead to more natural stands, with less economic, but with other kinds of conservational values: exceptional forest types, new kinds of habitats, higher biodiversity, unique research possibilities. Finding a proper equilibrium between managed and unmanaged forests, providing the highest landscape-level diversity is the responsibility of all decision-makers in any sectors dealing with forests. ACKNOWLEDGEMENT Zahvala Authors express gratitude to local foresters, especially E. Burián, T. Molnár, A. Pyber and I. Várady for help in fieldwork. Students of the UP, R. Hollós, V. Magyaros, E. Rogács, V. Sass, D. Varga-Tiffán and A. Vida essentially contributed to fieldwork. We are grateful to Dragica Purger for the Croatian translation and also correcting the English text. REFERENCES Literatura Annighöfer, P., P. Beckschäfer, T. Vor, C. Ammer, 2015: Regeneration patterns of European oak species (Quercus petraea (Matt.) Liebl., Quercus robur L.) in dependence of environment and neighborhood. PLoS ONE 10(8): e0134935. doi:10.1371/journal.pone.0134935 |