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ŠUMARSKI LIST 1-2/2017 str. 43     <-- 43 -->        PDF

spontaneous regeneration potential. The phenomenon is documented in Hungary at the very beginning of 20th century by Illés (1905) and by many other authors since that. Although seedlings may be abundant in the herb layer, they extremely rarely grow up into the shrub layer.
Quercus robur is a light-demanding species, this is why poor light conditions under closed canopy and in small-sized gaps hinder its regeneration (Miklin and Cizek, 2014; Annighöfer et al. 2015). An impressive amount of empirical evidence on the spontaneous regeneration of oaks in open or semi-open natural or anthropogenic landscapes, but not under closed stands, has been published by Vera (2000).
Browsing of too high game populations (Cervus elaphus, Capreolus capreolus) is also often-mentioned cause of insufficient oak forest regeneration (Hobza et al. 2007). High game populations may cause a change of stand structure not directly by oak damage, but with creating advantageous competition situation for less browsing-sensitive woody species, especially for thorny ones. The change of old-growth oak stands into thickets of shrubs had been largely overseen by researchers. Shrub species are usually excluded from forest structure studies; an exception is Tanentzap et al. (2012), who studied the interaction of deer browsing and survival of Crataegus and Corylus in the understorey of a mixed ash woodland.
Shrubs are documented in phytosociological works (e.g. Wendelberger, 1973; Drescher, 1985 and references therein), but usually only as a layer in the forest. Shrub communities of Crataegus monogyna (and Cornus sanguinea) are classified into Prunetalia spinosae order (Ellenberg, 1996; Borhidi, 2012), though being considered as an anthropogenic, temporary, degraded phase seldom phytosociologicaly studied in details. Kevey (1998) stresses that this issue should be investigated in more details and notes that they may be elements of natural vegetation dynamics. Thorny thickets, especially of Prunus spinosa and Crataegus monogyna protect juvenile oaks from browsing, so enable them to „grow out of the mouth of game”: they are key elements of the shifting mosaic cycle of oak regeneration on the landscape scale (Vera, 2000).
Knowledge gaps and research goals – Nepoznanice i ciljevi istraživanja
Natural regeneration processes in pedunculate oak forests are especially difficult to study, due to the lack of strict forest reserves, where natural processes are able to manifest.
Existing studies describe stand structure with special attantion to Quercus robur, but seldom refer to other woody species. Our aim is thorough a detailed description of the stand structure of a floodplain Quercus robur strict forest reserve after 17 years of unmanagement, with special respect of mass regeneration of shrub species.
MATERIALS AND METHODS
Materijal i metode
Study area – Područje istraživanja
Our study was performed in Bükkhát Forest Reserve, situated in the floodplain of Drava river (southern Hungary), in an elevation 96-100 m a.s.l.; GPS coordinates: N45°52’43.83’’; O18°00’28.27’. It is the largest floodplain oak forest reserve in Hungary with its area of 452 ha, declared as a forest reserve in 2002; but some parts are unmanaged since 1997. It has two strictly protected core areas of 58 ha, free of any kind of human intervention; and a buffer zone managed by close-to-nature silvicultural method (gap management), required by the Hungarian Forestry Act of 2009. Most stands are above 80 years, with a maximum of 120 years dominated by pedunculate oak (Quercus robur L.) high forest. The climate is moderately warm – moderately wet, mean annual temperature is 10.4 oC, mean annual precipitation is between 730-760 mm, with summer drought period occasionally up to two months. Soils are Histosols and Luvisols (FAO-UNESCO, 1997). Groundwater table is at 2-4 m (Dövényi, 2010). Depressions (former watercourse beds) are covered by water in springs up to three-six weeks. Dominant phytosociological associations are hornbeam-pedunculate oak forests (Circaeo-Carpinetum Borhidi em. Kevey 2006), and nemoral floodplain forests of large rivers dominated by Fraxinus angustifolia Vahl (ass.A-8 in Douda et al., 2015) with Quercus robur, and Ulmus laevis Pall., (formerly known as oak-ash-elm hardwood alluvial forests (Carici brizoidis-Ulmetum Kevey, 2008), with a transitional zone between them (Ortmann-Ajkai, 2002). The shrub layer is dense in the oak-ash, but may be missing in the oak-hornbeam stands. Characteristic herb species are: Circaea lutetiana L., Carex sylvatica Huds., C. remota L., Stachys sylvatica L., Pulmonaria officinalis L., Galium odoratum (L.) Scop., Rubus fruticosus agg. and R. caesius L. Both associations are natural habitats of European Community importance (NATURA 2000 habitats), listed as „Riparian mixed forests of Quercus robur, Ulmus laevis and Ulmus minor Mill., Fraxinus excelsior or Fraxinus angustifolia, along the great rivers (Ulmenion minoris; 91F0)” and „Illyrian oak-hornbeam forests (Erythronio-Carpinion; 91L0)” (EC, 2007). For a more detailed description see also Ortmann-Ajkai (1998). Current even-aged stands originate from planting of pedunculate oak on clear-cut areas, intensively managed in the first years, then thinned in about ten-years periods, as usual in Hungary. No viable spontaneous regeneration of Quercus robur can be observed, although seedlings are abundant. The region is one of Hungary’s most reputed red deer (Cervus elaphus) hunting area. Populations of red deer, and also of wild boar, are very high.
Field sampling – Uzorkovanje
Sampling was made in the 15 ha southern core area, in stands above 120 years, unmanaged since 1997.