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Vegetation sampling was implemented in 2012, according to the Hungarian Forest Reserve Protocol (Horváth et al., 2012). A systematic 50x50 m grid of sampling points was permanently marked in the field (54 points in all). Three surveys have been carried out in each sampling points: of the tree stand structure, of the regeneration and shrub layer and of the herb layer. Tree layer survey included all individuals above 5 cm diameter at breast height (DBH) in a circle of 6,92 m radius (250 m2 area) plus those included in a horizontal point sampling (DBH>25.2 cm, gauge constant k=2; basal area factor=2). Species, DBH, crown position compared to neighbouring trees, and health on a five-grade scale were recorded for each individual tree. Survey of shrub and regeneration layer included individuals of all woody species above 50 cm height and under 5 cm diameter at soil surface. Eight sampling circles of 4 m2 (1.13 m radius), were surveyed at each sampling point, positioned systematically in 6 m distance around the centre. Species, height in two categories (50-130 cm and above 130 cm) and browsedness of the apical shoot (Y/N) were recorded for each individual rooting in the circles. Herb layer study was implemented in 30 circles of 0.5 m2 positioned systematically inside a circle of 6 m radius around the center of the sampling point. Cover of each herb species, and of woody species under 50 cm height were recorded on the Braun-Blanquet scale (Braun-Blanquet, 1928; cited in Whitaker, 1980). Stand height, cover of upper (A1) and lower (A2) crown layer, of shrub (B) and herb layer (C) and gap size (four categories) were also recorded for all sampling points. For the analysis of structural changes, data of 23 phytosociological releves were also included. Releves of 20x20m were made according to the Braun-Blanquet method in 1995-96 in the Bükkhát Forest reserve (Ortmann-Ajkai, 1998). Statistical analysis – Statističke analize Regeneration of seven woody species was analyzed trees: Quercus robur, Fraxinus angustifolia, Carpinus betulus L., Ulmus minor and Acer campestre L.; shrubs: Crataegus monogyna Jacq. and Cornus sanguinea L. Current stand structure, including vertical distributions, horizontal patchiness, gaps and some abiotic conditions, was characterized by principal component analysis (PCA) using CANOCO 4.55 (ter Braak & Smilauer, 2002). Change of stand structure was analysed comparing current abundance data with those of 1995-96. Total cover of layers A1, A2, B and C, and specific cover values for Quercus, Crataegus and Rubus were obtained from 23 phytosociological releves. As data have no normal distribution, non-parametric Mann-Whitney u-tests were calculated, using PAST (Hammer et al., 2001). RESULTS Rezultati Stand structure and change – Struktura sastojine i promjene As length of eigenvalues in a trial DCA implied linear data structure, PCA was selected for unconstrained ordination. Some structural characteristics of the study site are presented on the PCA scattergram (Figure 1). Majority of points represent hornbeam-oak stands (Circaeo-Carpinetum), characterized by Quercus robur in the upper and Carpinus betulus in the lower canopy and juveniles of Carpinus and Acer campestre in the shrub layer. There are many transitional points towards elm-ash stands (Carici brizoidis-Ulmetum). Some points characterised by Crataegus monogyna both in canopy and shrub layer, deviates from this broadly known situation. The first two PCA axes accounts for 54% variability (30/24% respectively), all PCA axes for 79;5%. Species in the tree layer separate well along the first two axes into three groups. The hornbeam-oak – elm-ash continuum was represented along the first axis. Typical grove forest species (Fraxinus in A1 and A2; Ulmus and Cornus in A2) have high or medium positive scores on both axes (0.20 – 0.56), while oak and hornbeam have both negative ones (-0.58 – -0.19). Crataegus monogyna in A2 forms a separate group. Crataegus has the highest scores for both axes (0.70 and -0.70), positive for the first but high negative for the second one. Carpinus betulus and Acer campestre in the shrub and herb layer also have high negative values on both axes, while juvenile individuals of grove forest species have lesser positive scores, Fraxinus even small negative ones. Note that juveniles of Quercus robur (in B2 and C) show remarkable positive values on Axis 2; characteristic of grove forest species (Figure 1). The spontaneous stand development, creating this present structure, was characterized comparing data of 1995 and 2012 (Table 1). The most remarkable change is the increase of the total cover of the second canopy (from 27% to 54%), and especially that of Crataegus monogyna (its total change from 2% to 14%, its proportional change in the canopy is from 6% to 24%). Cover of the shrub layer also has been increased to twofold (22% to 42%), but proportions of the species have not changed significantly. Cover of herb layer is high (about 80%) at both times, but cover of the regrowth of the seven studied woody species decreased from 12% to 4%. At the same time, cover of Crataegus monogyna in the herb layer has risen to sixfold (although only 0,4% even now), and that of Rubus with 45%. All these results are significant at p=0,01. Other changes see in Table 1. DISCUSSION Rasprava PCA scattergram (Figure 1) partly presents a characteristic picture of floodplain oak forests of Hungary, well-known |