UVOD
Oak-hornbeam forests (alliances Carpinion betuli Issler 1931 and Erythronio-Carpinion betuli (Horvat 1958) Marinček in Wallnöfer et al. 1993) are among the most phytosociologically researched forests in Croatia; nevertheless, vast areas of these forests have never been investigated.
The syntaxomonical status of the Erythronio-Carpinion betuli alliance has not yet been fully resolved (Vukelić 2012). However, Illyrian forests, which are considered to be a part of this alliance, are still present in the official classification of higher-rank syntaxa of Croatia (Škvorc et al. 2017) as well as in the Croatian National habitat classification (Topić and Vukelić 2009) and European EUNIS Habitat Classification (Chytrý et al. 2020). In Croatia, within the Erythronio-Carpinion betuli alliance, ass. Epimedio-Carpinetum betuli (Horvat 1938) Bohridi 1963 is the most studied, and traditionally, six subassociations of this association have been described, namely caricetosum pilosae, erythronietosum, asperuletosum, staphyletosum, castaneetosum, and tilietosum tomentosae (Vukelić 2012). Among them, subass. staphyletosum occurs on soils rich in carbonates and without a highly illuviated horizon (Gračanin 2003). However, Vukelić et al. (2015) questioned the division of EpimedioCarpinetum betuli into the traditionally described subassociations and showed that the criteria for this classification are not as solid as considered before.
The ass. Epimedio-Carpinetum betuli is present in both the Dinaric and Pannonian parts of Croatia (Horvat 1938, Anić 1940, Horvat et al. 1974, Šegulja 1974, Pelcer 1975, 1979, Vukelić 1991, Baričević 2002, Škvorc 2006, G. Horvat 2011, and others). However, to date, none of the oak-hornbeam forests in the region of Ozalj and along the middle reach of the Kupa River have ever been phytosociologically investigated.
The whole reach of the Kupa River, from its source in the Risnjak National Park to the city of Ozalj, is mostly surrounded by limestone and dolomite canyons, which were deeply carved by the river. When it reaches the city of Ozalj, Kupa leaves the Dinaric ecoregion end enters the Pannonian ecoregion, and this area is the approximate edge of the former Pannonian Sea (see ter Borgh et al. 2013). Some phytosociological research has been carried out on the vegetation along the Croatian side of the Kupa River (Horvat 1951, Vukelić et al. 2012, Vukelić et al. 2018, Kovačić 2019), but only in the upper part of its stretch in the area of Gorski Kotar. However, no phytosociological data exists for the vegetation in the middle stream of the Kupa River. In general, phytosociological research in the whole Ozalj area is scarce, and to date, only one locality has been investigated within a research project on sweet chestnut ( Castanea sativa Mill.) forests (Medak 2011).
In the present study, phytosociological data on the oakhornbeam forest inhabiting the limestone cliffs along the Kupa River canyon (Kamanje, W-Central Croatia; Fig. 1.), which are included in the Natura 2000 ecological network, is provided for the first time.
MATERIJALI I METODE
Study area – Područje istraživanja
Figure 1. Study area (Kamanje, West-Central Croatia). Map source:www.croatiamap360.com
Slika 1. Istraživano područje (Kamanje, zapadni dio središnje Hrvatske). Izvor karte:www.croatiamap360.com
The investigated oak-hornbeam forest (Fig. 2) is situated on cliffs composed of biolithitic and bioclastic limestones and carbonate breccias and conglomerates (Bukovac et al. 1984), spanning along the shore of the Kupa River, along a transect that passes through Petrinski kut, Orljakovo, and Kamanje. A detailed geomorphological assessment of the Kamanje landscape is provided in Bregni (2020). In this area, the Kupa River serves as the state border between Croatia and Slovenia. In the present study, we investigated the forest on the Croatian side. The studied forest covers an area of around 15 hectares. The underlying soil is calcocambisol developed from the abovementioned rocks, with many limestone rocks protruding from the soil surface (see Husnjak 2014). According to the World Reference Soil Base (2015), this soil type corresponds to calcic cambisol. In the last century, the forest was rich in nose-horned vipers ( Vipera ammodytes) and common European viper snakes ( Vipera berus). However, within the last 20 years, their population greatly decreased, with the reason remaining unknown. In addition, this forest harbors a geomorphological nature monument, the Vrlovka cave.
Field vegetation survey – Terensko istraživanje vegetacije
A total of 14 relevés (13 relevés of ~400 m2 in size and one relevé of ~250 m2 in size) were surveyed using a classical six-degree Braun-Blanquet scal e (Braun-Blanquet 1964). Some of the relevés were placed on the top of the limestone cliffs with low inclination, while others were placed on their highly inclined edges. The field phytosociological research was conducted partly in 2020 and mainly in 2021. Surveys were conducted during early spring, late spring, and summer. During the surveys, all seedlings were assigned as part of the herb layer.
Figure 2. Various aspects of the oak-hornbeam forest in the karst canyon of the Kupa River in the study area; a – view of the forest from the river (Slovenian forest on the left side and Croatian forest on the right side of the Kupa River); b – part of the Kamanje forest on the top of the limestone cliffs; c and d – parts of the Kamanje forest on the slopes of limestone cliffs
Slika 2. Različiti aspekti hrastovo-grabove šume u krškom kanjonu Kupe na istraživanom području; a – pogled od rijeke prema šumi (slovenska šuma s lijeve strane i hrvatska s desne strane Kupe); b – dio Kamanjske šume povrh vapnenačkih litica; c i d – dijelovi Kamanjske šume na padinama vapnenačkih litica
Data analyses – Analize podataka
The relevés obtained in the present study were compared to those of various Epimedio- Carpinetum betuli stands from the Dinaric and Pannonian areas. In the analyses, besides our 14 relevés, 97 relevés from other authors were used, including 30 from Horvat (1938), 25 from Vukelić (1991), 26 from Šegulja (1974), and 16 from Pelcer (1975). The main matrix combining all these relevés was made using the Turboveg 2.0 software (Henekens and Schaminée 2001), in which the Braun-Blanquet cover values were converted into mean percentage cover values. Moss species were excluded from the analyses as not all authors recorded them, and different vegetation layers were merged.
Agglomerative hierarchical clustering using complete linkage on the Bray–Curtis species × site dissimilarity matrix was used to assess the relationships between the analyzed forests. This was performed in R with the package ‘stats’ using the hclust function. Prior to the cluster analysis, species percentage cover values were square-rooted. In addition, non-metric multidimensional scaling (NMDS) set for 2D solution was used to see if the clustering patterns can be confirmed with the ordination method. NMDS was done in the package ‘vegan’ (Oksanen et al. 2013) using the metaMDS function (ordination plot not shown). The nomenclature of plant taxa follows Kew’s Plants of the World Online (POWO 2023).
REZULTATI
Clustering results – Klasterska analiza
According to the results of the cluster analysis (Fig. 3), the oak-hornbeam forest analyzed in the present study was separated into a unique cluster. In the other large cluster, two main groups could be recognized. One group contained the subass. staphyletosum, erythronietosum, and caricetosum pilosae, which were grouped into two, mostly author-dependent, clusters. The other group was also composed of two clusters, one containing the subass. asperuletosum and luzuletosum albidae, and the other containing the subass. seslerietosum and luzuletosum albidae, the Veratrum album variant, as well as four of the caricetosum pilosae relevés. NMDS gave a similar clustering pattern of the subassociations with a stress of 22%.
Plant community composition and structure of the studied forest – Florni sastav i struktura istraživane šume
The oak-hornbeam forest investigated in the present study comprised a total of 164 vascular plant species (Table 1). The number of species in a relevé ranged from 42 to 80, with a mean ± standard error of 63 ± 3.
According to the traditional classification of sessile oak and common hornbeam forest subassociations in Croatia, this oak-hornbeam forest of Kamanje can be classified as ass. Epimedio- Carpinetum betuli subass. staphyletosum. Among the diagnostic species of this subassociation, Staphylea pinnata occurred in 78% of the relevés in the shrub layer and in 65% of the relevés in the herb layer. Hacquetia epipactis occurred in 78% of the relevés, followed by Carex digitata occurring in 71% of the relevés.
Figure 3. Dendrogram produced by hierarchical clustering using complete linkage
Slika 3. Dendrogram dobiven hierarhijskom klasterskom analizom koristeći complete linkage metod
Table 1. Relevés of the studied oak-hornbeam forest (Kamanje, West-Central Croatia)
Tablica 1. Fitocenološki snimci istraživane hrastovo-grabove šume (Kamanje, zapadni dio središnje Hrvatske)
Herb layer species occurring in only one relevé (number in the parentheses represents relevé number): Neottia nidus-avis (1), Luzula forsteri (1), Veronica chamaedrys (2), Eupatorium cannabinum (2), Erigeron annuus (2), Acer obtusatum (3), Poa pratensis (4), Orobanche sp. (4), Corydalis bulbosa (5), Pilosella officinarum (5), Melittis melissophyllum (6), Helleborus atrorubens2 (6), Asplenium ruta-muraria (6), Juniperus communis (6), Daphne mezereum (7), Gentiana asclepiadea (7), Cardamine waldsteinii (9), Viburnum opulus 10), Cardamine impatiens (10), Euphorbia verrucosa (10), Chelidonium majus (10), Stellaria nemorum (10), Paris quadrifolia (10), Doronicum cf. austriacum (10), Hieracium sp (10), Allium ursinum (11), Galinsoga ciliata (11), Aconitum lycoctonum (11), Polystichum lonchitis (12), Viscum album (13), Taraxacum officinale (13), Impatiens glandulifera (13), Lycopus europaeus (13), Dryopteris carthusiana (13), Cystopteris fragilis (13), Centaurea jacea (13), Symphytum tuberosum (13), Salvia glutinosa (14), Ranunculus ficaria (14), Lathraea squamaria (14), Anemone ranunculoides (14), Alnus glutinosa (14)
Legend: A – tree layer, B – shrub layer, C – herb layer (including the seedlings)
* Species of the Illyrian-Balkan floral geoelement (according to Vukelić 2012)
Among the diagnostic species of the ass. Epimedio- Carpinetum betuli, Epimedium alpinum was present in all relevés, whereas Lonicera caprifolium and Primula vulgaris were present in 57% and 50% of relevés, respectively. Many Illyrian species characteristic for the Erythronio- Carpinion betuli and Aremonio-Fagion alliances were also frequent, especially Staphylea pinnata, Galanthus nivalis, Cardamine enneaphyllos, Cyclamen purpurascens, Lamium orvala, Helleborus dumetorum, Erythronium dens-canis, Omphalodes verna, Isopyrum thalictroides and Hacquetia epipactis.
In the tree layer, Carpinus betulus was the most abundant and most frequent tree species, followed by Quercus robur and Tilia cordata. In terms of frequency, these species were followed by Fraxinus ornus, Acer campestre, and A. pseudoplatanus. In the tree layer, Q. petraea appeared in only 29% of the relevés. Here, we must emphasize that many of the recorded Q. petraea individuals highly resembled the intermediates between Q. robur and Q. petraea. However, for the purpose of our phytosociological survey, the possible hybrids having similar morphological features to that of Q. petraea were recorded as Q. petraea. In the shrub layer, Corylus avellana was the most abundant and frequent species, followed by Euonymus verrucosa, Staphyllea pinnata, Cornus mas, Fraxinus ornus, and Acer campestre. In the herb layer, Epimedium alpinum was the dominant species in more than half of the relevés. Moreover, many spring-flowering geophytes and hemicryptophytes appeared in all or almost all relevés, including Galanthus nivalis, Cyclamen purpurascens, Cardamine enneaphyllos, Anemone nemorosa, Hepatica nobilis, Asarum europaeum, Lamium galeobdolon, Pulmonaria officinalis, Omphalodes verna, and Stellaria holostea, among others. Some ferns also appeared in more than 90% of relevés in the herb layer, including Asplenium trichomanes, A. scolopendrium, Polypodium vulgare, and Dryopteris filix-mas. Among the seedlings, the most frequent were those of Q. robur, A. campestre, A. pseudoplatanus, and E. verrucosa.
RASPRAVA I ZAKLJUČCI
The oak-hornbeam forest investigated in the present study belongs to ass. Epimedio-Carpinetum betuli (all. ErythronioCarpinion). At the traditional subassociation level, this forest is edaphically and floristically the closest to staphyletosum (Gračanin 2003, Horvat 1938); however, the current classification of Epimedio-Carpinetum betuli into various subassociations still needs to be resolved (Vukelić et al. 2015).
Our findings showed many differences between the studied forest from Kamanje and those of the same association researched by other authors. Firstly, one very peculiar difference is the high frequency of species characteristic for the Asplenietea trichomanis class in the Kamanje forest, which was related to the high abundance of stony microhabitats favorable for their growth. This especially refers to the ferns Asplenium trichomanes and Polypodium vulgare, appearing in 100% and 93% of the relevés, respectively. In addition, other species characteristics for this class were present in this forest, such as Saxifraga cuneifolia, Moehrinigia mucosa, and Cardaminopsis arenosa. These three species were not highly frequent in our relevés; however, they grow on extremely inclined stony microhabitats within the forest, and parts of these microhabitats were not accessible in this phytosociological survey.
Secondly, an interesting feature of this forest is that Q. robur replaces Q. petraea. Even though the limestone cliffs on which the forest occurs are 15–25 meters higher than the fluvisols in which the Q. robur usually grows near the Kupa River, it is still the dominant oak species in the tree layer. It seems that it is able to protrude its roots through the limestone crevices and reach the groundwater favorable for its growth while also taking advantage of the seepage water intruding from the upper hills. Mixed Q. robur and Q. petraea stands are not as rare in Croatia as it could be interpreted from the literature. They are very common in northern parts of Croatia on gentle slopes and transitional areas from colline to planar belts. Examples include the areas of Bjelovarska Bilogora, Kalnik, lower slopes of Slavonian hills, Medvednica Mountain, and others.
Furthermore, a specific floristic feature of the studied forest is the high abundance of certain ferns that were not recorded in the forests studied by other authors. These include Asplenium scolopendrium, which appeared in 93% of the relevés, followed by Polystichum setiferum and P. aculeatum appearing in 57% and 43% of the relevés, respectively. Other species occurring in more than half of the relevés that were also recorded only in the Kamanje forest were Omphalodes verna and Ulmus minor.
In addition, many species that were also present in the forests investigated by other authors were much more frequent in the Kamanje forest. In the herb layer, these include Hepatica nobilis, Galanthus nivalis, and Isopyrum thalicthroides. Among the shrubs, these include Cornus mas and Euonymus verrucosa, and among the tree species Tilia cordata. Indeed, E. verrucosa appeared in all the relevés of the forest studied by Pelcer (1975), which he assigned to the seslerietosum subbasociation. However, his study included only four relevés per subassociation; thus, his observations cannot be used for floristic generalization about the Epimedio-Carpinetum betuli forests in the whole Lika plateau, where his research was conducted.
Another interesting feature of the Kamanje forest is the high frequency of Carex pilosa, H. nobilis, and E. dens-canis. The first two are traditionally considered as diagnostic for subass. caricetosum pilosae, and E. dens-canis is considered as diagnostic for subass. erythronietosum. The high occurrence of these species in the Kamanje forest, despite it being most similar to the subass. staphyletosum, supports the discussion of Vukelić et al. (2015), who questioned the traditional classification of these subassociations.
As mentioned before, if we followed the trend of classifying the Epimedio-Carpinetum betuli association into the traditional subassociations, the Kamanje forest investigated here would be classified as subass. staphyletosum. In addition, because of the high occurrence of ferns, which were not present in any of the previously studied forests, the Kamanje forest could be considered as a fern-rich staphyletosum subassociation. Because of the high limestone abundance and frequently high inclination, there are many rock crevices serving as microhabitats for these species. Thus, A. trichomanes and P. vulgare are the species that best reflect the stony habitat conditions in the Kamanje forest in contrast to other Epimedio-Carpinetum betuli forests. In addition, high air and soil moisture because of the proximity of the Kupa River, abscence of wood cutting, and high canopy closure also favor the growth and reproduction of other ferns, such as D. filix-mas, A. scolopendrium, Athyrium filix-femina, P. setiferum, and P. aculeatum.
We also want to note that the oak-hornbeam forests along the Kupa River canyon span farther than the area of our research, and these other areas should also be phytosociologically studied (including those on the Slovenian side of the river). This includes the canyon parts stretching further from Kamanje to the city of Ozalj, as well as the oak-hornbeam forests in the broad area of Vodenica hill. Most parts of these forests are not as preserved as the one in Kamanje because they are used for wood extraction, but research on these forests would still provide valuable insights about the oak-hornbeam forests on the south-west edge of the Pannonian basin, which are highly understudied.