|ORIGINAL SCIENTIFIC PAPERS|
|Vukelić, J., D. Baričević, Z. List, M. Šango||UDK 630* 188 (001)|
|A contribution to the phytocoenological research of forests of common alder (Alnus glutinosa Geartn) in Podravina pdf HR EN||479|
|Summary: Forests of common alder in Croatia cover approximately 8000 ha (Prpić and Milković 2005). The most valuable stands are found in the management unit Đurđevačke nizinske šume in Podravina. In this area common alder forms pure or mixed stand extending over about 2000 ha. The stands manifest distinctly regular growth of good quality. From a phytocoenological aspect they are characterized by a natural, diverse and rich composition. The two associations dominated by common alder were studied and described in detail by M. Glavač in his dissertation of 1960. These associations include the phytocoenosis of common alder and elongated sedge (Carici elongatae-Alnetum glutinosae) growing in floodplain sites with stagnant water and micro-depressions, and a much drier community of common alder and narrow-leaved ash with birdcherry (Pruno-Fraxinetum) continuing onto the former community.|
There have been many changes in the past fifty years elapsing since the research by Glavač, including the construction of hydropower stations on the river Drava, drastic ecological transformations and regeneration of the majority of common alder forests. We have undertaken phytocoenological research in order to assess the consequences of the changes on the structure, dynamics and composition of these two associations. The planned field research was carried out in the vegetation period of 2006 using the standard method of the Braun-Blanquet school. In doing so, we were able to analyze and compare the findings, since former research was also conducted using the same method. Stands with a stable floral composition aged 50 and over were used for relevés. The relevés were sized 20 × 20 m on average. A total of 28 relevés were made: 7 in the forest of common alder with elongated sedge and the rest in the association Pruno-Fraxinetum. Mosses were not considered.
The forest of common alder with elongated sedge (Carici elongatae-Alnetum glutinosae W. Koch 1926) in Podravina grows on peat and base-rich, humus gleyic soils saturated with water, which separate this community from the adjacent ones. The basic factor in the development of this community is the dynamics of groundwater oscillations. Here, alder is in its optimal stage of development. Preserved here since the glacial period, it has a relict character. It is classified into three subassociations: iridetosum pseudacori occurring in the wettest sites, where alder forms high cones surrounded with water almost throughout the year, polygonetosum hydropiperis growing in wet soils with a highly developed humus layer, and cornetosum sanguineae growing in relatively drier soils in progression. The floral composition of the community is given in Table 1, which contains 7 of our own relevés and 13 relevés by Glavač from 1960 showing a degree of participation.
According to the research, the range of the forest Carici elongatae-Alnetum glutinosae has decreased manifoldly. Only some fragments have remained in water-covered depressions smaller than half a hectare from which water cannot escape. A comparison of the community’s floral composition some fifty years ago and today (Table 1) reveals changes not only in the size of the area but also in the composition of the plant species. There are fewer hygrophytes and species of swampy depressions with long-lasting stagnant water, such as Carex elongata, Poa palustris, Lythrum salicaria, Valeriana dioica, Rorippa amphibia, Calystegia sepium, Mentha aquatica, Carex vesicaria, and Glyceria fluitans, whereas some species, e.g. Spiraea salicifolia, have already disappeared. The natural syndynamic development has also stopped, so that only a subassociation with Iris pseudacorus can currently be identified. The other stands have already been converted to Pruno-Fraxinetum.
The forest of common alder and narrow-leaved ash with birdcherry (Pruno-Fraxinetum Oberdorfer 1953) is formed by the succession from the previous association. It grows on drier and elevated terrains. It belongs to the order Fagetalia due to the lack of hygrophytes and the prevalence of mezophilic species. The floral composition of the community is given in Table 2. The first column shows 15 relevés by Glavač and the second 19 relevés from this research.
A comparison of the phytocoenological relevés shows an increased proportion of mezophilic species and an absence or a reduced number of the species that indicate swampy and very wet forests. The species occurring less frequently are Galium palustre, Ranunculus repens, Lycopus europaeus, Poa trivialis, Valeriana dioica, and Potentilla reptans, while on the other hand Sambucus nigra, Corylus avellana, Prunus padus, Mercurialis perennis, Paris quadrifolia, Anemone nemorosa, Leucojum vernum, Galeobdolon luteum and Lamium purpureum have grown in number. Some particular sites currently feature Acer pseudoplatanus and Daphne mezereum, species that are not typical for the lowland area of Croatia. In the past fifty years, the area under these associations has increased considerably.
The name and the description of the association originate in Central Europe, where these stands contain Fraxinus excelsior instead of Fraxinus angustifolia. For reasons of this and other differences, this community must be analyzed in detailed and compared with the Central European community. Its position in the phytocoenological nomenclature and systematics must also be determined.
Changes in the stands of common alder occurring in the past fifty years are primarily the consequence of the synergy of altered ecological factors and anthropogenic factors, i.e. stand management. Although the groundwater levels in the forest stands being investigated are still relatively favourable (Fig. 3), the newly built hydropower stations in Slovenia and Croatia have generally altered the water regime. The main problem facing the remaining wet areas of the river Drava in Slovenia and Croatia is the low water level amounting to only 2.4 % of the average water flow. Additional problems include significant oscillations of the river and the erosion of the riverbed. In the past 100 years the water surface area has decreased by 65 %, whereas open sites such as river islands and pebbly and sandy shoals have decreased by as much as 92 % (Schneider-Jackoby 2006).
Another important factor in the analysis of the condition and changes in the forests of the study area is the type of management, or more accurately, the regeneration method applied on the stands of common alder. Stands over 80 years old are cut in a single treatment and the site is cleared and regenerated with seedlings of common alder in the same year. Although such an approach is highly efficient, cheap and good from a silvicultural standpoint, it does not take sufficient account of the ecological, vegetational and other specific features and differences. This results in the uniformity of the stands and sites and a decrease or disappearance of some species – primarily hydrophytes and hygrophytes. Moreover, the natural onset, participation and growth of other species, such as the narrow-leaved ash and even pedunculate oak, are slowed down. Changes caused by the construction of roads and the adjacent canals, as well as a direct human impact, constitute another important factor.
Key words: a change in the floral composition; Carici elongatae-Alnetum glutinosae; Common alder; Podravina; Pruno-Fraxinetum
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