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ŠUMARSKI LIST 1-2/2018 str. 22     <-- 22 -->        PDF

Alnus incana (L.) Moench subsp. incana (hereafter Alnus incana), family Betulaceae, is a diploid (2n = 2x = 28), monoecious and wind-pollinated, relatively short-lived, small to medium-sized deciduous tree (Tallantire 1974; Mandák et al. 2016; Vít et al. 2017). It is a light-demanding, hygro-mesophilous species which tolerates low winter temperatures. The grey alder can be found on stream banks, lake shores, damp meadows and in wet mountain environments at elevations of up to 1800 m (Schütt and Lang 2014; Houston Durrant et al. 2016). Along with all other alder species, it is unique within the Betulaceae family since it can fix nitrogen in symbiotic root nodules with the actinomycetes bacteria from the genus Frankia (Benson et al. 2004; Vanden Heuvel 2011). In addition, it is an important pioneer species with the ability of fast regeneration and colonization of disturbed or damaged areas. Similar to the Norway spruce (Picea abies /L./ H.Karst), the range of the grey alder is divided into a northern (subsp. kolaensis /N.I.Orlova/Á.Löve et D.Löve) and a southern area, which meet in the Polish lowlands (Douda et al. 2014). The distribution of subsp. incana in the southern part of the range is linked with a patchy mountain occurrence (in the Alps, the northern Apennines, the Hercynian mountains, the Carpathians, the Bulgarian mountains, the Dinaric Alps, the Caucasus, and mountains in Turkey) and continues eastward across European Russia to western Siberia (Mandák et al. 2016). Grey alder is represented by two subspecies in North America: subsp. rugosa (Du Roi) R.T.Clausen and subsp. tenuifolia (Nutt.) Breitung.
In Croatia, A. incana inhabits north-western and north-eastern parts of the country at the southern border of its natural range of the Central European mountains (Vukelić and Rauš 1998; Vukelić 2012; Trinajstić 2008; Poljak et al. 2014). This part of the grey alder’s natural distribution is completely separated from its southern natural range in the Balkan Peninsula. Here, the grey alder appears in two biogeographical regions with contrasting climates: the continental region, along the course of the Drava river; and in the mountainous Alpine-Dinaric region in Gorski kotar, along the course of the Kupa river and its tributaries. This biogeographical divergence caused by strong climatic gradients and complex topography resulted in a clear floristic differentiation of grey alder stands in Croatia.
In the continental region of Croatia, the grey alder occurs mainly in riparian and floodplain forests along the main watercourse of the river Drava, where it forms smaller and isolated populations. These populations are included within the association Equiseto hyemali-Alnetum incanae Moor 1958 (Trinajstić 1964; Franjić et al. 1999). This association is characterized by the presence of hygrophilous species, such as: Fraxinus angustifolia Vahl subsp. oxycarpa (Willd.) Franco et Rocha Afonso, Ulmus laevis Pall., U. minor Mill., Prunus padus L., Equisetum hyemale L., and others. The former forest areas that were studied by Trinajstić (1964) are today no longer covered by forest. Riparian and floodplain forests close to urban centres are being dried out, and their habitats are transformed into residential areas and municipal infrastructure so that grey alder forests along the course of the Drava river are nowadays in strong regression (Vukelić 2012; Poljak et al. 2014).
Stands from the north-western Dinarides are defined within the association Lamio orvalae-Alnetum incanae Dakskobler 2010, where a geographic variant with Helleborus dumetorum is described (Vukelić et al. 2012, 2017). In general, the main characteristic of these stands is floristic heterogeneity and high richness of species, mainly as a consequence of the biogeographical position and floristic development of the western Dinarides (Bennett et al. 1991; Petit et al. 2003; Magri et al. 2006; Liepelt et al. 2009; Temunović et al. 2013) – one of the floristically richest forest areas in Europe. The following species are particularly prominent: Fraxinus excelsior L., Ulmus glabra Huds., Fagus sylvatica L., Acer pseudoplatanus L., Lamium orvala L., Scopolia carniolica Jacq., Helleborus dumetorum Waldst. et Kit., Lunaria rediviva L., Knautia drymeia Heuff. subsp. drymeia, and others. The occurrence and growth of grey alder forests in this region is conditioned by occasional flooding of the river Kupa and its tributaries over a length of about thirty kilometres. The river partially flows through a canyon and partially extends along to horizontally elongated terraces, on which forests were cut down in the past. In the last 50 years, land cultivation has been gradually abandoned, giving place to natural succession of vegetation.
To date, several genetic studies have been carried out on the grey alder. Microsatellite primers specific for the A. glutinosa (L.) Gaertn and A. incana, developed by Lepais and Bacles (2011), were used to evaluate the population genetic structure and diversity of 65 populations, and to reconstruct the historical pattern of postglacial biogeographical range expansion of the boreal tree species A. incana in Europe (Mandák et al. 2016). Furthermore, genetic diversity and clonal structure of 24 populations covering the Central European part of the species range from the Scandinavian Peninsula to the Balkans were analysed utilizing four nuclear microsatellite markers (Dering et al. 2016). Additionally, several studies have addressed the morphological variability of grey alder populations (Krauze-Michalska and Boratyńska 2013; Poljak et al. 2014). Krauze-Michalska and Boratyńska (2013) revealed a high level of morphological variation of leaves and significant differences between the Scandinavian and all other European populations of A. incana. By contrast, in our previous research of grey alder populations in Croatia (Poljak et al. 2014), along the upper course of the Drava river, the absence of inter-population variability was observed. However, no research of the grey alder’s morphologic variation on a larger geographical scale in Croatia has been performed thus far.