Ass.: SALICI ELEAGNI-JUNIPERETUM COMMUNIS Poldini, Francescato, Vidali & Castello ass. nov. (Tab. 1)

Holotypus : rel. 2 of Tab. 1 in this paper.

Diagnostic species : Salix eleagnos, Achnatherum calamagrostis, Carex alba, Pinus nigra subsp. nigra, Pinus sylvestris, Populus nigra, Gypsophila repens, Hippophaë fluviatilis.

Structure and composition : Dense, rather impenetrable scrub, dominated by Juniperus communis, Salix eleagnos, Fraxinus ornus accompanied by Hippophaë fluviatilis, Ligustrum vulgare and many other shrubs, along with single small trees of Pinus nigra and P. sylvestris. The herbaceous layer is characterized by Achnatherum calamagrostis and Carex alba. Juniperus communis occurs with plants up to 4 m tall, with very elongated, down-curved, pendulous branches and rather long and spaced needles, corresponding to the controversial var. intermedia (Schur) Sanio, found in the submontane and montane areas in the foreland of South-Eastern Alps.

Syntaxonomy : The assignment to Rhamno-Prunetea is provided by the high incidence of shrubs of this class; the community is included in the endemic suball. Fraxino orni-Berberidenion for the high frequency of Ostrya carpinifolia and Fraxinus ornus. Compared to the other associations of this suballiance distributed from the South-Eastern Alps to the Dinarides, Salici-Juniperetum stands out for the entities related to soil moisture and loose gravel deposits correlated to fluvisols, and the strong dealpinism. The occurrence of glareicolous elements of Thlaspietea rotundifolii (Achnatherum calamagrostis, Gypsophila repens, Lomelosia graminifolia, Petasites paradoxus) in a community of Rhamno-Prunetea should be highlighted. It is a riverside ecotonal shrub community between the classes Rhamno-Prunetea and Salicetea purpureae.

Salici-Juniperetum differs from the analogous scrubs with Hippophaë fluviatilis described from Italy for the absence of Quercetea ilicis species differential of Pruno-Rubion ulmifolii (Tab. 2). Spartio juncei-Hippophaetum fluviatilis (col. 3 in Tab. 2) is a mantle vegetation of riparian woods on pebbly-gravelly substrates of recent terraces described from the River Taro (Emilia-Romagna region) by Biondi et al. (1997), which is differentiated by species with southern European distribution of the alliance Cytision sessilifolii, while Junipero-Hippophaetum fluviatilis (Pruno-Rubion ulmifolii) (cols 1, 2) is typical of the dune habitats of the North-Adriatic coasts (Géhu et al. 1984; Gamper et al. 2008). These thermophilous communities are both rich in Quercetea ilicis elements. The peculiar position of Salici-Juniperetum (col. 4) is given also by the presence of several dealpine species. Finally, Salici-Juniperetum is similar to the Central European Hippophao-Berberidetum (Berberidion) (col. 5), of which it constitutes the phytogeographical parallelism south of the Alps, being differentiated by the large presence of Juniperus communis, the scarcity of Berberis vulgaris and the high incidence of southern elements (Fraxinus ornus, Ostrya carpinifolia, etc.).

Compared to Salici incanae-Hippophaetum, a community of river gravel banks dominated by Hippophaë fluviatilis included in Salicetea purpureae and reported from Friuli Venezia Giulia by Oriolo and Poldini (2002), the new association is readily distinguished physiognomically by the prevalence of Juniperus communis and the absence of Salix daphnoides and S. purpurea.

Synecology : It grows on primitive, cobble-gravel soils on recent low river terraces subject to episodic flooding. It is found at about 100 m a.s.l., in the middle course and in the last part of the upper course of the River Tagliamento. Compared to the other scrub communities of river gravels it represents the least wet term. It is therefore possible to identify a sequence of scrub communities ordered along a gradient of decreasing soil moisture represented by Salicetum incano-purpureae, Salici-Hippophaetum and Salici-Juniperetum, establishing a typical topographic-catenal sequence.

Dynamic contacts : It may perhaps represent the result of the evolution of Stipetum calamagrostis and the initial stages of shrub encroachment of Centaureo dichroanthae-Globularietum cordifoliae (Festuco-Brometea).

Catenal contacts : In contact with pioneer communities of Epilobio-Scrophularietum caninae, communities with Xanthium italicum and elements of Dauco-Melilotion (Artemisietea).

Synchorology : Upper and middle course of the River Tagliamento, from Venzone to Valvasone (Friuli Venezia Giulia) (Suppl. material 1, Fig. S1), in the Temperate macrobioclimate, oceanic variant, upper mesotemperate to lower supratemperate thermotypes and lower humid to upper humid ombrotypes (according to Pesaresi et al. 2017).

Annex I Habitat (92/43/EEC Directive) : 3240.

Ass.: ULMO MINORIS-PALIURETUM SPINAE-CHRISTI Poldini & Vidali ass. nov. (Tab. 3)

Holotypus : rel. 8 of Tab. 3 in this paper.

Corresponding names : “Fitocenon a Paliurus spina-christi e Ulmus minor” in Poldini and Vidali (1995); Rubo ulmifolii-Ligustretum vulgare rubetosum caesii Poldini 1989.

Diagnostic species : Ulmus minor subsp. minor, Rubus caesius.

Structure and composition : Medium-tall, dense scrub characterized by Ulmus minor, always occurring as a shrub, Rubus caesius and Paliurus spina-christi, generally accompanied by Rhamnus cathartica, Prunus spinosa, Crataegus monogyna, Cornus sanguinea subsp. hungarica and Ligustrum vulgare. The dense shrub layer overshadows the undergrowth, and the herbaceous layer is poorly developed. The cover of Ulmus minor and Paliurus spina-christi follows the gradient of soil moisture with an inverse trend: where Ulmus minor is more frequent, Paliurus spina-christi decreases and vice versa.

Syntaxonomy : This community was interpreted by Poldini and Vidali (1995) and Poldini et al. (2002a) as a hygrophilous mantle connected to termophilous hedgerows included in the alliance Berberidion vulgaris (Prunetalia spinosae) and attributed to Fraxino orni-Berberidenion, a suballiance which is characterized by a mixture of Mediterranean, Central-European and Illyrian elements and represents a transition between the Central European Berberidenion and the Mediterranean Pruno-Rubion ulmifolii. In their treatment of the class Rhamno-Prunetea in Italy, Poldini et al. (2002b) included the community in the submesophilous coenoses of Fraxino orni-Berberidenion, encompassing the mantles of mesophilous and submeso-hygrophilous mixed deciduous forests.

Biondi (1999) suggested a possible inclusion of this community in Pruno spinosae-Rubion ulmifolii (Pyro spinosae-Rubetalia ulmifolii), including termophilous Mediterranean and submediterranean scrub communities with abundant Rubus ulmifolius occurring on moist soils and characterized by the presence of a large group of Mediterranean species (Biondi et al. 2014a; Biondi and Blasi 2015). The coenosis was not analysed in the subsequent revision of the Paliurus spina-christi-dominated vegetation of Europe by Casavecchia et al. (2015) because of the limited cover values of Paliurus spina-christi in the relevès published by Poldini and Vidali (1995).

Due to the preponderance of Illyric submediterranean xeric elements and the scarceness of Mediterranean elements the coenosis is maintained within the suballiance Fraxino orni-Berberidenion, of which it represents the least arid element of transition towards Pruno-Rubion. We therefore prefer to maintain the position discussed in Poldini et al. (2002a), which point out that the arrangement in Pruno-Rubion suggested by Biondi (1999) could be accepted from an ecological point of view, but it is not supported by floristic features.

Synecology : It is found in the highest areas of the banks of the karstic Lake Doberdò which are subject to episodic floods, representing the outermost situation influenced by the presence of water. It is an Illyric submediterranean thermophilous meso-hygrophilous scrub community that constitutes the landward mantle of the meso-hygrophilous Rhamno catharticae-Ulmetum minoris woodland introduced in this paper. Two aspects can be distinguished: a more hygrophilous one with abundant Ulmus minor, Rubus caesius and Rhamnus cathartica, and a more arid one dominated by Paliurus spina-christi, Fraxinus ornus and other elements of Prunetalia.

Dynamic contacts : In dynamic contact with Rhamno catharticae-Ulmetum minoris.

Catenal contacts : In contact landward with thermophilous aspects of karstic deciduous mixed oak woodlands (Aristolochio luteae-Quercetum pubescentis).

Synchorology : Karst Lake Doberdò (Friuli Venezia Giulia) (Suppl. material 1, Fig. S1).

Annex I Habitat (92/43/EEC Directive) : -

Ass.: GALIO PALUSTRIS-Salicetum albae Rauš 1976 (Tab. 4)

Lectotypus hoc loco : rel. 4 of Tab. F4 in Rauš 1976: 53.

Syntaxonomic synonym : Carici elatae-Salicetum albae Kevey 2008.

Corresponding names : “Aggr. a Salix alba” in Lastrucci et al. (2008); “Facies a Salix alba dell’ordine Alnetalia glutinosae” in Merloni and Piccoli (2001); Salicetum albae Issler 1926 subass. phragmito-caricetosum Jurko 1958 var. Carex elata in Bolpagni et al. (2007); Salicetum albae subass. phragmito-caricetosum Jurko 1958 in Šilc (2003).

Diagnostic species : vs Alnetea glutinosae: Salix alba, Salix purpurea; vs Salicetum albae s.l. (Salicion albae): Galium palustre s.l., Carex elata.

Structure and composition : Softwood forest with the tree layer dominated by Salix alba and the shrub layer poorly developed or absent including besides S. alba other willows such as S. cinerea, S. purpurea and S. triandra. The herbaceous layer can be well developed and it includes a large number of marsh elements ingressive from Phragmito-Magnocaricetea; common species are Galium palustre s.l., Carex elata, Limniris pseudacorus, Lysimachia vulgaris (Rauš 1976; Rauš et al. 1985). Given the wide extension of its distribution area there is a high variability in the floristic composition of the coenosis, affected by dynamic-catenal contacts dependent on territorial characteristics. In the Hungarian territory there is a greater participation of helophytic species (see Tab. 5).

As for Italy (Tab. 4), this community is rather rich in species, with Salix alba forming a rather open tree layer, where Alnus glutinosa occurs on more developed soils. In the shrub layer, Salix alba is often associated with Frangula alnus, Cornus sanguinea, Rhamnus cathartica, S. cinerea, S. purpurea. The floristic structure of the herbaceous layer is rather variable and similar to that of the Croatian and Hungarian stands: it is usually dominated by tall sedges, namely Carex elata and C. acutiformis, accompanied by other Phragmito-Magnocaricetea elements or by Rubus ulmifolius and various other shrubs in more degraded stages.

Syntaxonomy : The syntaxonomic treatment of Salix alba swamp woods is still critical: in many studies the hygrophilous woods dominated by the white willow have been designated with the name “Salicetum albae Issler 1926”, a riverine association described for Central Europe and assigned to the class Salicetea purpureae. Salicetum albae, however, is both floristically and ecologically different from the Salix alba swamp community assignable to the class Alnetea glutinosae here reported, which can be attributed on the whole to Galio palustris-Salicetum albae, an association described from the Danube River basin in North Eastern Croatia (Rauš 1976) (col. 2 in Tab. 5), and reported also from the Drava and other rivers of that part of Croatia (Rauš 1992; Rauš et al. 1985; Karadžić et al. 2015). Galio-Salicetum albae grows in depressions subject to long, frequent, up to 2-4 m high floods, in the swamps and oxbows of the great river systems in the southern Pannonian Plain, on pseudogley or gley soils (Rauš 1976). Rauš (1976) is the first author who distinguished the peculiar features of this white willow swamp at the association level, but he classified it in Salicion albae. In spite of the floristic-ecological differences, Šilc (2003) and Vukelić (2012) treated Galio-Salicetum albae as a syntaxonomic synonym of Salicetum albae Issler 1926.

Yet, Kevey (2008) is the first to recognize in the syntaxonomic classification the particular ecology of the Salix alba swamps that he investigated in the Great Hungarian Plain. He attributed these stands to the new association Carici elatae-Salicetum albae (col. 1 in Tab. 5), described from the Hungarian side of the River Drava along the border with Croatia, and assigned to Alnion glutinosae (Alnetea glutinosae).

However, Carici-Salicetum is both floristically and ecologically very similar to Galio-Salicetum. The two associations share in addition almost the same distribution; furthermore, they are both reported respectively by Rauš (1992) and Kevey (2008, 2019) from the same sector of the River Drava along the Croatian-Hungarian border, where incidentally the holotype of Carici-Salicetum is located and that is close to the Danube stretch from which Galio-Salicetum was described. In our opinion Galio-Salicetum and Carici-Salicetum may correspond to the same forest type found within the same fluvial systems. As a matter of fact, the floristic analysis of the synoptic table (Tab. 5) clearly suggests that Carici-Salicetum, Galio-Salicetum and the Italian relevès should be referred to the same association, but the name Galio-Salicetum has the priority; this way, the Kevey’s (2008) name must be considered as a syntaxonomic synonym. Carici-Salicetum seems to include stands associated to long-lasting stagnant water, characterized by a good expression of helophytic elements of Phragmito-Magnocaricetea, which can be considered as the most hygrophilous term of the association, opposed to stands rich in Rubus ulmifolius found in Italy and discussed hereafter.

The ecological features of Galio-Salicetum support the interpretation of Kevey (2008) and the association is therefore here referred to Alnetea glutinosae. Therefore, Galio-Salicetum represents the swamp counterpart of the riverside Salix alba woods belonging to the riparian alliance Salicion albae (Salicetea purpureae). Salicetum albae Issler 1926 (cols. 4-7 in Tab. 5) is distinguished from these white willow swamps by the lack of its own differential species, the lower participation of ingressive elements of Phragmito-Magnocaricetea and a quantitative variation of some elements such as Phalaris arundinacea and Urtica dioica, much better represented than in Galio-Salicetum; its floristic composition is influenced by catenal contacts leading to a good expression of Alno-Populetea and Rhamno-Prunetea elements.

Salix alba stands related to lentic habitats are reported from various areas of northern and central Italy (Tab. 4, col. 3 in Tab. 5). In Italy, white willow swamp vegetation had not been considered so far as an autonomous unit: the first author who realized the ecological difference of white willow woods of lentic habitats was Pirola (1968), who described the establishment of Salix alba woods on Magnocaricion communities in the oxbow lakes of the River Ticino, but their syntaxonomic treatment was not subsequently addressed. This woodland type probably corresponds to the white willow stands observed by Pedrotti (1988) at Lake Loppio, for which unfortunately no relevés are available. Also the stands with Salix alba and Carex elata reported by Tisi et al. (2007) from the banks of the lakes of the area “Cinque Laghi” of Ivrea, Piedmont region (SAC IT 1110021) could be attributed to this association: indeed, these coenoses are sometimes interpreted in the literature as dynamic stages of helophytic communities with S. alba.

A synoptic table of the association in Europe is provided in Tab. 5.

Synecology : Galio-Salicetum is found along the banks of lowland lentic habitats with stagnant or very slow-flowing water, in frequently and long-flooded sites with prevalent vertical water movements and high water table, in shallow depressions, backwaters, oxbow lakes and lentic channels in connection with great river systems and lacustrine/palustrine habitats; it grows on waterlogged hydromorphic soils with a great content of slightly decomposed organic matter, even moderately peaty; however peat accumulation is more limited than in other swamp forests (Kevey 2019).

In Italy, Galio-Salicetum albae is found along the banks of lakes and minor water bodies, on alluvial and colluvial, muddy, fine-textured, hydromorphic soils with a good content of organic matter. The Italian stands show a certain variability, and three main aspects of the coenosis can be distinguished, mainly related to different flooding and soil moisture conditions. The Carex elata-rich stands (rels. 1-9, Tab. 4) correspond to a definitely hygrophilous vegetation growing on frequently flooded soils enriched with organic matter (hydromor); Carex acutiformis-rich stands (rels. 10-17) represent a dynamic stage of shrub encroachment and eutrophication, differentiated by this large sedge and with a lesser expression of C. elata, mostly found in abandoned quarry pits and other artificial water bodies; finally the stands enriched in Rubus ulmifolius, accompanied by other woody species (rels. 18-22) are the most termophilous and the least hygrophilous ones corresponding to a more advanced stage of shrub encroachment.

Dynamic contacts : The association can be considered as a permanent community being the final product of a dynamic evolution of Magnocaricion communities driven by infilling processes, as realized by Pirola (1968).

Catenal contacts : It comes in contact with aquatic communities (Lemnetea, Potametea), helophytic and hygro-nitrophilous herbaceous vegetation (Phragmito-Magnocaricetea, Bidentetea tripartitae and Agrostietea stoloniferae), and shrub communities (mainly Frangulo-Salicetum cinereae, which may be locally considered its functional mantle).

Synchorology : Croatia and Hungary (area of the Danube, Drava, Sava, Tisza rivers in the Pannonian Plain), Slovenia and Italy. In Italy it is recorded from Friuli Venezia Giulia, Veneto, Lombardy, Emilia-Romagna, Tuscany (Suppl. material 1, Fig. S1); probably it also occurs in Trentino and Piedmont.

Annex I Habitat (92/43/EEC Directive) : this Salix alba swamp woodland can be attributed to habitat 91E0*. Therefore, it is suggested to integrate Galio palustris-Salicetum albae in the vegetation types included in this Natura 2000 forest habitat.

Ass.: DIOSCOREO COMMUNIS-POPULETUM NIGRAE Poldini & Vidali in Poldini, Sburlino & Vidali 2017 (Tab. 6)

Pseudonyms : Salici-Populetum nigrae sensu Auct. Ital. p.p. non Meijer Drees 1936.

TYPICUM subass. nov. (typus of the subassociation: the holotypus of the association: rel. 1 of Table I of Poldini & Vidali in Poldini, Sburlino & Vidali 2017: 1113, corresponding to rel. 8 of Tab. 6 in this paper)

VAR. Alnus incana (rels. 21-23 of Tab. 6 in this paper)

POPULETOSUM ALBAE (Biondi, Vagge, Baldoni & Taffetani 1999) Poldini, Vidali & Castello comb. nov.

Basionym : Salici-Populetum nigrae populetosum albae Biondi, Vagge, Baldoni & Taffetani 1999.

Holotypus : rel. 6 of Tab. 16 in Biondi et al. 1999: 78, corresponding to rel. 7 of Tab. 6 in this paper.

VAR. LIGUSTRUM VULGARE (rels. 1-3 of Tab. 6 in this paper)

Diagnostic species : Dioscorea communis, Corylus avellana, Robinia pseudoacacia, Juglans regia, Parietaria officinalis, Aegopodium podagraria (Poldini et al. 2017).

Structure and composition : Riverine woods dominated by Populus nigra (including hybrids), with a high frequency of Salix alba, accompanied by different tree species such as Robinia pseudoacacia and Ulmus minor; Populus alba can be generally found in higher sites. The shrub layer is rather developed and rich in species, many of which belonging to Rhamno-Prunetea. Lianas are characteristically well represented; common species are Dioscorea communis, Hedera helix, Clematis vitalba. The herbaceous layer is rather poorly developed and discontinuous: common species are Brachypodium sylvaticum, Aegopodium podagraria, Parietaria officinalis, and there are many hygro-nitrophilous species. Various alien species may occur such as Robinia pseudoacacia, Amorpha fruticosa, Solidago gigantea, Buddleja davidii, Reynoutria spp.

Syntaxonomy : The Dioscoreo-Populetum nigrae association recently described by Poldini et al. (2017) is here reconsidered adding relevés related to the most external Holocene river terraces and some impoverished aspects occurring on river islands and elevated lateral gravel bars of the great Alpine rivers, in particular of the braided section of the River Tagliamento. The relevés along the River Po published by Tomaselli (1959) were not considered due to their extreme floristic impoverishment; the author himself suggests that they are heavily human-degraded stands.

Synecology : Dioscoreo-Populetum is a riparian woodland that thrives mainly in the middle and lower reaches of rivers. It is found on sandy-gravelly to sandy-silty mineral calcareous, excessively drained soils. It grows in sites with high water table on recent terraces and their scarps reaching the upper parts of the floodplain, and also on river islands in the active channel of gravel-bed rivers with torrential character of the Po Plain; moreover it can reach the external ancient river terraces with peculiar xerophilous aspects. Compared to willow woodland (Amorpho-Salicetum albae) it typically thrives in upper sites which are still prone to flooding during normal high discharge, but are less frequently or occasionally inundated.

On the basis of the present analysis, Dioscoreo-Populetum has been divided into two subunits: the subassociations typicum and populetosum albae (Tab. 6).

The subass. populetosum albae (rels. 1-7 in Tab. 6) was described by Biondi et al. (1999) for Populus nigra and Salix alba woodland designated as Salici-Populetum from the River Stirone and is characterized by the dominance of Populus alba. Similar stands are here reported from the lower reaches of the River Tagliamento, on higher river terraces subject to less frequent and shorter floods than the typical subass. A particular aspect was observed on the most external old Holocene river terraces, which have been almost completely destroyed by agriculture activity: along the Tagliamento it has been possible to identify a vegetation richer in Rhamno-Prunetea shrubs and Acer campestre, with an impoverishment of both riparian (Salix alba) and gravel banks (S. purpurea, S. eleagnos) willows, a reduction of Robinia pseudoacacia and absence of Sambucus nigra. It is here described as a variant with Ligustrum vulgare and Fraxinus ornus (rels. 1-3), which is (potentially) related to middle and lower river reaches.

The subass. typicum (rels. 8-23) grows on recent terraces and their scarps towards the floodplain, more internally than the subass. populetosum albae. It encompasses most of the relevés published by Poldini et al. (2017) and corresponds to the most typical riparian vegetation, in which willows are more common; it has no differential species. A group of relevés with Apennine origin (rels. 10-13), is distinguished by a more thermophilous character (Viola alba subsp. dehnhardtii), abundant Robinia pseudoacacia and elements such as Prunus spinosa, P. avium indicating more developed soils. Within the typical subassociation, two aspects can be distinguished.

An aspect is represented by the stands with Populus nigra and Salix eleagnos observed in the braided sections of the middle course of the River Tagliamento between Osoppo and Morsano al Tagliamento (rels. 14-20), on river islands (braid bars) and also on high lateral bars (c. a few meters elevated with respect to the river bed) in the active channel, on sandy-silty soil deposited by less frequent floods. In braided river reaches, the formation of vegetated islands is greatly supported by a natural flood regime, a sufficient source of sediments, an unconstrained channel and large woody debris: tree trunks and branches promote the subsequent accumulation of coarse sediments, the establishment of pioneer fast-growing woody species able to resprout such as willows and poplars, and the increasing stability of river islands, where seeds can germinate (see Tockner et al. 2003). Here the community occurs with a more primitive aspect and a tall-shrub structure: it is still dominated by Populus nigra accompanied by river bed willows of Salicetea purpureae, but it is negatively characterized by the considerable reduction of Rhamno-Prunetea (Berberidion) elements, due to the lower development of soils and higher exposure to hydrodynamics and river erosion.

A variant with Alnus incana accompanied by Ostrya carpinifolia includes the lowland stands of the Alpine foreland, located in the transition area between the upper and the middle course of the River Tagliamento (around Osoppo Field, or “Piana di Osoppo”) (rels. 21-23). This area of the High Plain is still influenced by the inflows of elements from the prealpine, colline-submontane grey alder riparian woods of Primulo vulgaris-Alnetum incanae: here Populus nigra can get mixed with Alnus incana.

Synchorology : North-Eastern and Central Po Plain (Friuli Venezia Giulia, Veneto, Emilia Romagna), from upper mesotemperate to lower supratemperate horizons in the Temperate oceanic and continental bioclimates.

Annex I Habitat (92/43/EEC Directive) : 92A0. Submediterranean riverside woodlands rich in Populus spp. of the alliance Dioscoreo-Populion found in the Po Plain should be included in this habitat. 

All.: DIOSCOREO COMMUNIS-ULMION MINORIS Poldini & Vidali in Poldini, Sburlino & Vidali 2017

This alliance has recently been introduced by Poldini et al. (2017) to include meso-hygrophilous, riverine Ulmus minor-rich woods that grow on upper river terraces in the North-Eastern and Central Po Plain.

In light of the new alliance, a survey of the hardwood forests rich in Ulmus minor found along the river systems of the Po Plain was carried out in order to verify their syntaxonomic treatment and their possible inclusion into Dioscoreo-Ulmion. Indeed, the survey considered Po Plain mesophilous and meso-hygrophilous Ulmus minor-rich communities, which were originally attributed to Alno-Padion Knapp 1942 (syn.: Alno-Ulmion, Fraxino-Carpinion), currently regarded as a synonym of Alnion incanae (see Biondi et al. 2015; Biondi and Blasi 2015; Mucina et al. 2016); however, the occurrence of the critical Alnion incanae alliance (and in particular of the Ulmenion suballiance) in the lowland areas of the Po Plain has to be reconsidered in light of Biondi et al. (2015) and Mucina et al. (2016).

The analysis mainly took into account the synthesis of riparian and swamp forests of Italy of Pedrotti and Gafta (1996), and was based on published relevés of woods with elm, oak, ash and white poplar from the central-western Po Plain area assigned in the literature to Alno-Padion, taken from Cavani et al. (1981), Sartori and Zucchi (1981), Bracco et al. (1984), Sartori (1984), Guglielmetto Mugion and Montacchini (1993-94), Assini (1998, 2011a). Furthermore, unpublished relevés of Ulmus minor lakeshore forest stands from the Karst were considered. These Ulmus minor-rich stands were compared to the analogous Po Plain riverine and alluvial plain forest types already described as Lamio-Ulmetum (the type of the Dioscoreo-Ulmion alliance) from Poldini et al. (2017), Asparago-Quercetum roboris (Erythronio-Carpinion) from Lausi (1967), Rubo caesii-Ulmetum minoris (Carici remotae-Fraxinion oxycarpae) from Corbetta and Censoni Zanotti (1974, sub “Carici-Fraxinetum angustifoliae Pedrotti 1970”), as well as Dioscoreo-Populetum (Dioscoreo-Populion) from the original table in Poldini et al. (2017).

The cluster analysis of the relevés (Fig. 1) highlights 3 main clusters that are well connected to ecological differences of the stands.

Cluster 1 includes the mesophilous oak-hornbeam stands occurring in the low Po Plain on deep alluvial soils with high water table, namely Asparago-Quercetum from the eastern Po Plain (group A) and Polygonato-Quercetum (group B) from the central-western Po Plain. The relevés of these two forest types are clearly separated, confirming their autonomy basically related to distinctive biogeographical features that lead to their assignment to different alliances.

Cluster 2 includes the stands of meso-hygrophilous forests from the Po Plain rivers and the Karst lakes. The relevés of Lamio-Ulmetum, Rubo-Ulmetum and Dioscoreo-Populetum are clearly distinguished as three different groups (D, G, F respectively). The dendrogram allows the identification of three other main groups of elm-rich stands: groups C and H include riparian forests from the western Po Plain that are described in this paper as the new associations Vinco minoris-Ulmetum minoris and Salvio glutinosae-Quercetum roboris; group E encompasses the woods from the Karst lakes attributed to the new association Rhamno catharticae-Ulmetum minoris. The PCA of the relevés of this cluster (Fig. 2) shows a remarkable separation of Rubo-Ulmetum from the other stands. This is a forest type with Ulmus minor, Quercus robur and Fraxinus angustifolia subsp. oxycarpa reported from a wide loop of the River Reno (Romagna) (Corbetta and Censoni Zanotti 1974; Brullo and Spampinato 1999): the statistical analysis excludes a possible relationship with other elm-rich stands and supports the independence of this association which is included in Carici remotae-Fraxinion oxycarpae (Poldini and Sburlino 2018). Indeed, this coenosis has a contradictory floristic composition, showing a high expression of Ulmus minor, Quercus robur and Carex pendula, and it therefore takes an intermediate position between Carici remotae-Fraxinion oxycarpae and Dioscoreo-Ulmion: its poverty in species and strong degradation, stressed by Corbetta and Censoni Zanotti (1974), do not allow a univocal interpretation.

Furthermore, the cluster analysis does not support a possible relationship of the stands rich in Populus alba, which are included in group H, with the Dioscoreo-Populion forests (group F), as confirmed by the results of the PCA, in which Dioscoreo-Populetum is clearly separated from all the other relevès of cluster 2 along the third axis (not shown: 8.31 % of total variance).

Cluster 3 gathers the stands recorded by Guglielmetto Mugion and Montacchini (1993-94) from Lake Viverone (Piedmont), which are clearly separated from the other relevés (group I). These stands were originally attributed to Querco-Ulmetum minoris but they do not correspond to the association described by Issler (1924) for their completely different ecology and floristic structure. Indeed Querco-Ulmetum minoris is a riparian forest type occurring in the alluvial plains of the great rivers of Central Europe (Oberdorfer 1992), while the Quercus robur and Fraxinus excelsior woodland from Lake Viverone is definitely a lacustrine type with a swampy character, proved by the occurrence of Frangula alnus, Salix cinerea, Carex elata, Thelypteris palustris, Galium palustre, Thysselinum palustre. These lakeside stands deserve further investigations to clarify their syntaxonomical position.

On the whole, the statistical analysis allows the identification, besides Asparago-Quercetum (Erythronio-Carpinion) and Rubo caesii-Ulmetum minoris (Carici remotae-Fraxinion oxycarpae), of five alluvial/waterside elm-rich communities comprising the well-known Polygonato-Quercetum, Lamio-Ulmetum and three other forest types. The synthetic tables of the five Ulmus minor-rich woods from the Po Plain and the Karst were compared at the Italian and European level, considering corresponding elm-rich woods from central-southern Italy and Central Europe taken from the literature (Tab. 7). The synoptic table was subjected to hierarchical classification, which highlighted the affinity among the coenoses of northern Italy, grouping them into a single cluster (Fig. 3). Therefore, on the basis of floristic and ecological features, the five Po Plain-Karst forests rich in Quercus robur and/or Ulmus minor are included in the Dioscoreo-Ulmion alliance.

The synoptic table (Tab. 7) provides a representation of the floristic gradient of the coenoses and highlights the transitional character of the Po Plain-Karst elm-rich forests. Moreover, it allows a better characterization of Dioscoreo-Ulmion and the detection of the diagnostic species (preferential species sensu Biondi (2011)) of the associations. These hardwood forests are characterized by species of Fagetalia with a wide European distribution (such as Carex sylvatica, Daphne mezereum, Paris quadrifolia, Polygonatum multiflorum, Salvia glutinosa, Viola reichenbachiana), South-East European species of Erythronio dentis-canis-Carpinion betuli (such as Lonicera caprifolium, Primula vulgaris) and of Aremonio agrimonioidis-Fagion sylvaticae (such as Lamium orvala, Asarum europaeum s.l.), as well as by a group of differential species of Dioscoreo-Ulmion represented by Vinca minor, Asparagus tenuifolius, Aristolochia clematitis, Leucojum aestivum. In addition to these, there is a much larger group of Central European species (Glechoma hederacea, Rhamnus cathartica, Viola hirta), which, along with mesophilous elements (such as Corylus avellana) and sub-hygrophilous species in common with Central Europe (Rubus caesius, Aegopodium podagraria, Humulus lupulus, Prunus padus, Parietaria officinalis), differentiate these forests from southern elm woods. On the other hand, there is a group of southern species (Arum italicum, Dioscorea communis, Fraxinus ornus, F. angustifolia subsp. oxycarpa, Hedera helix, Ruscus aculeatus, Bryonia dioica) that can be considered as differential entities of the Dioscoreo-Ulmion forests from the Central European Fraxino-Quercion roboris ones. In the synoptic table these groups of species give rise to an imbricate (overlapping) arrangement, which reflects the temperature gradient that occurs in the contact areas between different macrobioclimates. Dioscoreo-Ulmion differs from the Mediterranean/submediterranean alliance Lauro nobilis-Ulmion minoris for the scarceness of Mediterranean species, and from Alnion incanae for the shortage of Fagetalia entities and the occurrence of southern lianas (such as Bryonia dioica, Dioscorea communis, Hedera helix) and Prunus spinosa (Poldini et al. 2017).

Dioscoreo-Ulmion was originally placed in the class Querco-Fagetea by Poldini et al. (2017). In the light of the new relevés, making prevail the concept of azonality in agreement with Mucina et al. (2016) and considering the weakening of Fagetalia elements compared to the ecologically corresponding coenoses of Central Europe, it is now considered appropriate to move the alliance into the class Alno glutinosae-Populetea albae and the order Populetalia albae.

Multivariate analysis of data of Tab. 7 highlights the macroclimatic gradient underlying the different coenoses. The dendrogram of Fig. 3 shows a clear separation of the Mediterranean group of coenoses from the submediterranean-Central European one: this indicates a greater affinity of Dioscoreo-Ulmion associations with Temperate communities than Mediterranean ones. Successively, the submediterranean Po Plain-Karst group is distinguished from the Temperate one. This clear separation is also confirmed by the indirect gradient analysis (Fig. 4), where the submediterranean communities take an isolated and intermediate position between the Mediterranean and Central European groups, which reflects their transitional floristic composition.

On the basis of these analyses, the content of the transitional climate alliance Dioscoreo-Ulmion is expanded to include meso-hygrophilous and mesophilous hardwood Ulmus minor-Quercus robur-rich forests occurring in the lowlands of the Po Plain along the river systems and their alluvial plains as well as around karstic lakes (Suppl. material 2, Tab. S1). They are elm or oak-elm forests with or without Fraxinus spp. that grow on clay soils mixed with fine gravel to fine sandy-silty or sandy-fine gravelly soils, with more or less superficial water table and variable soil moisture conditions between the inundations, which may be due to higher phases of normal high water or exceptional floods of rivers or lakes, or to the rising of the water table.

Lamio orvalae-Ulmetum minoris is the type association of the alliance described from the resurgence rivers of the Friulian Plain; further differential species can be deduced from Suppl. material 2, Tab. S1 in addition to those identified by Poldini et al. (2017).

Polygonato-Quercetum roboris and the two new associations Vinco minoris-Ulmetum minoris and Salvio glutinosae-Quercetum roboris correspond to forest types of the central-western Po Plain originally assigned to Alno-Padion or other synonyms of Alnion incanae. In the latest Vegetation Prodrome of Italy (Biondi and Blasi 2015) this critical alliance is classified in the class Querco-Fagetea and subdivided in the suballiances Alnenion glutinoso-incanae and Ulmenion minoris. Conversely, Mucina et al. (2016) include hardwood alluvial forests with azonal character in Alno glutinosae-Populetea albae, and separate the elm-ash and oak communities formerly included in Ulmenion in the Fraxino-Quercion roboris alliance, kept distinct from Alnion incanae. Therefore, while for the “nemoral” Europe Mucina et al. (2016) classify the azonal riparian floodplain forests in Alnion incanae and Fraxino-Quercion roboris (Alno-Fraxinetalia excelsioris), thus, symmetrically, in the lowlands of Northern Italy, these two alliances are replaced by Ligustro vulgaris-Alnion glutinosae, including riverside Alnus glutinosa-rich forests on organic-peaty soils, and Dioscoreo-Ulmion minoris, spread on clay to sandy-fine gravelly soils, the two alliances both belonging to Populetalia. Hence, Polygonato-Quercetum cannot be included in Alnion incanae nor in Erythronio-Carpinion, but it is well placed in Dioscoreo-Ulmion for its moderate southern character (Tab. 7); Suppl. material 2, Tab. S1 allows to identify the differential species of this forest type. The new riparian associations Vinco-Ulmetum and Salvio-Quercetum encompass stands that cannot be assigned to Alnion incanae for the same reasons.

Ass.: RHAMNO CATHARTICAE-ULMETUM MINORIS Poldini, Vidali & Castello ass. nov. (Tab. 8)

Holotypus : rel. 3 of Tab. 8 in this paper.

Pseudonyms : Carpino betuli-Quercetum roboris sensu Poldini 1989 non (Anić 1959) em. Rauš 1969; Salicetum albae sensu Poldini 1989 non Issler 1926.

Diagnostic species : Rhamnus cathartica, Bidens frondosa, Prunella vulgaris subsp. vulgaris, Clematis recta, Ranunculus repens, Frangula alnus subsp. alnus, Galium palustre subsp. elongatum, Campanula trachelium subsp. trachelium, Carex elata subsp. elata, Lysimachia vulgaris, Ruscus aculeatus.

Structure and composition : Mixed broadleaved forest with a rather closed-canopy dominated by Ulmus minor, Fraxinus angustifolia subsp. oxycarpa and Populus nigra. Ulmus minor is mainly concentrated in the lower tree layer, accompanied by Fraxinus ornus, Acer campestre, Salix alba and sporadic Quercus robur. The shrub layer is rather well developed and characterized by Rhamnus cathartica, Frangula alnus and Rubus caesius, accompanied by various Rhamno-Prunetea shrubs and small individuals of U. minor and F. ornus; U. minor shows a remarkable regeneration in the undergrowth. The liana layer is made up mainly of Clematis viticella and Hedera helix. The herbaceous layer is rather poorly developed and discontinuous, including species such as Aristolochia clematitis, Asparagus tenuifolius, Brachypodium sylvaticum, Clematis recta, Deschampsia cespitosa, Viola reichenbachiana and the exotic Bidens frondosa; at Lake Doberdò it is characterized by the extensive flowering of Leucojum aestivum in the spring.

Two main aspects of this lacustrine elm wood can be recognized. In areas closer to the water an aspect with abundant Populus nigra occurs (rels. 1-7, Tab. 8): in the tree layer the black poplar is often accompanied by Salix alba, Frangula alnus is often present and the herbaceous layer is characterized by abundant Aristolochia clematitis, Leucojum aestivum, and other hygrophilous and sub-hygrophilous entities such as Galium palustre subsp. elongatum, Carex elata, Ranunculus repens, Equisetum arvense, Prunella vulgaris. The second aspect (rels. 8-12) is found in more rarely flooded sites, where Populus nigra is less abundant or absent, Salix alba disappears, Ulmus minor may join with abundant Fraxinus angustifolia subsp. oxycarpa; the shrub layer is more developed and species-rich; in the herbaceous layer Carex sylvatica, Deschampsia cespitosa and Geum urbanum are frequent.

Syntaxonomy : The analysis of the Ulmus minor-rich forests at the Italian and European level (Figs 1–2) showed the autonomy of the karstic lakeshore stands, here described as a new association which can be assigned to Alno-Populetea on the basis of the considerable frequency of numerous elements of this class, and included in Dioscoreo-Ulmion on the basis of the presence of (sub-)mediterranean entities such as Dioscorea communis, Hedera helix, Arum italicum, Fraxinus ornus, Ruscus aculeatus, Rubus ulmifolius, Asparagus acutifolius, some species of Fagetalia and entities of the alliance (Tabs. 7, 8, and Suppl. material 2, Tab. S1).

Given the high frequency of Ulmus minor, Fraxinus angustifolia subsp. oxycarpa, Rubus caesius and Clematis viticella, a possible relationship with Rubo caesii-Ulmetum minoris from the River Reno was considered, but the statistical analysis (Figs 1–2) confirmed the independence of Rubo-Ulmetum from the karstic Rhamno-Ulmetum, which is well-differentiated by the high frequency of species not present in Rubo-Ulmetum such as Rhamnus cathartica, Aristolochia clematitis, Leucojum aestivum, Campanula trachelium, Crataegus monogyna, Viola reichenbachiana.

Compared to the other coenoses of the alliance, Rhamno-Ulmetum is characterized by elements linked to lentic habitats such as Frangula alnus, Carex elata and Galium palustre subsp. elongatum (Suppl. material 2, Tab. S1). Its peculiarity with respect to the other Dioscoreo-Ulmion forests can be well highlighted by including in the analysis swamp communities of the class Alnetea glutinosae, namely Valeriano dioicae-Fraxinetum oxycarpae, described from the same karstic wetlands (Poldini and Sburlino 2018) and Cladio-Fraxinetum oxycarpae (Frangulo-Fraxinion oxycarpae). The dendrogram of Fig. 5 shows the connection of Rhamno-Ulmetum with Dioscoreo-Ulmion woods; the PCA (Fig. 6) confirms the results of the cluster analysis, but highlights the peculiar position taken by Rhamno-Ulmetum towards the other coenoses, which are arranged in the diagram along a gradient of decreasing soil moisture and water availability, from the wettest extreme given by Cladio-Fraxinetum and Valeriano-Fraxinetum to the least wet one represented by the mesophilous aspects of Polygonato-Quercetum with Anemonoides nemorosa. In the PCA Rhamno-Ulmetum takes an intermediate position, revealing its transitional character towards truly swamp forests of Frangulo-Fraxinion. This peculiar transitional character is strongly conditioned by the particular hydrodynamic regime of the Karst lakes, which is characterized by strong and rapid vertical fluctuations of the water level, so that the entire lakeshore geosigmetum of the Karst lakes system is made up of elements belonging to the swamp woods of Alnetea glutinosae as well as to the fluvial ones of Alno-Populetea. Furthermore, compared to the other associations of the alliance, Rhamno-Ulmetum is distinguished by a greater thermophily that is expressed by entities such as Rubus ulmifolius, Asparagus acutifolius and Quercus pubescens, transgressive from Lauro nobilis-Ulmion minoris (Tab. 7).

This community shows a certain affinity with Ulmo-Fraxinetum angustifoliae ass. prov., a coenosis dominated by Fraxinus angustifolia and Ulmus minor identified by Horvat (1962) for karst dolines inundated in autumn and spring in north-western Croatia, which has never been formalized later. The two communities share various species such as Aristolochia clematitis, Brachypodium sylvaticum, Campanula trachelium, Clematis recta, Prunella vulgaris, Rubus caesius. The Croatian forest community appears to be a more mesic type, due to the presence of Carpinus betulus, Corylus avellana, Lamium orvala, Thalictrum aquilegiifolium.

Synecology : This is a meso-hygrophilous forest, found on the banks of karstic lakes and karstic springs (limnocrenes) in lowland areas, strongly conditioned by the particular hydrodynamics of the Karst lakes. It is spread in the parts of the banks that are periodically inundated, but not for long periods, at peaks of seasonal high water, mainly in spring and autumn. Water movements are fundamentally vertical, with large variations in height that can occur in a few days: at Lake Doberdò the fluctuations of water level can be higher than 6 m (Cucchi et al. 2000; Samez et al. 2005). Soils are neutral and correspond mainly to a complex of alluvial and colluvial silt loam, very poorly drained and temporarily saturated hydromorphic soils with no gravel and with a good content of organic matter (hydromor), and silty-clay loam or silt loam thin soils rich in gravel and excessively drained, developed on carbonate substrates (Michelutti et al. 2006); soils may be well drained during low water periods due to the underlying carbonate rocks and are not peaty.

The floristic variation within the association can be correlated with frequency and length of flooding and water-content of soil. Three main aspects can be observed.

The aspect with abundant Populus nigra (rels. 1-7 of Tab. 8), observed at Lake Doberdò, represents a more hygrophilous vegetation occurring in low-lying areas of the banks close to the water body and subject to more frequent and prolonged flooding; these sites are characterized by high water table and wetter and damper soils which, although highly fertile, may adversely affect the black poplar. P. nigra often dominates in the upper tree layer, and it may have been favoured by human activities in the past. However, at present the black poplar is not regenerating, while a remarkable vitality and recruitment of Ulmus minor, often accompanied by Fraxinus angustifolia subsp. oxycarpa, can be noticed: U. minor is currently abundant in the lower tree layer and the present wood with prevailing P. nigra is likely to turn into an U. minor wood within a short time, as result of the ongoing natural turnover of the black poplar with the field elm. Therefore this vegetation is interpreted as an aspect of an elm wood. A rather similar situation of lakeshore woods dominated by Populus nigra which may be accompanied by abundant Ulmus minor is reported by Lastrucci et al. (2014) from Tuscany. The aspect with Ulmus minor and Fraxinus angustifolia subsp. oxycarpa (rels. 8-12) is found in the outer parts of the banks of Lake Doberdò or bordering minor karstic water bodies; compared to the aspect with Populus nigra, it grows on soils which are more rarely flooded or becoming drier between flooding events. A further aspect with Crocus heuffelianus, Vinca minor and Thelypteris palustris (rels. 13-14) is found at the Karst lakes of Pietrarossa and Sablici.

Dynamic contacts : Rhamno-Ulmetum can be considered as the mature stage of the meso-hygrophilous dynamic series of vegetation of the upper banks of karstic lakes that are regularly but not long-flooded by seasonal high water, on alluvial/colluvial soils on carbonate substrates. The meso-hygrophilous Ulmo-Paliuretum introduced in this paper can be considered the mantle of this woodland.

Catenal contacts : It forms the hardwood forest at the back of swamp willow woods and scrubs with Salix alba and S. cinerea (Galio-Salicetum albae, Frangulo-Salicetum cinereae); in contact waterward also with helophytic communities of Phragmito-Magnocaricetea, landward with karstic deciduous woodlands (Aristolochio luteae-Quercetum pubescentis, Ornithogalo pyrenaici-Carpinetum betuli).

Synchorology : Karst area (Friuli-Venezia Giulia) (Suppl. material 1, Fig. S1).

Annex I Habitat (92/43/EEC Directive) : 91F0.

Ass.: Vinco minoris-Ulmetum minoris Poldini, Vidali & Castello ass. nov. (Tab. 9)

Holotypus : rel. 9 of Tab. 9 in this paper.

Pseudonyms : Querco-Ulmetum minoris sensu Auct. Ital. p.p. non Issler 1924; Polygonato multiflori-Quercetum roboris sensu Bracco, Sartori & Terzo 1984 non Sartori 1984.

Corresponding names : "Boschetti di Olmo e Farnia" in Sartori and Zucchi (1981); "Querceto misto a Quercus robur e Ulmus minor" in Cavani et al. (1981).

Diagnostic species : Vinca minor. It is also to highlight the high presence of Viburnum lantana in the shrub layer, which is shared with Salvio glutinosae-Quercetum roboris hereafter described.

Structure and composition : See the original works by Cavani et al. (1981), Sartori and Zucchi (1981), Bracco et al. (1984) and Assini (1998). The general structure of this hardwood oak-elm forest can be outlined as follows. The tree layer is generally medium developed and is dominated by Quercus robur and Ulmus minor, accompanied by Robinia pseudoacacia, Populus nigra and sometimes Alnus glutinosa, Populus alba and Salix alba. The shrub layer is rather well developed, with a discontinuous cover; common species are Rubus caesius, Crataegus monogyna, Cornus sanguinea, Ligustrum vulgare, Viburnum lantana, along with young individuals of Ulmus minor, Quercus robur and Robinia pseudoacacia. The climbing species are well represented, particularly by Dioscorea communis, Hedera helix, Clematis vitalba, along with Lonicera caprifolium and L. japonica. The herbaceous layer is often poorly developed and discontinuous, and is dominated by seedlings of the main woody species. Vinca minor stands out for its frequency and abundance; facies with Hedera helix, Aegopodium podagraria, Anemonoides nemorosa and Ficaria verna occur locally.

Syntaxonomy : The statistical analysis (Figs 1, 5) allowed to refer to a single new association the riverside oak-elm communities called "boschetti di Olmo e Farnia" by Sartori and Zucchi (1981) and "querceto misto a Quercus robur e Ulmus minor" by Cavani et al. (1981) (respectively along the rivers Oglio and Adda), originally classified in Alno-Padion, along with the disturbed woodland along the River Po near Frascarolo tentatively attributed to Polygonato-Quercetum roboris by Bracco et al. (1984), and the wood along to River Po near Bassignana identified as Querco-Ulmetum minoris by Assini (1998).

Indeed this forest type has been usually attributed in the Italian literature on a physiognomic-ecological basis to Querco-Ulmetum Issler 1924, described from southern Germany, with which it has in common the dominance in the tree layer of Quercus robur and Ulmus minor. The analysis of the synoptic table of the Ulmus minor woods at the European level demonstrates the autonomy of the oak-elm woods of the Po Plain (col. 9 in Tab. 7) from the Central European Querco-Ulmetum (col. 13). The presence of entities with a southern gravitation and the scarcity of Fagetalia justify a more appropriate inclusion of the coenosis within the Dioscoreo-Ulmion alliance. In particular, Vinco-Ulmetum differs from Querco-Ulmetum of Central Europe for the high frequency of Hedera helix and Dioscorea communis (absent in Querco-Ulmetum), and for the lack of Fraxinus excelsior, Acer pseudoplatanus, Primula elatior and Stachys sylvatica.

In this work we also addressed the issue of the presence of Querco-Ulmetum minoris Issler 1924 in Northern Italy. The presence of this Central European riparian floodplain association in the Po Plain was hypothesized by Hofmann (1981) at the suggestion of Pignatti, and taken up by Gentile (1981). Later, Pedrotti and Gafta (1996) listed under this name the pedunculate oak-elm woods reported by Sartori and Zucchi (1981) and Cavani et al. (1981) for Lombardy, in addition to other woods reported by Bracco et al. (1984) for Frascarolo, Lausi et al. (1978) for Friuli, and Guglielmetto Mugion and Montacchini (1993-94) for Lake Viverone (Piedmont). Various authors subsequently reported Querco-Ulmetum from different areas of the Po Plain, but already Assini et al. (2010) include this association among the syntaxa of doubtful presence in the area of the River Po. In the Italian interpretation manual of the 92/43/ECC Habitats Directive (Biondi et al. 2009), Querco-Ulmetum is taken up and attributed to the habitat 91F0.

On the basis of our analysis, the riverside oak-elm woods reported by Sartori and Zucchi (1981), Cavani et al. (1981), Bracco et al. (1984) and Assini (1998) are attributed to the new association Vinco-Ulmetum, while the peculiar swamp wood from Lake Viverone does not correspond to Querco-Ulmetum for both floristic and ecological features (see comment to Dioscoreo-Ulmion alliance). No relevés are available for the riverine woodlands dominated by Ulmus minor, Quercus robur and Acer campestre recorded from the High Friulian plain by Lausi et al. (1978). According to these authors, their treatment was strongly problematic already at that time due to their much reduced distribution and strong human-induced alteration: their classification using Central European models was tentative and can no longer be maintained based on current knowledge. A connection of these stands with Carici albae-Fraxinetum excelsioris described in this work from the same area is possible.

Therefore, almost all records of riverine oak-elm forests attributed to Querco-Ulmetum in the Italian literature were found to correspond to Vinco-Ulmetum, while the remaining ones are to be differently classified. As a result, the presence of Querco-Ulmetum minoris has not yet been demonstrated unequivocally in Italy.

Synecology : Vinco-Ulmetum is a riverside hardwood, meso-hygrophilous forest occurring in the Low Po Plain, found on the low river terraces near the river channel; it is not inundated during periods of normal high discharge, but it is regularly inundated during the most intense floods. It grows on mostly sandy-gravelly to sandy-silty mineral soils with a very high water table (see Cavani et al. (1981), Sartori and Zucchi (1981), Bracco et al. (1984) and Assini (1998)).

The community is found only as very reduced stands, suffering from strong fragmentation and heavy human disturbances which have affected the structural features of these woodlands. The high presence of shrubs of Prunetalia, along with the rather frequent reduced development of the tree layer can be correlated to human pressures on the tree component.

The coenosis shows a certain variability, which allows to distinguish two aspects: one aspect with Viburnum lantana on hydromorphic soils with greater quantity of Rubus caesius (rels. 1-17 of Tab. 9), and another aspect on deeper, more nutrient-rich soils with Sambucus nigra, Solanum dulcamara, Corylus avellana as well as Fagetalia species (Asarum europaeum, Salvia glutinosa) (rels. 18-35).

Synchorology : Piedmont and Lombardy, along the Rivers Po, Adda and Oglio (Suppl. material 1, Fig. S1); possibly to be extended also to Friuli, since in some spots along the River Tagliamento now intended for agricultural use there are frequent isolated individuals of Quercus robur associated with Viburnum lantana.

Annex I Habitat (92/43/EEC Directive) : 91F0.

Ass.: Salvio glutinosae-Quercetum roboris Poldini, Vidali & Castello ass. nov.

Holotypus : rel. 4 of Tab. 1 in Cavani et al. 1981: 22.

Corresponding names : “Boschi igrofili a Populus alba” in Cavani et al. (1981).

Diagnostic species : Populus alba, Neottia ovata, Salvia glutinosa, Aegonychon purpurocaeruleum (Suppl. material 2, Tab. S1).

Structure and composition : See Cavani et al. (1981). It is an open mixed wood, with the tree layer showing a modest cover, reaching up to 40%. The dominant species is Populus alba, usually joined with Quercus robur and Populus nigra, and more rarely Ulmus minor. Ulmus minor and Quercus robur are rather abundant in the shrub layer, along with the shrubs Crataegus monogyna, Corylus avellana, Ligustrum vulgare, Cornus sanguinea and Viburnum lantana, while Populus alba shows poor vitality. The climbing species Dioscorea communis and Hedera helix are very common, accompanied by Clematis vitalba. The herbaceous layer is discontinuous; the most frequent species are Salvia glutinosa, Asarum europaeum s.l., Neottia ovata and Primula vulgaris.

Syntaxonomy : The association includes the stands reported as “Boschi igrofili with Populus alba” by Cavani et al. (1981) from the River Adda and originally classified in Alno-Padion. The multivariate analysis confirmed the independence of this community (Figs 1, 2, 5), excluded affinities with woodlands of the Dioscoreo-Populion alliance and supported its assignment in Dioscoreo-Ulmion with other communities previously attributed to Alno-Padion (Tab. 7 and Suppl. material 2, Tab. S1). Also in this case the species of the Dioscoreo-Ulmion alliance Dioscorea communis and Hedera helix and of Fagetalia (Primula vulgaris, Isopyrum thalictroides, Salvia glutinosa, Symphytum tuberosum subsp. angustifolium) are well represented (Suppl. material 2, Tab. S1). We avoided considering Populus alba in the name of the association due to its failure in regeneration reported by the original authors. The new association is based on the original table of Cavani et al. (1981), from which however relevé number 8 is excluded.

Synecology : See Cavani et al. (1981). It is a hygrophilous riverine woodland, representing the wettest forest type after the riparian willow woodland observed in the area surveyed by these authors. It occurs on sandy-gravelly soil, with the water table always high (water at a depth of less than 1 m). It is more hygrophilous than Vinco-Ulmetum being found in sites closer to the water.

Catenal contacts : In contact with Vinco-Ulmetum.

Synchorology : Lombardy, lower course of the River Adda (Suppl. material 1, Fig. S1).

Annex I Habitat (92/43/EEC Directive) : 91F0.