A novel insight into the remnants of hygrophilous forests and scrubs of the Po Plain biogeographical transition area (Northern Italy)

Hygrophilous forests and scrubs are ecotonal habitats providing essential ecosystem services, especially in human-modified landscapes; nevertheless, they are among the most threatened habitats worldwide. A sound knowledge of waterside woody vegetation provides a valuable basis for interventions of renaturation of waterbodies. This paper focuses on peculiar communities that occur in riparian and swamp areas of the Po Plain, a broad ecotonal area between the Mediterranean and Temperate regions. The study allowed the description of six new associations. Moreover, it provides a detailed picture of Dioscoreo communis-Populetum nigrae (Populetalia albae) and Amorpho fruticosae-Salicetum albae (Salicetalia purpureae), an overview of Salicetum triandrae (Salicetalia purpureae) at the national and European level, and an update of the alliance Dioscoreo-Ulmion minoris, which is better characterized, classified in Populetalia albae and enlarged to include five associations of elm-oak-rich forests of the Po Plain lowlands and the Karst. Dioscoreo-Ulmion includes, besides Lamio orvalae-Ulmetum minoris, also Polygonato-Quercetum roboris and three new associations: Vinco minoris-Ulmetum minoris and Salvio glutinosae-Quercetum roboris from Po Plain rivers and the karstic lakeshore Rhamno catharticae-Ulmetum minoris. The new arrangement of Dioscoreo-Ulmion results from an analysis of Po Plain elm-rich forests including stands so far attributed to the critical alliance Alnion incanae; the presence of Querco-Ulmetum minoris in Italy is discussed. Two new associations are attributed to Prunetalia spinosae: Salici eleagni-Juniperetum communis and Ulmo minoris-Paliuretum spinae-christi. Stands from the Rivers Isonzo and Tagliamento referred to Veratro nigri-Fraxinetum excelsioris and to the new association Carici albae-Fraxinetum excelsioris represent the outermost expressions of the Ostryo-Tilion ravine forests extending towards the High Plain. A Salix alba swamp forest, Galio palustris-Salicetum albae, is reported for the first time in Italy and attributed to Alnetea glutinosae.


Introduction
Areas adjacent to rivers, lakes and other inland water bodies encompass complex ecosystems with a definitely ecotonal character between the truly aquatic and terrestrial habitats; they are characterized by distinctive abiotic and biotic conditions strongly influenced by the water, and are particularly sensitive to environmental change (Naiman and Décamps 1997;Verry et al. 2000;Naiman et al. 2005). These highly dynamic transitional zones provide a multitude of fundamental ecosystem services such as maintaining biodiversity, high primary productivity, transfer of materials and energy between terrestrial and aquatic ecosystems, nutrient filtering, flood-protection and reduction of natural disaster risks, water availability and quality improvement, climate change mitigation, rec-reation (see e.g. Tockner and Ward 1999;Ewel et al. 2001;Naiman et al. 2005;Hale and Adams 2007;Dimopoulos and Zogaris 2008;Strayer and Findlay 2010;Riis et al. 2020). Indeed, freshwater marginal areas and in particular riparian woodlands associated with river systems in human-altered landscapes could be considered as "keystone habitats" for biodiversity conservation, because they provide support to a large number of native species, structure biotic communities and help drive ecosystem functions with an importance far beyond their real, typically reduced extent (Dudgeon 2000;Parrott and MacKenzie 2000;Boutin et al. 2003;Jobin et al. 2004;McKinstry et al. 2004;Lovell and Sullivan 2006;Dimopoulos and Zogaris 2008;Wantzen et al. 2008). The Habitats Directive 92/43/ EEC regards rivers and their riparian areas as landscape features essential to maintain biological connections, while the Water Framework Directive (2000/60/EC) recognizes the value of riparian and shore zones as hydromorphological quality elements for surface water bodies (European Commission 2003). Despite their ecological and economic remarkable value wet habitats bordering water bodies are among the most human-altered and threatened habitat types worldwide.
Waterside habitats play a major role in biodiversity conservation in intensive agricultural landscapes because they represent remnants of both wetland and woody habitats available for wildlife (Jobin et al. 2004). The Po Plain is one of the most important industrial and agricultural areas in Europe, and one of the most human-altered as well: the Po Plain landscape has been almost completely altered by centuries of human presence.
From the bioclimatic and biogeographical point of view, the Po Plain constitutes a broad ecotonal band interposed between the Mediterranean and the Temperate regions. It is recognized in the classification of the ecoregions of Italy as one of the 7 Provinces of Italy, namely the "Po Plain" Province , and corresponds to the Padanian sector of the Biogeographic Map of Europe (Rivas-Martínez et al. 2004). Due to its geographic position, it has experienced, and has been shaped by, the effects of two main migratory routes of species, one lying along a south-north axis and the other along an east-west one. Although it lies within the Temperate macroclimate, its transitional bioclimatic character is shown by the fact that it largely belongs to ecotonal bioclimatic variants, namely the Submediterranean and Steppic variants according to Rivas-Martínez et al. (2011), with Submediterraneity features sensu lato (Pesaresi et al. 2017). The eastern part of the Po Plain area, corresponding to the Venetian-Friulian Plain, does not show values of the Submediterraneity index to be included in one of the two aforementioned variants (Pesaresi et al. 2017); nevertheless, with its ecological and biogeographical features, it still belongs to a transition, Temperate-Mediterranean ecoregion, and is often considered in the literature as submediterranean in a broad sense (see e.g. Mucina et al. 2016).
Riparian and swamp communities have a strong azonal character: indeed they are mainly conditioned by the water regime, hydrodynamics and soil features, and to a lesser extent by macrobioclimatic characteristics. As a result, variations occur only with marked macrobioclimatic changes (Biondi et al. 2004). However, the preconception that hygrophilous azonal vegetation is little influenced by the biogeographical context has led to the uncritical use of names of communities described from Central Europe for plant assemblages of the Po Plain, without an evaluation of their floristic and biogeographical content Sburlino et al. 2011). Indeed, in the past, the treatment of Po Plain hygrophilous woodlands and scrubs used to be performed adopting Central European syntaxa. To solve these problems of large-scale ecotonal variations of hygrophilous forest communities, in the recent literature new syntaxa have been already introduced at the level of alliances, such as Ligustro vulgaris-Alnion glutinosae, Frangulo alni-Fraxinion oxycarpae , Dioscoreo communis-Populion nigrae, Dioscoreo-Ulmion minoris .
The aim of this paper is to discuss some peculiar hygrophilous and meso-hygrophilous woodlands and scrubs associated with lotic and lentic freshwater water bodies of the Po Plain. These communities often represent what little remains of the natural habitats in the current intensive agricultural, industrial and densely populated areas of the Po Plain landscape. We therefore address the issue of clarifying the syntaxonomic position of various woody hygrophilous communities, many of which at least potentially widespread, of this broad ecotonal area between the Mediterranean and Temperate regions, complying with the basic ecological distinction between hygrophilous communities of flowing and standing waters.
The heavy human impacts in the Po Plain result also in a high level of hemeroby in hygrophilous communities: indeed, invasive exotic plants are one of the major threats to biodiversity in this area (Assini et al. 2010;Poldini et al. 2011). Consequently, woody communities undergo human-induced regression and degradation, up to the substitution of physiognomically relevant elements in the extreme cases. A further problem addressed in this paper is that of the need to use invasive transformer species in the syntaxonomic treatment of plant coenoses because of the considerable modification of the characteristics of riparian communities, which are particularly exposed to the effect of alien species (e.g. Richardson et al. 2007;Schnitzler et al. 2007). In this work therefore some syntaxonomic units are defined also using naturalized alien species.
This paper aims to fill a gap of knowledge about riparian and swamp woody vegetation in the Po Plain area, including the Karst sector, and to provide basic knowledge to produce reference schemes for interventions of restoration of wet habitats in order to preserve their ecological and biogeographical specificities.
Relevés of plant communities were carried out according to the Braun-Blanquet (1964) approach and organized in a database. For a better treatment of some syntaxa, the relevés from the Po Plain were compared to published data (synthetic tables) from various areas of Italy and Europe.
Statistical analyses were performed using SYN-TAX 2000 (Podani 2001). The main community types were analised separately, by means of agglomerative hierarchical clustering using Similarity ratio as the resemblance measure, and Principal Component Analysis (PCA, covariance method).
The analysis of the matrices of relevés and species was performed on cover data, which were transformed according to Van der Maarel (1979). Analytic tables were arranged according to the results of the hierarchical clustering. Sporadic species (i.e. occurring in 1 relevé of an analytic table) were excluded from statistical processing.
When considered appropriate to characterize a syntaxon, the synthetic tables of the Po Plain communities derived from the analytic ones were compared by means of multivariate analysis with those of coenoses from other parts of Italy and/or Europe. The analyses of synthetic tables were based on percentage frequency values; species occurring with a frequency less than 20% were excluded from data processing.
Data concerning the analytic tables are quoted in the Appendices. The sources of the relevés and the original syntaxa names of the communities in the synoptic tables are listed in their captions. Syntaxonomic nomenclature up to the level of alliance follows Biondi et al. (2014b) and further updates by Biondi and Blasi (2015), except for the class Salici purpureae-Populetea nigrae that is substituted by Alno glutinosae-Populetea albae in order to include only forest communities, and the alliances Ostryo carpinifoliae-Tilion platyphylli and Fraxino excelsioris-Acerion pseudoplatani, which are in accordance with Mucina et al. (2016). The phytosociological nomenclature follows Weber et al. (2000), taking also into account Theurillat et al. (2020). Communities are presented according to the syntaxonomic hierarchy.
Diagnostic entities of vegetation classes fundamentally follow Mucina et al. (2016) and Aeschimann et al. (2004). For the identification of the diagnostic species of the as-sociations, the entities having frequency classes from V to III in the synthetic tables were considered. The concept of differential species is in accordance with Mucina (1993) and Biondi (2011).
Taxonomic nomenclature follows Bartolucci et al. (2018) and Galasso et al. (2018), with the exception of Asarum europaeum, for which we maintain the distinction of the subsp. caucasicum, which replaces the nominal subspecies in southern European regions (Fischer et al. 2008). Subspecies are indicated in the text only when they are different from the nominal subspecies or when one or several subspecies occur besides the nominal one.
The bioclimatic characterization is in accordance with Rivas-Martínez (2008), Rivas-Martínez et al. (2011) and follows Pesaresi et al. (2017). However, the term "submediterranean" is here used in a broad sense, often adopted in the literature for transition areas with a tendency of summer aridity, including the areas of the Po Plain lying in the Submediterranean and Steppic bioclimatic variants (with positive values of the Submediterraneity Index), as well as areas of the Venetian-Friulian Plain, which belong to other variants of the Temperate macrobioclimate according to Pesaresi et al. (2017). nos, 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).
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.  ci et al. (2008); "Facies a Salix alba dell' ordine Alnetalia glutinosae" in Merloni and Piccoli (2001); Salicetum albae Issler 1926subass. phragmito-caricetosum Jurko 1958var. Carex elata in Bolpagni et al. (2007; Salicetum albae subass. phragmito-caricetosum Jurko 1958 in Šilc (2003).  (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  Fr.

Diagnostic species of upper units (Alnion glutinosae, Alnetea glutinosae)
Frangula alnus Mill. subsp. alnus Alien species Bidens frondosa L.  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 (2008Kevey ( , 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).
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.
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  (Rauš 1976); 3: Italy (Tab. 4 in this paper); 4: Slovenia (Šilc 2003); 5 -7: Germany (Oberdorfer 1992).  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  to include meso-hygrophilous, riverine Ulmus minor-rich woods that grow on upper river terraces in the North-Eastern and Central Po Plain.
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.
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. 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).
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 con-   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][9][10][11][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.
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 , 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.
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  Syntaxonomy: The association includes the stands reported as "Boschi igrofili with Populus alba" by  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 Table 9. Vinco minoris-Ulmetum minoris ass. nov. Relevés are arranged according to cluster analysis (cover data, Similarity ratio, Complete linkage).

Diagnostic species of the association
Vinca minor L.  Table 9. Continuation.
(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.

Annex I Habitat (92/43/EEC Directive): 91F0.
Willow open forests and scrubs of the class Salicetea purpureae  Poldini et al. (2011), in which the analytic table had not been included, is here presented in more detail. In the comparison with other Salix alba woods at the Italian and European levels, Amorpho-Salicetum stands out for the transitional character between the Mediterranean and the truly Temperate communities and the high incidence of exotic elements. As already discussed by Poldini et al. (2011), when an invasive perennial exotic and/or synanthropic plant species structurally modifies a community replacing various original species, and also changes the environmental characteristics thus becoming a species that transforms the environment and builds the community (i.e. transformer and edificator species) (Pyšek et al. 2004), it is permissible to use it as a diagnostic species in the formulation of the name and characterization of the syntaxon.
The current situation of riverine Salix alba woodlands in Italy can be summarized as follows. The coenoses of southern Italy converge into the central-western Mediterranean alliance Salicion pedicellatae, Rubo-Salicetum albae has been established for the woodlands of central Italy, while the vicariant Amorpho-Salicetum albae has been described for the Po Plain area. While not excluding that other natural or near natural situations attributable to Salicetum albae Issler 1926 may exists in the Po Plain, its presence remains to be ascertained.
Synecology: Softwood hygrophilous forest that thrives on the river floodplain flanking the channel, frequently inundated for long periods at times of high discharge, but well-drained during low water periods, and therefore subject to significant fluctuations of the water level. It occurs on mainly sandy to silty-clayey alluvial soils (fluvisols), along the middle and lower courses of rivers in lowland and hill areas of the Po Plain.
Amorpho-Salicetum is characterized by a strong level of hemeroby, due to the interaction of two main factors: the association occurs in an area heavily affected by intense human activities (industry, agriculture) and demographic concentration, and it thrives on highly unstable, dynamic fluvial areas subject to frequent flood disturbance but also to a large nutrient supply that enhance the vulnerability of these habitats to the invasion by exotic species.
The analytic table of Amorpho-Salicetum (Suppl. material 3, Tab. S2) highlights the variability of the coenosis, already identified by Assini et al. (2010) and , which may be referable to two main aspects, here described as new subassociations, namely populetosum nigrae and urticetosum dioicae. The interpretation of the variability of the coenosis is based above all on the occurrence of woody species or ecological groups of species, as in these habitats, which are very dynamic and unstable being conditioned by strong hydrodynamics, the herbaceous species are more subject to strong fluctuations, which often interfere with primary successions (Dierschke 1996). Conversely, woody species are elements that once established tend to maintain themselves, so they are less subject to marked variations. The subassociations are related to remarkable soil variations on wide areas and correspond to the two fundamental ecologies, one still torrential and the other fluvial.
The subass. populetosum nigrae (rels.  corresponds to the aspect rich in Populus nigra (and hybrids) in which Salix triandra and S. purpurea occur as well, including the stands that seem less altered, linked to the stretches still with a torrential character. It is spread in Friuli Venezia Giulia, Emilia Romagna and Veneto, in the stretches lying in the transition between the High and Low Plain, where a braided structure of the channel still persists and there is a mineral gravel fraction in the sandy-silty matrix of the soil. This situation occurs both on the Alpine side and the Appenine one. The diagnostic species of the subass. populetosum nigrae are: Populus nigra, Salix triandra subsp. triandra, S. purpurea, S. eleagnos. Two variants are identified: a) a variant with Rubus caesius (rels. [1][2][3][4][5][6][7][8][9][10][11][12][13][14], basically bound to the Alpine watercourses of NE Italy (Tagliamento); b) a variant with Lythrum salicaria and Agrostis stolonifera and without Rubus caesius (rels. 15-24), on more sandy-silty soils, of the right tributaries (Apennine watercourses) of the River Po, and more rarely of the Alpine rivers (Piave). The subass. urticetosum dioicae (rels. 25-87) is connected to stretches with fluvial regime, finer sediments and higher eutrophication, where various hygrophilous woody species fail. It is characterized by the rarefaction of shrub willows and the reduction of Populus nigra; it is found along the river reaches of the Low Plain, on sandy-silty to silty-clayey soils with strong water level fluctuation, as pointed out by the presence of Phalaris arundinacea, and eutrophication, as indicated by Urtica dioica. The diagnostic species of this subassociation are: Urtica dioica, Phalaris arundinacea subsp. arundinacea, Poa trivialis, Convolvolus sepium, Limniris pseudacorus. The subass. can be further subdivided on the basis of the water regime in two variants that correspond to those already recognized by Assini et al. (2010) which are confirmed with an enlargement of the differential entities (Suppl. material 3, Tab. S2): a) a variant with Sambucus nigra and Cucubalus baccifer (rels. 25-42), along with Humulus lupulus, a little drier, still with a certain condition of naturalness, located on positive undulations of the floodplain with lower soil moisture, characterized by a reduced occurrence of ruderal and exotic species. These stands tend towards the formation of fluvial nitrophilous mantles (cfr. Bryonio-Sambucetum). b) a variant with Bidens frondosa and Persicaria dubia (rels. 43-87), strongly disturbed and invaded by exotic species such as Sicyos angulatus, in which shrub willows disappear, Populus nigra becomes rare, and the tree layer is markedly depleted in species; conversely, there is an increase in frequency and cover values of Phalaris arundinacea as well as synantrophic species given by neophytes and hygro-nitrophilous ruderals of Bidentetea tripartitae, Stellarietea mediae and Artemisietea vulgaris. This is a typical aspect of the lowland stretches heavily affected by intensive human activity (with high polyhemerobic level). Catenal contacts: It is found in slightly higher positions of the river floodplain with respect to the willow scrub Salicetum triandrae. It can come in contact landwards, towards higher, more sandy-gravelly sites with Dioscoreo-Populetum nigrae.
Synchorology: Lowlands of almost the entire Po Plain, in the Temperate oceanic and continental bioclimate variants, from upper mesotemperate to lower supratemperate thermotypes.

Annex I Habitat (92/43/EEC Directive):
This Salicion albae vegetation includes riparian white willow woods that potentially match with the Annex I habitat 91E0* representing a more or less degraded and alien-invaded Salix alba habitat type in urgent need of conservation measures, although often with low possibility of restoration.
Ass.: SALICETUM TRIANDRAE Malcuit 1929 (Tab. 10) Diagnostic species: in Italy Salix triandra. Structure and composition: Riparian scrub or mesoforest. There may be a discontinuous tree layer dominated by Salix alba and Populus nigra and sometimes P. alba. The shrub layer is well developed and dense, dominated by Salix triandra, which may be joined by S. purpurea and S. alba; Amorpha fruticosa can be present with thick complexes. The herbaceous layer is discontinuous and rather poor, characterized by hygro-nitrophilous species: Agrostis stolonifera is often present, accompanied by Phragmites australis in the sites from central Italy and Calamagrostis pseudophragmites in those from North-Eastern Italy. The community is characterized by a high level of hemeroby (Amorpha fruticosa, Bidens frondosa, Erigeron canadensis, Table 10. Salicetum triandrae Malcuit 1929 from different parts of Italy. Relevés are arranged according to cluster analysis (cover data, Similarity ratio, Ward's method). Cl: species of Salicetea purpureae; All: species of Salicion triandrae.

Diagnostic species of association in Italy
Cl, All Salix triandra L. subsp. triandra 5 5 4 4 3 3 4 5 3 4 4 5 4 5 4 3 3 3 5 5 2 3 2 4 3 1 4 4 3 3 2 2 2 100.0 Helianthus tuberosus, Symphyotrichum squamatum, Xanthium italicum, etc.). Syntaxonomy: The relevés of Salicetum triandrae s.l. from Italy were compared by multivariate analysis with those of similar formations from various areas of Europe. The analysis did not allow to detect particular variations within the relevés at national level (Tab. 10), which can therefore be treated as one single association. Not even the comparison at the European level (Tab. 11), recognizes particular features in the Italian stands to justify their inclusion in a distinct, new association. Constant elements in common with the other European Salix triandra scrubs are Convolvolus sepium and Phalaris arundinacea, sub-hygrophilous companion species which in the Italian relevés reach much lower values of presence compared to the other European communities.

Species of Salicetea purpureae and Salicion triandrae
Salicetum triandrae from Italy (col. 4 in Tab. 11) differs from the other willow scrubs of Europe for the following three main characters: 1) absence of Salix viminalis for biogeographical reasons; 2) high presence of Populus nigra (incl. hybrids) and in subordinate way of Populus alba, which gives the vegetation stands a southern character; 3) large presence of Xanthium italicum, invasive neophyte widely spread in Italian river habitats, especially on silty sediments. Besides riparian woodlands, this entity has already been used to describe other hygrophilous coenoses (see e.g. Markovic (1981)) such as Polygono lapathifolii-Xanthietum italici and Xanthietum italici.
The statistical analysis (Figs. 7,8) highlights the peculiar position of the communities of Serbia and Croatia. The relevés from Serbia are those of Salicetum albo-triandrae Slavnić 1952, described from Vojvodina and considered by Horvat et al. (1974) a synonym of Salicetum triandro-viminalis Tüxen (1931Tüxen ( ) 1951, that is Salicetum triandrae: they show a floristic composition that is different from the other coenoses, revealing an ambiguous situation between fluvial and swamp systems.
Synecology: It is a riverine, pioneer, dense tall scrub community, which constitutes the first front of woody riparian vegetation towards the water on loamy sediments, thriving in the lowest areas of the floodplain close to the river channel that are regularly inundated for long periods at high water. It occurs in the lower courses of rivers and is linked to slow-flowing water that allows the sedimentation of fine deposits. It has been strongly compromised by human interventions of flow regulation and use of water resources, and has now become a rare vegetation with fragmentary presence.
Catenal contacts: In contact with Salix alba woodlands (Salicion albae), which are found on higher sites of the river floodplain.
Synchorology: North and Central Italy up to Abruzzo (Biondi and Blasi 2015).
Annex I Habitat (92/43/EEC Directive): -. Although it is not included in the Annex I of the Habitats Directive, in Italy this community is by now rare and often reduced to linear fragments, more or less disturbed by human ac-Relevé number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29   Synecology: Xero-thermophilous alluvial Fraxinus excelsior forest related to river systems that occur on outer, stabilized fluvial terraces prone to extreme flood events in river stretches with a torrential character. It represents the outermost expression of ravine forests on fluvisols along torrential rivers at the their opening into the lowlands and the High Friulian Plain, on alluvial coarse-grained alkaline deposits.
Carici-Fraxinetum excelsioris is mainly found in the upper course of the River Tagliamento, in the stretch that flows through the wider part of the Tagliamento Valley lying at lower elevations (below 400 m a.s.l.). It grows on the gravelly parts of the outer terraces, on coarse-textured brunified soils with a thin layer of sand. Abundant Pinus sylvestris can be observed in some sites lying in lower positions along the River Tagliamento and its tributary the stream But; this could indicate that this woodland is the result of an evolution of the Alno incanae-Pinetum sylvestris floodplain forest of river islands, possibly favoured by river regulation interventions.
Other examples of colonization of outer, marginal parts of river valleys with a torrential character by forests dominated by noble ravine trees with Carex alba are represented by Carici albae-Carpinetum betuli, a pioneer woodland described from Slovenia on alluvial young terraces of the upper River Nadiža (Natisone) (Čušin 2002), and Carici albae-Tilietum cordatae, described from Germany and reported from other areas of Central Europe (e.g. France, Germany, Austria, Slovenia, Italy). The latter association    has a still controversial syntaxonomic position: it is found both in ravines on steep, sunny slopes and in rarely flooded, relatively dry fluvial areas, and is placed by some authors in Carpinion betuli (e.g. Müller 1992; INPN 2019) and by others in Alnion incanae (Willner 2007). Assini (2011b) reports meso-thermophilous woods referred to as "aggruppamento a Tilia cordata e Carex alba" and classified within Tilio-Acerion from Lombardy on south-facing slopes at elevations between 650 and 735 m a.s.l. Assini and Verde (2007), quoting Andreis et al. (2002), list Carici albae-Tilietum cordatae for Lombardy. Schubert et al. (2001) cite this association from steep slopes exposed to the south of the Kaiserstühl, but also from gravelly fluvial terraces in the upper Rhine and in the eastern part of Lake Constance, classifying it in Carpinion betuli.
The new association would represent an ecological convergence south of the Alps of Carici-Tilietum cordatae of Central Europe. This would be a further confirmation of how ravine forests can also colonize gravelly alluvial terraces of watercourses in montane-hilly areas, expanding the ecological definition of the Ostryo-Tilion and Tilio-Acerion/Fraxino excelsioris-Acerion pseudoplatani alliances.  [1][2][3][4][5] and Veratro nigri-Fraxinetum excelsioris (rels. 6-9). Relevés are arranged according to cluster analysis (cover data, Similarity ratio, Complete linkage).  Synchorology: Upper and middle reaches with torrential character of the River Tagliamento at its opening into the lowlands and the High Plain and lower course of its montane tributaries (stream But) (Friuli Venezia Giulia) (Suppl. material 1, Fig. S1).

Annex I Habitat (92/43/EEC Directive):
From a formal syntaxonomic point of view this woodland would be part of 9180* -Tilio-Acerion forests of slopes, screes and ravines. From an ecological point of view this is a riverine, mixed forest dominated by hardwood, "noble" trees growing on alluvial recent deposits. It is therefore attributed to 91F0.
Syntaxonomy: The floristic structure of these stands rich in Fraxinus excelsior and Allium ursinum found along the River Isonzo near Gorizia suggests their inclusion into the xero-thermophilous broad-leaved ravine Ostryo-Tilion alliance. This forest type is dominated by Fraxinus excelsior and Tilia cordata accompanied by Ulmus glabra, but is differentiated by elements related to the fluvial environment such as Fraxinus angustifolia subsp. oxycarpa, Populus nigra, Salix eleagnos and hygro-nitrophilous species such as Parietaria officinalis; it is characterized by a large number of thermophilous elements such as Ruscus aculeatus and many of the diagnostic entities of the Ostryo-Tilion alliance according to Košir et al. (2008). It is interpreted as a fluvial, most extreme, species-impoverished aspect of Veratro nigri-Fraxinetum excelsioris in its variant with Allium ursinum described by Dakskobler (2007) from the submediterranean-pre-Alpine region of Western Slovenia, including the Central Soča (Isonzo) Valley.
With regard to the occurrence of Fraxinus angustifolia subsp. oxycarpa, a recent study on narrow-leaved ash in North-Eastern Italy by Belletti et al. (2015) showed that in this area (specifically in the area of Farra, in which rels. 6-8 of Tab. 12 are located) many individuals have intermediate genetic and morphological characteristics between Fraxinus angustifolia subsp. oxycarpa and F. excelsior, which possibly originated by introgression: many individuals originally identified as narrow-leaved ash showed genetic characteristics attributable to the common ash. Hybridization could be favoured by the fact that the populations grow at the margins of the distribution area of both species and their co-presence is therefore frequent. However, these phenomena lead to strong difficulties in the identification of Fraxinus angustifolia subsp. oxycarpa in this area.
Synecology: This community is found along the torrential stretch of the River Isonzo that reaches the High Friulian Plain from Gorizia southwards, on the marginal, occasionally flooded areas of the first river terraces, on brunified fluvial soils that according to Michelutti et al. (2006) are alkaline, sandy loamy to loamy, thin to moderate deep, coarse-grained and excessively drained. Compared to Carici albae-Fraxinetum excelsioris it grows on soils with finer grain size. It can be considered the outermost expression of the ravine forests of the Julian pre-Alpine sector extending towards the High Plain.

Conclusions
The present study provided a broader and better articulated vision of three major communities of willows and poplars that constitute typical elements of the floodplains and the first river terraces of the Po Plain river systems: the Salicetum triandrae willow scrub, the Amorpho-Salicetum albae white willow forest which is the secondary association shaped by the high occurrence of invasive species that substitutes Salicetum albae along the whole River Po, and the poplar-rich forest Dioscoreo-Populetum nigrae, which substitutes in Northern Italy the Mediterranean Populion albae forests and has been often named in the literature as Salici-Populetum.
The Ulmus minor and Quercus robur-rich forests of the Po Plain are grouped in the submediterranean alliance Dioscoreo-Ulmion minoris, which is enlarged to include meso-hygrophilous and mesophilous hardwood forests with oak and/or elm occurring in the lowlands along the higher terraces of rivers and their alluvial plains as well as around karstic lakes. The alliance includes forest types that substitute in the Po Plain the Central European Fraxino-Quercion roboris riparian forests. The study led to the description of new associations and the reclassification of two hardwood forest communities typical of the central-western Po Plain: the well-known Polygonato-Quercetum roboris is moved from Alnion incanae to Dioscoreo-Ulmion, while the oak-elm woods often attributed to Querco-Ulmetum Issler 1924 on the basis of the application of Central European schemes to the submediterranean area of Northern Italy are merged into the new association Vinco-Ulmetum minoris. As a result, the presence of Querco-Ulmetum has not yet been demonstrated unequivocally in Italy.
The study highlights the presence of Fraxinus excelsior forests on fluvial terraces along torrential stretches of rivers that represent the outermost expressions of the Ostryo-Tilion noble hardwood ravine forests going down up to the High Plain.
Finally, a peculiar Salix alba swamp forest, Galio palustris-Salicetum albae described from the Balkan Peninsula, is reported for the first time in Italy and attributed to the class Alnetea glutinosae, its distribution extending to Northern and Central Italy.
Waterside woodlands and scrubs of the Po Plain, althought facing severe alterations caused by human action and high alien species pressure, are still fundamental elements providing essential ecosystem services. In such an altered territory, these fragments of native vegetation, even if degraded, could be recovered and enhanced as ecological corridors or stepping stones for maintaining and promoting biodiversity and ecosystem functions at the landscape level. We addressed the remnants of waterside woody communities up to the meso-hygrophilous oak-elm woodlands, as they are valuable landmarks to delimit the areas of fluvial or lacustrine pertinence, to improve basic knowledge useful for renaturation of river ecosystems, environmental requalification in agricultural areas and actions to promote the sustainable development of agriculture.

Acknowledgements
We are grateful to Pierpaolo Merluzzi who accompanied us in sites along the River Isonzo, Stefano Zanini (Forest Station of Coseano) and the staff of the Forest Station of Tolmezzo for logistic support and precious information on some sites along the River Tagliamento. Special thanks are due to Leonardo Ghirelli and Luciano Giovagnoli for making available to us some unpublished relevés from the Veneto Region, to Cristiano Francescato for unpublished relevés from the River Tagliamento, and to Silvia Assini, Mariacristina Villani and Francesco Bracco for unpublished relevés of Amorpho-Salicetum albae. We also thank Cristiano Francescato and Francesco Liccari for their help in field work. We would like to thank Jorge Capelo and an anonymous reviewer for their suggestions and comments.