Aims: (a) To revise the previous classification systems for the Alnetea glutinosae vegetation in Ukraine using a representative dataset; (b) to define the vegetation types at the association level and outline their geographical distribution and species composition; (c) to describe the relationships between the defined associations and the main environmental factors. Study area: Ukraine. Methods: 5818 vegetation plots of floodplain forests, alder carr, and birch forests were collected in the field and digitized from the literature. We used PC-ORD beta-flexible clustering (beta = –0.3, Bray-Curtis dissimilarity, log-transformed percentage abundances) to distinguish the vegetation types. We plotted the classification results using an NMDS ordination diagram to visualize the similarity patterns among the vegetation types. Results. We distinguished six associations within three alliances: mesotrophic regularly flooded alder carr (Thelypterido palustris-Alnetum glutinosae, Carici elongatae-Alnetum glutinosae, Carici acutiformis-Alnetum glutinosae), basiphilous birch forests on mesotrophic mires (Salici pentandrae-Betuletum pubescentis) and acidophilous wet birch forests (Thelypterido palustris-Betuletum pubescentis, Eriophoro vaginati-Betuletum pubescentis). Using NMDS-ordination and Ellenberg-type indicator values we demonstrated that the main gradients for vegetation differentiation are the soil moisture and soil reaction, nutrients, and light availability. Conclusions: Using our comprehensive data set, we critically revised the previous concept and syntaxonomic structure of the class Alnetea glutinosae in Ukraine. We distinguished six phytosociological associations and highlighted the main environmental drivers determining vegetation differentiation and variability. Our study also provides an overview of the current distribution for each association of the Alnetea glutinosae in Ukraine.<br> Taxonomic reference: Euro+Med (2006-) for vascular plants, Hodgetts et al. (2020) for bryophytes.<br> Syntaxonomic reference: Higher syntaxa follow Mucina et al. (2016).<br> Abbreviations: db-RDA = distance-based redundancy analysis; EIVs = Ellenberg indicator values; EVC = EuroVegChecklist, GIVD = Global index of vegetation plot databases, ICPN = International code of phytosociological nomenclature (Theurillat et al. 2021); NMDS = Non-metric multidimensional scaling; NNP = National nature park.
