Hardwood riparian forests in northern Iberian Peninsula: classification and diversity patterns

Aims : Temperate hardwood riparian forests constitute an endangered habitat occurring mostly on the floodplains of major European rivers, and included in habitat T13 of the European Nature Information System (EUNIS). In this study we aim to 1) confirm their presence in small rivers of northern Spain, 2) revise their classification at association level, and discuss it at alliance level, 3) evaluate their diversity patterns and ecological preferences. Study area : Inner temperate and submediterranean valleys in northern Spain. Methods : We sampled 72 nested series of vegetation plots with grain sizes ranging from 0.0001 m 2 to 200 m 2 , where the presence of vascular plants was recorded in all grain sizes and their cover estimated in 100 m 2 . 200 m 2 plots were submitted to the EUNIS Expert System (EUNIS-ESy) and to agglomerative hierarchical clustering. Non-metric dimensional scaling was used for their ordination in the multivariate space. Diagnostic species were assessed by means of modified phi index and differences among forest types regarding their diversity and ecological preferences through analyses of variance. Results : The EUNIS-ESy classified five plots in habitat T13, 19 were classified in other deciduous forest types, and 48 were classified only at level 1 (forests) or remained unclassified. Three main clusters emerging from the agglomerative clustering corresponded to three associations from the alliance Alnion incanae : Carici pendulae-Fraxinetum excelsioris , Galio laevigati-Fraxinetum excelsioris , and Viburno lantanae-Ulmetum minoris . The mean species richness per plot was 75.3, with the highest values in the western Pyrenean association Galio-Fraxinetum , while Shannon diversity was lowest in the submediterranean Viburno-Ulmetum . Conclusions : These riparian forests provide refuge for several endangered species and might mark the southwestern distribution of hardwood riparian forests in Europe. It is therefore crucial to clarify their classification at alliance level, for which a comprehensive analysis of European temperate riparian, alluvial and mesic oak forests is required. Taxonomic reference : Euro+Med PlantBase (Euro+Med 2024). Syntaxonomic reference : Mucina et al. (2016), updated in Chytrý et al. (2024), for high rank syntaxa; Biurrun et al. (2016) for associations and suballiances. Abbreviations : CWM = Community Weighted Means; DIV = Disturbance Indicator Values for European plants; EDGG = Eurasian Dry Grassland Group; EIVE = Ecological Indicator Values for Europe; EUNIS = European Nature Information System; EUNIS-ESy = EUNIS Expert System; NMDS = Non-Metric Multidimensional Scaling; SAR = Species Area Relationship.