Hidden Riverside Forests of Northern Spain May Mark Europe’s Southwestern Edge

Deep in the valleys of the Basque Country and Navarre, where rivers such as the Baia, Zadorra, Ultzama and Irati wind southward toward the Ebro, grows a forest type that European conservationists thought belonged almost exclusively to the great floodplains of Central Europe. Temperate hardwood riparian forests—tall, multi-layered woodlands of ash, elm and oak with an understory bursting with vernal geophytes—are listed as an Endangered habitat on the European Red List of Habitats. Yet Spain was never included in that evaluation, and until recently the only evidence for their presence on the Iberian Peninsula came from vegetation plots mapped by the EUNIS Expert System. A new study, published in the journal Vegetation Classification and Survey, now provides the first standardized field confirmation that these endangered forests thrive in the small rivers of northern Spain, and suggests the region may mark the southwestern distribution limit of the entire habitat type in Europe.

The research team set out to answer three questions: whether temperate hardwood riparian forests, classified as habitat T13 in the European Nature Information System (EUNIS), truly occur in the inner valleys of the Basque-Cantabrian mountains and the western Pyrenees; how these forests should be classified at the level of association and alliance; and what their diversity patterns and ecological preferences reveal. To do so, they sampled 72 nested series of vegetation plots with grain sizes ranging from a minuscule 0.0001 square meters—about the area of a fingernail—to 200 square meters, recording the presence of vascular plants at every scale and estimating cover in the largest plots. The plots were established as permanent installations, marked with red-painted wooden stakes, so that future monitoring can track how these forests respond to global change.

The sampling design was a technical feat in itself. The team adapted the standard protocol of the Eurasian Dry Grassland Group, originally developed for grasslands, to the linear shape of floodplain forests. Instead of square plots, they laid out rectangles of 10 by 20 meters, placing two contiguous 100-square-meter plots parallel to the riverbed. The herb layer was sampled in two replicated nested series, one starting at the river corner and the other at the inner corner, capturing everything from tiny seedlings to dominant hemicryptophytes. Because many vernal geophytes—low-growing spring plants such as wood anemones and scillas—vanish by summer, the researchers conducted additional spring samplings in 2023 and 2024 to avoid dismissing these fleeting species from the record.

The floristic results were striking. Across all plots, the team identified 456 vascular plant taxa, with only 48 occurring in at least half of the plots. Three species—Acer campestre, Crataegus monogyna and Hedera hibernica—were present in every single plot. Mean species richness per 200-square-meter plot reached 75.3, with a minimum of 39 species in a submediterranean ash forest and a maximum of 124 in a western Pyrenean ash forest dominated by Fraxinus excelsior with hazel, alder, beech and wych elm. This richness clearly exceeds what models predict for deciduous forests of the region, confirming northern Iberia as a secondary biodiversity hotspot for European deciduous forests, after the main hotspots in Central and southeastern European mountain ranges.

Among the 456 taxa were five endangered species included in regional catalogues of the Basque Country and Navarre: Carex strigosa, Circaea alpina, Galanthus nivalis, Lathraea squamaria and Narcissus pseudonarcissus subsp. nobilis. Galanthus nivalis, the snowdrop, is also listed as Near Threatened on the European red list of vascular plants, while the Iberian daffodil subspecies appears in Annex II of the EU Habitats Directive. Beyond the threatened flora, the sampled forests sheltered eleven Cantabrian-Pyrenean endemics, including Fritillaria pyrenaica, Meconopsis cambrica and Scilla lilio-hyacinthus, as well as species common in Central European forests and high mountains but rare in the lowland forests of northern Spain, such as Aconitum napellus subsp. vulgare and Adenostyles alliariae. These riparian corridors clearly function as refuges for some of the region’s most vulnerable plants.

When the 200-square-meter plots were submitted to the EUNIS Expert System, the results were surprising. Only five plots were classified in habitat T13, the temperate hardwood riparian forest; nineteen fell into other deciduous forest types, including mesic Carpinus and Quercus forests and alder-dominated riparian woodland; and 48 were classified only at the coarsest level as forests or remained unclassified. More than half of the plots thus failed to receive a valid assignment at EUNIS level 3, a result the authors describe as unacceptable for regional-scale application. The expert system, optimized for habitat definitions across the whole of Europe, evidently needs further adjustment before it can reliably classify these Iberian forests.

The unsupervised classification told a clearer story. Agglomerative hierarchical clustering of the floristic data, supported by permutational multivariate analysis of variance, yielded three main clusters corresponding to three associations within the alliance Alnion incanae: Carici pendulae-Fraxinetum excelsioris in the subcantabrian valleys, Galio laevigati-Fraxinetum excelsioris in the western Pyrenean valleys, and Viburno lantanae-Ulmetum minoris in the submediterranean valleys. Diagnostic species, assessed with a modified phi index, separated the types convincingly. The western Pyrenean association boasted 60 diagnostic species, including Melica uniflora, Veronica chamaedrys and Fagus sylvatica, plus Pyrenean specialties such as Acer opalus and Pulmonaria affinis. The submediterranean association was defined by Fraxinus angustifolia and Ulmus minor, along with ruderal companions like Lapsana communis and Melissa officinalis. The subcantabrian association stood out for Quercus robur, absent from both Pyrenean and submediterranean valleys, and for wet-soil geophytes and hemicryptophytes such as Filipendula ulmaria and Ornithogalum pyrenaicum.

Diversity patterns differed significantly among the three forest types. Stands of Galio-Fraxinetum contained significantly more vascular plant species than the other two associations, with mean richness of 91.8 species per plot, while Shannon diversity was significantly lowest in Viburno-Ulmetum at 2.33. Whittaker’s beta diversity, a measure of species turnover among plots, was highest in the submediterranean forests at 4.7 and lowest in the subcantabrian forests at 3.2. Fine-scale turnover, expressed as the z value of the species-area relationship, was highest in the western Pyrenean forests at 0.32, though differences were not significant after correction. The authors attribute the exceptional richness of Galio-Fraxinetum to its mountainous character, linked to lower site productivity, higher flooding intensity and greater soil heterogeneity, with cover of stones, rocks and gravel significantly exceeding that of the other forest types. Consistent with the hump-shaped model of the productivity-richness relationship, soil nitrogen indicator values were significantly lower in the most species-rich association.

Climatic and structural variables reinforced the ecological split. The submediterranean Viburno-Ulmetum experienced the warmest annual temperatures, the lowest precipitation, the widest annual temperature range and the driest summers, with a mean summer ombrothermic index of 2.14—close to the threshold of 2 that defines Mediterranean climate. This result aligns with earlier findings that the replacement of Fraxinus excelsior by F. angustifolia coincides with the onset of true Mediterranean conditions. Disturbance indicator values were also highest in the submediterranean forests, both for the whole community and for the herb layer, reflecting the more open canopies and greater human influence of these agricultural valleys. Notably, alien plant invasion remained modest in these inner valleys: only 28 alien species, or 6.1 percent of the species pool, were recorded, with a mean invasion level of 1.6 per plot—values comparable to European hardwood forests overall and far below the 15.7 percent recorded in nearby Cantabrian streams, where warmer, more disturbed conditions favor subtropical invaders.

The classification at association level replicates the framework established for the entire Iberian Peninsula, but the alliance-level question remains open. The three associations were traditionally placed in Alnion incanae, within a new suballiance Buxo sempervirentis-Alnenion glutinosae, yet their floristic composition—rich in Acer campestre, Ligustrum vulgare and mesic forest species—draws them toward the Central European alliance Fraxino-Quercion roboris, which gathers elm and ash forests on episodically flooded lowland plains. Whether the Iberian suballiance should be transferred to Fraxino-Quercion, to the submediterranean Alno-Quercion roboris, or retained in Alnion incanae cannot be resolved without a comprehensive pan-European analysis of temperate riparian, alluvial and mesic oak forests. What is certain, the authors conclude, is that these hardwood riparian forests—endangered, exceptionally diverse, and sheltering threatened and endemic flora—stand at the fragile southwestern limit of their European distribution, where climate change will test their resilience first. Clarifying their identity within both the EUNIS habitat classification and the European phytosociological system is therefore not an academic exercise but a prerequisite for effective conservation.

Subject of Research: Ecology

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

Article References: Biurrun, I., Belmonte, J., Sanz-Zubizarreta, I., & Campos, J. A. (2025). Hardwood riparian forests in northern Iberian Peninsula: classification and diversity patterns. Vegetation Classification and Survey, 6, 1-21. https://doi.org/10.3897/vcs.145406

Image Credits: AI Generated

DOI: 10.3897/VCS.145406

Keywords: riparian forests, EUNIS habitat T13, phytosociology, biodiversity, northern Spain, Alnion incanae, nested-plot sampling, endangered habitats, Hardwood, riparian, forests, northern