Distribution and morphological differentiation of native alder taxa (Alnus Mill.) in the Iberian Peninsula

• Comprehensive sampling reveals distribution insights of Iberian populations of Alnus glutinosa and A. lusitanica. • The study identifies new diploid-tetraploid contact zones, with no triploid hybrids detected. • Specific morphological traits distinguish Alnus glutinosa and A. lusitanica with 87.5% accuracy . • An integrative approach improves the knowledge of riparian Iberian Alnus species. The riparian environments of Europe host a remarkable richness of plant communities, often dominated by a single tree species. Understanding the identity of this species, including its morphology and distribution, is essential for the effective management and conservation of biodiversity. In Iberian alder forests, Alnus lusitanica Vít, Douda & Madák appears to be more common than Alnus glutinosa (L.) Gaertn., which dominates most of western Europe. Previous studies based on limited sampling suggested A. lusitanica was widespread in the western and southern Iberian Peninsula, while A. glutinosa was scattered in the Pyrenees and across the Cantabrian Range. Although some morphological differences have been observed, the two species are mainly differentiated by their ploidy levels: A. lusitanica is tetraploid, and A. glutinosa is diploid. This study aims to document the detailed distribution and morphological differentiation between A. lusitanica and A. glutinosa in the Iberian Peninsula, and determine whether putative hybrids (triploids) exist. Fresh and herbaria samples covering the entire Iberian range of Alnus , plus others from Europe, were collected. Ploidy levels were determined by flow cytometry. A morphometric study was also carried out with 26 variables and ratios. The results indicate that A. glutinosa is more widespread than previously reported and no triploid hybrids were detected. The distinction from A. lusitanica can be made using a set of characters. These are described in a new identification key that successfully identifies 87.5% of specimens.