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The degradation of riparian ecosystems occurring throughout the past decades has motivated efforts aimed at the restoration of these ecosystems. The success of active revegetation approaches to restoration requires appropriate selection of reproductive material, which in turn requires knowledge of seed traits and germination. Alnus glutinosa (L.) Gaertn. (common alder) is a key riparian tree widely used in restoration projects, and has recently been classified as comprising three species: A. glutinosa; A. lusitanica Vít, Douda, & Mandák; and A. rohlenae Vít, Douda, & Mandák. To help guide restoration species selection, we assessed differences among populations of these species by (1) investigating seed weight, morphology, and germination success from a large population set and (2) modeling germination success in each species in relation to morphological traits and environmental conditions. Seeds were collected from 12 populations encompassing the latitudinal extremes of the species complex, and were then characterized and germinated. Ploidy levels and species were distinguished using cytometric analysis. Site-level climatic data and seed morphology data were used to model germination success for each species. All seed traits differed between populations and one morphological-trait (seed weight-to-area ratio) differed significantly between the three species. Germination modeling showed that the southwestern species, A. lusitanica, responded positively to high temperature extremes, suggesting tolerance to the climate changes projected for southern Europe. Populations of A. lusitanica located at the latitudinal rear edge of common alder's distribution appear to show establishment-facilitating adaptations, and therefore may contribute to ecological restoration efforts under a range of environmental conditions.