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Populations located at the rear-edge of a species’ distribution may have disproportionate ecological and evolutionary importance for biodiversity conservation in a changing global environment. Yet genetic studies of such populations remain rare. This study investigates the evolutionary history of North-African low latitude marginal populations of Alnus glutinosa Gaertn., a European tree species that plays a significant ecological role as a keystone of riparian ecosystems. We genotyped 551 adults from 19 populations located across North Africa at 12 microsatellite loci and applied a coalescent-based simulation approach to reconstruct the demographic and evolutionary history of these populations. Surprisingly, Moroccan trees were tetraploids demonstrating a strong distinctiveness of these populations within a species otherwise known as diploid. Best-fitting models of demographic reconstruction revealed the relict nature of Moroccan populations that were found to have withstood past climate change events and to be much older than Algerian and Tunisian populations. This study highlights the complex demographic history that can be encountered in rear-edge distribution margins that here consist of both old stable climate relict and more recent populations, distinctively diverse genetically both quantitatively and qualitatively. We emphasize the high evolutionary and conservation value of marginal rear-edge populations of a keystone riparian species in the context of on-going climate change in the Mediterranean region.

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´tude phytoe ´cologique des aulnaies du nord-est alge ´rien montre que ces habitats d'affinite ´septentrionale pre ´sentent une tre `s grande richesse spe ´cifique (> 400 espe `ces) et une structure complexe, qui plaident pour leur origine ancienne. Sur le plan phytosociologique, elles sont rattache ´es a `deux syntaxons, le Campanulo alatae-Alnenion glutinosae (aulnaies riveraines) et le Rusco hypophylli-Alnetum glutinosae (aulnaies mare ´cageuses). Leur e ´tat de de ´gradation et leur dynamique re ´gressive, observe ´es au cours des 14 anne ´es de l'e ´tude, re ´ve `lent leur statut extre ˆmement pre ´caire et leur de ´clin rapide sous l'influence de perturbations anthropiques incontro ˆle ´es (de ´frichement, incendies, drainage, pollution. . .). Au vu de leur importance e ´cologique, historique et patrimoniale, les aulnaies de Numidie alge ´rienne doivent rapidement faire l'objet de mesures de protection, dans le but d'assurer la pe ´rennite ´des corte `ges floristiques exceptionnels qui les constituent.

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