The relict communities of Alnus glutinosa on the southern range edge of its biogeographical distribution in the southern Mediterranean are vital for biodiversity conservation but are threatened by human activities and climate change. To develop effective management and conservation strategies, a comprehensive understanding of their floristic composition and plant community dynamics is essential. We conducted field surveys across the Rif Mountains, collecting 36 vegetation relevés from 11 populations. Rectangular sample plots of 200 m² were established in homogeneous river strips dominated by Alnus glutinosa with over 50% coverage. Our analysis revealed a rich diversity, with 219 vascular plant species belonging to 74 botanical families. Noteworthy findings include the presence of 14 regional endemics and 28 red-listed species. Herbaceous species dominated the communities, with hemicryptophytes being prevalent among the life forms. Phytogeographical analysis indicated a predominance of native species (96%), with 8% classified as alien. The Mediterranean broad-ranging chorotype was the most common (31.5%), followed by Paleartic, global, extended, and regional chorotypes. TWINSPAN classification identified two distinct plant communities, influenced primarily by altitude and their associated climatic conditions, as well as anthropogenic factors. Syntaxonomical analysis revealed distinct characteristics of black alder communities, predominantly associated with the Osmundo-Alnion glutinosae alliance. These communities exhibit two main groups, differentiated by species composition and environmental variables: (i) the Alnus glutinosa-Nerium oleander group, thriving in both moist sandy and gravelly deep soils under thermophilic conditions at lower altitudes; (ii) the Alnus glutinosa-Prunus lusitanica group, found on rocky clay soils on steep slopes, characterized by mesoclimatic and hydrological conditions at medium-to-high altitudes, primarily under mesomediterranean and lower supramediterranean conditions. Our findings highlight the importance of local abiotic and anthropogenic factors in shaping black alder communities in Morocco. It is therefore imperative to implement targeted conservation efforts to preserve these unique and ecologically valuable black alder communities in the southern Mediterranean.