Phenotypic variation and genetic diversity in European Alnus species

The recent taxonomic distinction between the European common alder species Alnus glutinosa and A. lusitanica strengthens the need for studies on phenotypic and genetic variation. Understanding such variations requires the use of populations encompassing the total latitudinal distribution and environmental envelope of the distribution range of A. glutinosa and A. lusitanica. This study aims to describe the genetic diversity and assess the phenology, morphology, physiology and biochemistry of alder seedlings from populations reaching out to the latitudinal extremes of both species. We grew 1579 2-year-old seedlings from seven A. glutinosa and six A. lusitanica populations in a common garden in Oeiras, Portugal. By using simple sequence repeat markers, two genetically distinct groups in A. glutinosa and three in A. lusitanica were identified. The species presented marked morphological and biochemical differences. The southern populations of A. lusitanica showed higher genetic diversity than A. glutinosa populations. Significant associations between four alleles from three loci, and one morphological and three biochemical traits were detected in several genetic groups from both alder species. Traits from southern populations of A. lusitanica were associated with low water availability. Inter-specific trait variation and association of traits with molecular markers suggest that A. lusitanica might have an adaptive advantage if subjected to drought and heat stressors. This study strengthens the recent taxonomical differentiation between the two species and emphasizes the need for preserving southern A. lusitanica populations and their genetic diversity in Mediterranean riparian corridors.