Landscape genetics of the tropical willow Salix humboldtiana: influence of climate, salinity, and orography in an altitudinal gradient

Gene flow in riparian ecosystems is influenced by landscape features such as orography, climate, and salinity. The downstream increase in genetic diversity (DIGD) hypothesis states that the unidirectionality of the watercourse causes an accumulation of genetic diversity toward downstream populations, while upstream populations are more structured and less diverse, especially in water-dispersed organisms.We used chloroplast and nuclear microsatellites to characterize genetic diversity, structure, and gene flow patterns among populations of Salix humboldtiana across an elevation and salinity gradient on three rivers (Actopan, Antigua, and Blanco) in Mexico. We used optimization of resistance surface methods to determine whether genetic distances between populations are correlated with landscape features.Positive FOur results depict that the DIGD and gene flow patterns in S. humboldtiana result from the complex interaction among physiography, climate, river salinity, and life-history traits of the species.