The tough, the wet and the hidden: Evolutionary strategies of a polyploid tropical tree in a changing environment

Abstract The Zambezian Floristic Region (ZFR) in south-central Africa experienced major environmental changes in the past and is nowadays home to numerous woody taxa of wet-tropical provenance. Within many taxa, we observe adaptations to multiple habitats, but know little about their environmental drivers. In order to gain knowledge about adaptation and speciation processes in this region, we chose the tree species complex Syzygium guineense (Willd.) DC. s. l. as a model. Using microsatellite genotyping and multivariate analysis of functional traits of nine phenotypes occupying various habitat types, we analyzed phenotypic and genetic differentiation with regard to environmental factors. We found polyploidy, weak correlation between genetic and spatial distances and three admixed genetic ancestries. Distinct growth forms and habitats characterize each of them. We identified several groups, i.e. ecotypes, which most probably mirror evolutionary strategies to overcome environmental changes happening in the ZFR over the last 10 Ma. Strong environmental filters favored the persistence of conservative ecotypes in refugia, and the evolution of ecotypes adapted to newly emerged habitats with increased disturbance regimes. Polyploidy and phenotypic plasticity possibly promoted these processes, which until now were seldom documented in adaptation to drought stress of tropical tree species. Our findings highlight the value of the Zambezian vegetation mosaic as a unique evolutionary experiment.