Parallel and convergent diversification in two northern hemispheric species-rich Carex lineages (Cyperaceae)

Carex species are dominant and abundant plants in boreal and arctic landscapes, typically covering large wetland areas. Most of these vegetation-characteristic species are from Carex sections Phacocystis and Vesicariae, frequently growing together, but also forming monodominant stands. Here we study these species in a phylogenetic framework to infer whether this co-occurrence pattern results from convergent evolution. In both sections, we observed a Northern clade consisting only of arctic to boreal species, a Mixed clade of northern and more southerly distributed species and a Southern grade of species mainly from temperate or further southern zones. The species of the Northern clades of both sections that are the focus of the study may be of similar young age and are rather equally diversified in terms of molecular divergence and morphology, suggesting a replicate adaptive radiation in boreal to arctic habitats. Morphological characters possibly linked with functional importance are not significantly different between the respective clades of the two sections, whereas reproductive structures may be phylogenetically constrained. The evolution of salt tolerance and ongoing diversification in the Northern clade of sect. Phacocystis suggests that the observed replicate adaptive radiation might be a transitional state in the diversification of species and may explain why such radiations are so rarely documented.