Older Lineages of Vascular Plants in Wetlands Dominate in Habitats That Are More Ubiquitous Across the Region: A Case Study in Southern Africa.

Vascular plants that presently grow and often dominate wetland habitats are mostly derived from ancestors adapted to dry terrestrial environments. We hypothesize that recently evolved wetland lineages dominate more challenging or novel wetland environments, whereas older lineages tend to dominate widespread and more stable wetland types, likely due to greater dispersal capabilities, as wetland habitats are geologically transient. The regional wetland flora of southern Africa is investigated by listing all species that are flood-tolerant without specific adaptations, mesophytic, helophytic, and aquatic. The age in which species moved from one state to another was calculated by projecting the traits onto the phylogenetic tree of all vascular plants. Diversification rates were calculated for eight different plant orders in order to determine and compare levels of preadaptation. The occurrence of wetland plants of different age groups (the period in geological history in which they transitioned from the flood-tolerant stage to the mesophytic stage) was mapped across the different current wetland habitats found in the region and correlated to environmental variables by means of a redundancy analysis. The orders Alismatales and Poales have the most extant wetland species and have been proven to transition to states of wetland adaptation the most frequently in the course of their evolution. Lineages in the monocots generally tend to be more preadapted towards wetland environments than dicots. Some wetland habitats in southern Africa are currently occupied by species that have only recently transitioned towards wetland adaptations, such as saline pans. Wetlands in savanna and grassland vegetation are dominated by species that have spent a much larger part of their evolutionary history in wetland environments. This study emphasizes the contribution to the wetland flora from a regional perspective, but it should be noted that many of the transitions would have taken place outside of the actual region of study.