Correlated evolution of leaf shape and physiology in the woody Sonchus alliance (Asteraceae: Sonchinae) in Macaronesia

The Sonchus alliance represents an oceanic island radiation that includes six genera and 31 species of herbs, shrubs, and trees occurring in diverse habitats ranging from cool, upland cloud forests to hot, dry, exposed cliff faces and coastal deserts. To investigate the role of photosynthetic adaptations during radiation into diverse habitats, we measured leaf morphological and physiological traits and relationships among 15 representative species of the woody Sonchus alliance grown under uniform conditions. Correlated evolution was observed between photosynthesis per mass with leaf size and specific leaf area and between photosynthesis per area with dark respiration per area and stomatal conductance. In general, species from exposed habitats exhibited smaller, more dissected leaves with greater photosynthetic rates compared with those of species native to relatively shaded pine and laurel cloud forest. We also observed that sister taxa in contrasting habitats can develop substantially divergent leaf forms in relatively short evolutionary time. The data serve as a striking example of the role of photosynthetic adaptation in exploiting habitats with contrasting light regimes during rapid evolutionary radiation on isolated oceanic islands.