Correlated evolution in traits influencing leaf water balance in Dendrobium (Orchidaceae)

A favorable leaf water balance can be achieved through coordinated variation in multiple leaf traits. However, the role of evolutionary history in producing this trait coordination is not well understood, especially in epiphytic species. To address these limitations, we measured 11 leaf traits for 19 Dendrobium species grown in the same environment, and used phylogenetically independent contrasts to test how the co-evolution of these traits has contributed to leaf water balance in epiphytes. Our results show that more closely correlated species had similar habitats, geographical distribution, and some leaf traits (leaf density, upper cuticle thickness, and stomatal index), whereas distantly related species did not exhibit such similarities. Species originating from forests below the altitude of 1,500 m exhibited smaller leaf area, stomatal area, and stomatal index, but thicker leaves than those from forests above the altitude of 1,500 m. Stomatal density was evolutionarily correlated with vein density, upper and lower epidermal thicknesses, and leaf density. Leaf thickness was significantly correlated with stomatal density and vein density, while leaf area was significantly correlated with epidermal cell thicknesses, stomatal density, and stomatal area. These results found that both the environment and evolutionary history significantly affected leaf functional traits and ecological characteristics in Dendrobium, and supported the hypothesis that co-evolution in leaf traits is an important contributor to leaf water balance in Dendrobium. This study provides new insights into the evolution of ecological strategies in epiphytic orchids.