Structural versus functional leaf trait coordination in the adaptive radiation of Hawaiian violets

Background/Question/Methods Plants function in given environments based on coordination and optimization of numerous leaf traits. We studied leaf trait integration for eight rare taxa of Hawaiian Viola, an adaptive radiation that includes herbs, shrubs and treelets, ranging from bog to forest to cliff habitats. We quantified leaf size and shape; composition; venation architecture; mesophyll, xylem and epidermal anatomy; hydraulic conductance; photosynthetic CO₂ response parameters, and drought tolerance traits. 

Results/Conclusions We found strong leaf diversification, and tested hypotheses for structural and functional leaf trait coordination. Structural or anatomical trait linkages held strongly across all eight taxa (e.g., correlation of xylem vessel, guard cell and epidermal cell sizes). Functional trait linkages typically held only among the five taxa of wet areas (3-4 m mean annual rainfall), with the three species of warmer, drier, and/or extremely wet sites breaking from these trends. For the five wet area taxa, we found strong linkage among carbon and water flux-related traits, including leaf hydraulic conductance, stomatal pore area, venation density and photosynthetic rate per area. We also found a coordination among traits relating to nutrient and carbon economics, and among traits related to drought tolerance. Our findings demonstrate the rapid evolution of trait coordination for taxa radiated within a narrow range of given environmental variables, and the decoupling of traits when species radiated outside of that range. Leaf trait relationships can also inform conservation, indicating the range of shared and specialized habitats important for preservation of the whole lineage.