Leaf Trait Diversification and Design in Seven Rare Taxa of the Hawaiian Plantago Radiation
Abstract
Linkages among leaf traits and environment have most often been tested across communities but infrequently within lineages. We studied seven endemic Hawaiian Plantago taxa radiated across elevations, climates, and habitats. We grew plants of six taxa in controlled conditions for 1–2 yr and collected leaves from a seventh in the field. For all taxa, we measured 46 leaf traits and tested hypotheses for trait-environment and trait-trait associations. Because of the rarity of the study plants, our study included low replication within taxa and multiple growth locations; despite these limitations, given reasonable assumptions, our analyses pointed to genetic differentiation among taxa. The leaves of bog taxa were smaller and thicker than those of woodland taxa, with higher leaf mass per area (LMA), stomatal pore area per leaf area, and carbon isotope discrimination (δ13C). Taxa from higher elevations had thicker leaves and higher LMA, as well as lower nitrogen per mass and higher adaxial stomatal distribution. Taxa of drier sites had higher vein density and δ13C. Many traits were allometrically related to leaf area, including stomatal density, major vein densities, and xylem conduit numbers and dimensions. Stomatal and xylem traits were correlated, indicating a matching of hydraulic supply and demand. Leaf diversification in Hawaiian Plantago involved coordinated trait shifts, generating strong, apparently adaptive trait linkages.
