Wood Anatomy of Dubautia (Asteraceae: Madiinae) in Relation to Adaptive Radiation

Abstract

Qualitative and quantitative features are reported for stem wood of 13 collections of 12 species of the Hawaiian genus Dubautia. Although the species share a basic wood plan, quantitative expressions range widely, especially with respect to vessel element dimensions, vessel density, vessel grouping, length of libriform fibers, and dimensions of multiseriate rays. Ecology and habit explain most of the diversity. Variations in the ratio between vessel element length and libriform fiber length are correlated with habit both within Dubautia and when Dubautia is compared with Argyroxiphium and Wilkesia. Other variation in wood is related mostly to ecology. The Dubautia species of wet forest have high mesomorphy ratio values. Low mesomorphy ratio values occur in species of recent or dry lava (e.g., D. scabra) or dry alpine areas (D. menziesii); mesomorphy ratio values in the xeric species are comparable with those in Argyroxiphium. Highly xeromorphic wood in the bog species D. waialealae may reflect recent immigration from a dry habitat or peculiar features of the bog habitat. The lianoid D. latifolia has notably xeromorphic wood, which may reflect recent entry into wet forest or else the tendency for lianas in general to have xeromorphic features that confer conductive safety. All species of Dubautia show fiber dimorphism. Dubautia is a superb example of adaptive radiation, in contrast to the Hawaiian Schiedea (Caryophyllaceae), which has shifted into various habitats with little change in wood anatomy, or the Galapagos genus Scalesia, all species of which must survive periods of drought and have xeromorphic wood.