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

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. lati/olia 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. Wood anatomy has proven to be a sensitive indicator of adaptation to diverse ecological regimes and may be seen with special clarity in families and genera that have diversified in recent geological time, with little attendant extinction (Carlquist 1975). Thus, we would expect the wood anatomy of Dubautia to show adaptive radiation in wood exceptionally well. Dubautia ranges from subshrubs on hot, dry lava (D. linearis, D. scabra) to shrubs of mesic forest (D. microcephala, D. plantaginea), rain forest trees (D. knudsenil), bog shrubs (D. waialealae), alpine shrubs (D. menziesii), and a lianoid shrub (D. lati/olia). The habitats and localities for Dubautia species are well described by Carr (1985, 1990). Earlier studies that included data on wood of Dubautia (Carlquist 1958, 1974, 1994) dealt with a small number of the species and 356 HAWAllAN MADIINAE (Argyroxiphium, Dubautia, Wilkesia) constitutes a monophyletic group (Baldwin et al. 1990, Baldwin and Robichaux 1995) often cited as a premier example of adaptive radiation in plants (e.g., Carlquist 1965, 1970, Carr 1985, Robichaux et al. 1990). Argyroxiphium, with five species, and Wilkesia, with two (Carr 1985), show distinctive aspects of this adaptive radiation, but the diversity of the 21 species now recognized within Dubautia (Carr 1985, 1990) illustrates entry into a wide range of habitats with attendant change in morphological, anatomical, and physiological expressions. 1 Manuscript accepted 2 April 1998. 2Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, California 93105. Address for correspondence: 4539 Via Huerto, Santa Barbara, California 93110-2323. Wood Anatomy of Dubautia-CARLQUISf with limited data sets. Therefore, a more detailed series of studies is attempted here. Companion studies, on wood of Argyroxiphium (Carlquist 1997) and Wilkesia (Carlquist in press), lend perspective to the observations on Dubautia wood. Adaptation by wood of dicotyledons to varied ecological regimes is primarily related to vessel features: vessel diameter, vessel element length, and vessel density (Carlquist 1975). To this list, degree of vessel grouping should be added (Carlquist 1984). Presence and nature of growth rings (Carlquist 1980) and occurrence of vasicentric tracheids (Carlquist 1985a) in wood are also indicative of ecology. Degree of woodiness (subshrubs, shrubs, trees) is indicative of adaptation to various habitats in a phylad in which secondary woodiness has occurred, as appears to be the case in Dubautia and other island genera (Carlquist 1974). Secondary woodiness is characteristic of the Hawaiian Madiinae, judging by their terminal position in cladograms (Baldwin et al. 1990, Baldwin and Robichaux 1995). Basic to the Hawaiian genera are subshrubs of very limited woodiness, such as the Californian montane genus Raillardiopsis, which has been included in this study for purposes of comparison. Adaptive radiation in wood of Dubautia should be understood in the context of the Madiinae as a whole. MATERIALS AND METHODS The wood samples studied represent mostly wood samples I collected over a period of years from naturally occurring specimens. To these have been added stems from a few herbarium specimens. The sample of D. linearis wood was small (3 mm diameter), but the sample of D. waialealae, although ca 6 mm in diameter, was from the basal stem of a mature plant. The stem of D. latifolia was taken from a dead 3-yr-old branch and does not represent a basal stem, but the scarcity of this species does not permit harvesting of larger wood samples. For all other species, the wood samples represent 357 basal stems or major branches, portions obtainable when these plants were relatively common 40 or more years ago. Wood samples were prepared by drying. Voucher specimens are listed in Table 1. Summaries of habitat information for the various species given by Carr (1985) are the best source for comparisons with wood anatomy, and thus localities of the individual specimens are not given here. The wood of Raillardiopsis muirii (A. Gray) Rydb. has not been included in Table 1; it was collected in Tehipite Valley, Middle Fork of the Kings River, 1400 m, Fresno County, California, J. T. Howell 33960 (RSA). The specimen of D. plantaginea (Carlquist 1928), collected in Ko'olau Gap, is referable to D. plantaginea var. platyphylla Hillebrand, a variety now placed into synonymy under D. plantaginea subsp. plantaginea by Carr (1985). Portions of dried wood samples were boiled in water and stored in 50% aqueous ethanol. Wood was sectioned on a sliding microtome. Sections were stained either with safranin or with a safranin-fast green combination. Macerations for measurement of vessel element length and libriform fiber length were prepared with Jeffrey's Fluid and stained with safranin. Means shown in Table 1 are based on 25 measurements per feature except for rather variable ones: vessel wall thickness, pit diameter, and libriform fiber wall thickness figures are based on conditions judged to be typical. Number of vessels per group is computed on the basis of a solitary vessel = 1, a pair of vessels in contact = 2, etc. Vessel lumen diameter is used rather than outside vessel diameter because lumen diameter is considered a better indicator of conductive characteristics. The lumen diameter of vessels oval in transection was measured as an average between widest and narrowest diameter. Wood anatomy terms follow the IAWA Committee on Nomenclature (1964), except for the terms vasicentric tracheid and vascular tracheid, which follow earlier usages (Carlquist 1984, 1985a). Radial sections of all species were prepared, as is typical in studies on wood anatomy, but radial sections have not been illustrated here; they yield