Ecological and Genetic Implications of Foliar Polymorphism inMetrosideros polymorphaGaud. (Myrtaceae) in a Habitat Matrix on Mauna Loa, Hawaii

Abstract

Glabrous and pubescent foliar phenotypes ofMetrosideros polymorphaoccur sympatrically on Mauna Loa, Hawaii. This coexistence has been suggested, by earlier workers, to represent distinct genotypes, each of which produces progenies with a homogenous foliar phenotype. We re-examined the question of to what extent siblings from the same parents varied in phenotype when grown under the same common-garden conditions. Seeds were collected from parental trees in a matrix of two substrate ages (114–140 yrvs.approx. 1200–3200 yr old) at five elevations from lowland (100 m) to treeline (2470 m) on Mauna Loa. Seeds were sown and grown under identical environmental conditions at 1200 m. In field parental populations, two forms converged into pubescent forms with decreasing rainfall (<2000 mm) on both substrates, and into glabrous forms with increasing soil age where rainfall was ample. The 4-yr old seedlings from pubescent parents invariably expressed both glabrous and pubescent phenotypes for all altitudes and for both substrate ages except for the young 1980-m site. The seedlings from glabrous parents tended to express only the glabrous phenotypes except for the young 1280-m site. Overall, all populations on the young lava flow (glabrous and pubescent individuals combined if coexisting) could produce progenies of both phenotypes except at 1980 m. Cuticle thickness on the adaxial lamina surface was positively correlated with the magnitude of pubescence (P=0.05) among siblings from the same parents, ranging from a mean cuticle thickness of 6.0 μm for glabrous to 15.7 μm for the most pubescent individuals. Greater magnitudes of pubescence were associated with more negative osmotic potentials at turgor loss point, and with greater maximum CO2assimilation rates at the point of light saturation. This physiological and anatomical polymorphism, together with spatially and temporally varying selective forces, appears to result in elevationally and successionally patternedM. polymorphapopulations on the wet slope of Mauna Loa.