S. Cordell á G. Goldstein á D. Mueller-Dombois D. Webb á P.M. Vitousek Physiological and morphological variation in Metrosideros polymorpha, a dominant Hawaiian tree species, along an altitudinal gradient: the role of phenotypic plasticity Received: 12 March 1997 / Accepted: 27 August 1997 Abstract Metrosideros polymorpha, a dominant tree species in Hawaiian ecosystems, occupies a wide range of habitats. Complementary ®eld and common-garden studies of M. polymorpha populations were conducted across an altitudinal gradient at two dierent substrate ages to ascertain if the large phenotypic variation of this species is determined by genetic dierences or by phe- notypic modi®cations resulting from environmental conditions. Several characteristics, including ecophysio- logical behavior and anatomical features, were largely induced by the environment. However, other charac- teristics, particularly leaf morphology, appeared to be mainly determined by genetic background. Common garden plants exhibited higher average rates of net as- similation (5.8 lmol CO 2 m )2 s )1 ) and higher average stomatal conductance (0.18 mol H 2 Om )2 s )1 ) than their ®eld counterparts (3.0 lmol CO 2 m )2 s )1 , and 0.13 mol H 2 Om )2 s )1 respectively). Foliar d 13 C of most common- garden plants was similar among sites of origin with an average value of )26.9&. In contrast, mean values of foliar d 13 C in ®eld plants increased substantially from )29.5& at low elevation to )24.8& at high elevation. Leaf mass per unit area increased signi®cantly as a function of elevation in both ®eld and common garden plants; however, the range of values was much narrower in common garden plants (211±308 g m )2 for common garden versus 107±407 g m )2 for ®eld plants). Nitrogen content measured on a leaf area basis in common garden plants ranged from 1.4 g m )2 to 2.4 g m )2 and from 0.8 g m )2 to 2.5 g m )2 in ®eld plants. Photosynthetic nitrogen use eciency (PNUE) decreased 50% with in- creasing elevation in ®eld plants and only 20% in plants from young substrates in the common garden. This was a result of higher rates of net CO 2 assimilation in the common garden plants. Leaf tissue and cell layer thickness, and degree of leaf pubescence increased signi®cantly with elevation in ®eld plants, whereas in common garden plants, variation with elevation of origin was much narrower, or was entirely absent. Morphological characteristics such as leaf size, petiole length, and internode length decreased with increasing elevation in the ®eld and were retained when grown in the common garden, suggesting a potential genetic basis for these traits. The combination of environmentally induced variability in physiological and anatomical characteristics and genetically determined variation in morphological traits allows Hawaiian M. polymorpha to attain and dominate an extremely wide ecological distribution not observed in other tree species. Key words Metrosideros polymorpha á Phenotypic plasticity á Photosynthesis á Carbon isotope ratios á Photosynthetic nitrogen use eciency Introduction The extreme polymorphism of the Hawaiian tree species Metrosideros polymorpha Gaud. (Myrtaceae) has long interested biologists. On the island of Hawaii M. poly- morpha is abundant across gradients of substrate age, soil moisture, elevation, precipitation, and temperature, extending from sea level to 2500 m. M. polymorpha is the dominant woody species on new lava ¯ows, as well as on mature, well developed soils, and from sites with <400 mm annual rainfall to one of the wettest places on earth (Dawson and Stemmermann 1990). Morphologi- cal variation of M. polymorpha has been associated with elevation and substrate age. Young ¯ows and high-ele- vation sites are inhabited predominantly by relatively small M. polymorpha plants with densely pubescent leaves, whereas old ¯ows and lower-elevation sites are inhabited by M. polymorpha plants that are overstory Oecologia (1998) 113:188±196 Ó Springer-Verlag 1998 S. Cordell (&) á G. Goldstein á D. Mueller-Dombois á D. Webb Department of Botany, University of Hawaii, 3190 Maile Way, Honolulu, HI 96822, USA P.M. Vitousek Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA