PHYSIOLOGICAL AND MORPHOLOGICAL VARIATION IN METROSIDEROS-POLYMORPHA,A DOMINANT HAWAIIAN TREE SPECIES, ALONG AN ALTITUDINAL GRADIENT - THEROLE OF PHENOTYPIC PLASTICITY

Metrosideros polymorpha, a dominant tree species in Hawaiian ecosystem s, occupies a wide range of habitats. Complementary field and common-g arden studies of M. polymorpha populations were conducted across an al titudinal gradient at two different substrate ages to ascertain if the large phenotypic variation of this species is determined by genetic d ifferences or by phenotypic modifications resulting from environmental conditions. Several characteristics, including ecophysiological behav ior and anatomical features, were largely induced by the environment. However, other characteristics, particularly leaf morphology, appeared to be mainly determined by genetic background. Common garden plants e xhibited higher average rates of net assimilation (5.8 mu mol CO2 m(-2 ) s(-1)) and higher average stomatal conductance (0.18 mol H2O m(-2) s (-1)) than their field counterparts (3.0 mu mol CO2 m(-2) s(-1), and 0 .13 mol H2O m(-2) s(-1) respectively). Foliar delta(13)C of most commo n-garden plants was similar among sites of origin with an average valu e of -26.9 parts per thousand. In contrast, mean values of foliar delt a(13)C in field plants increased substantially from -29.5 parts per th ousand at low elevation to -24.8 parts per thousand at high elevation. Leaf mass per unit area increased significantly as a function of elev ation in both field and common garden plants; however, the range of va lues was much narrower in common garden plants (211-308 g m(-2) for co mmon garden versus 107-407 g m(-2) for field 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 field p lants. Photosynthetic nitrogen use efficiency (PNUE) decreased 50% wit h increasing elevation in field plants and only 20% in plants from you ng substrates in the common garden. This was a result of higher rates of net CO2 assimilation in the common garden plants. Leaf tissue and c ell layer thickness, and degree of leaf pubescence increased significa ntly with elevation in field 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 lengt h, and internode length decreased with increasing elevation in the fie ld and were retained when grown in the common garden, suggesting a pot ential genetic basis for these traits. The combination of environmenta lly induced variability in physiological and anatomical characteristic s and genetically determined variation in morphological traits allows Hawaiian M. polymorpha to attain and dominate an extremely wide ecolog ical distribution not observed in other tree species.