Leaf chemical, biophysical, and optical properties were measured in 13 populations of Metrosideros polymorpha across gradients of soil fertility and climate in Hawaii. Climate (predominantly temperature) caused large changes in specific leaf area (SLA) and SLA-linked traits, including nitrogen (N) and pigment contents, as did conditions of highest soil fertility on 20 ky old substrates. When averaged by site, chemical constituent ratios containing chlorophyll (Car/Chl, Chl/N) varied more across climate than substrate gradients, while the Chl a/b ratio was similarly influenced by climate and substrate. Variations in Chl a/b ratios and SLA were similar to those found previously in a common garden of M. polymorpha taken from our climate gradient, suggesting strong genetic control over these traits. Optical reflectance indices related to photosynthetic function were closely correlated to pigment changes, varying three times more in response to climate than across substrate ages. Combined, our results suggest that variation in leaf structure, composition, and function of M. polymorpha is a result of genetic and phenotypic adaptation to environmental differences, and that these variations are greater in response to climate (especially temperature) than to soil fertility.