Metrosideros polymorpha, a dominant tree species in the Hawaiian Islands, shows an extreme phenotypic polymorphism both across gradients of climatic/edaphic conditions and within populations, making it a potentially useful model species for evolutionary study. In order to understand how the phenotypic diversity is maintained within populations as well as across populations, we examined the diversities of several leaf and stem functional traits across five elevations and two soil substrates on the volcanic mountain of Mauna Loa, on the island of Hawaii. Leaf dry mass per area (LMA), a key leaf functional trait, was particularly focused on and analyzed in relation to its underlying components—namely, tissue LMA and trichome LMA (LMA = tissue LMA + trichome LMA). Across populations, tissue LMA increased linearly with elevation while trichome LMA showed unimodal patterns with elevation, which were better correlated with temperature and rainfall, respectively. Substantial phenotypic variations were also found within populations. Interestingly, the variations of tissue LMA were often negatively correlated to trichome LMA within populations, which contrasts with the cross-populations pattern, where a strong positive correlation between tissue LMA and trichome LMA was found. This suggests that phenotypic variations within populations were substantially influenced by local ecological processes. Soil depth (an indicator of local water availability) and tree size (an indicator of colonized timing) modestly explained the within-population variations, implying other local environmental factors and/or random processes are also important in local phenotypic diversity. This study provides an insight about how phenotypic diversity of plant species is maintained from local to landscape levels.