Metrosideros polymorpha, a dominant tree species on Hawaiian Islands, shows an enormous variation in the amount of leaf trichomes. Pubescent leaves are more abundant in drought and/or low-temperature conditions, whereas glabrous leaves are common in less-stressful conditions. In general, the adaptive significances of leaf trichomes vary depending on plant species and their habitats; therefore, multiple functions of leaf trichomes should be considered. We evaluated three possible functions of leaf trichomes: (i) increasing diffusion resistance, (ii) prolonging leaf wetness, and (iii) defense against gall-making insects. We measured various morphological and physiological characteristics of leaves at various locations on the island of Hawaii covering wide environmental gradients. Leaf trichomes could slightly increase the CO₂ assimilation through increasing leaf temperature in the dry alpine area only; however, in turn, leaf trichomes decreased water-use efficiency at all elevations due to the increased water loss through increased leaf temperature (i). Leaf trichomes increased the duration of leaf wetness and promoted foliar water uptake, which could prolong the period of photosynthesis (ii). The amount of leaf trichomes was negatively associated with the number of large galls that increased water loss; thus, leaf trichomes could contribute to the avoidance of extra water stress by impeding gall formations (iii). Our study demonstrated that the leaf trichomes of M. polymorpha have multiple functions depending on environmental conditions, and such multi-functionality can be a driver of the large variation in leaf trichomes.