The amount of trichomes on the leaves of Metrosideros polymorpha varies enormously, ranging from 0 to ca 150 g/m2 across environmental gradients on the island of Hawaii. Pubescent individuals are abundant in dry areas or on young lava flows, whereas glabrous individuals are abundant in wet areas or on developed soils. To understand the adaptive advantages of pubescent individuals in arid environments, we addressed the following questions: (1) whether leaf trichomes increase the boundary layer resistance to gas diffusion, which in turn reduces the transpiration rate and increases water-use efficiency (WUE); and (2) whether pubescent individuals have other associated leaf and shoot traits that have adaptive significance in arid environments. We made detailed ecophysiological measurements on M. polymorpha in three populations in habitats that varied in aridity. We found a large allocation of leaf mass to trichomes, up to 33 percent at the arid site, but our analyses showed that trichomes had small effects (1–9%) on gas exchange and negligible effects on WUE, suggesting the trichomes may have roles beyond increasing WUE. However, pubescent individuals did have higher Rubisco amount and a lower leaf-to- sapwood area ratio, which are considered adaptive in arid environments. These results suggest that pubescent individuals of M. polymorpha are indeed adapted to arid environments with changes in a suite of traits. The adaptive significance of the enormous variation in amounts of trichomes remains unclear and may be related to functions other than increasing boundary layer resistance.