Dense leaf trichomes can directly decrease gas fluxes through increased gas diffusion resistance and indirectly increase gas fluxes through increased leaf temperature due to increased heat diffusion resistance, which may contribute to adaptation to dry and/or low‐temperature conditions. However, it remains unclear whether the leaf‐trichome resistance increases or decreases the gas‐exchange rates through combined direct and indirect effects. We focused on Metrosideros polymorpha, a dominant tree species inhabiting a large range of environmental gradients in the Hawaiian Islands, whose leaves have an enormous variation in trichome thickness on the lower surface. In five elevational sites, we measured leaf morphological and physiological traits including trichome thickness, gas‐exchange characteristics, and leaf temperature. The trichome thickness was largest in the coldest and driest site and thinnest at the wettest site. Leaf temperature was significantly increased with trichome thickness. With biophysical and physiological models, we show that leaf trichomes can increase the daily photosynthesis through increasing leaf temperature only in the cold alpine area. The daily water‐use efficiency can be lower with increasing leaf trichomes at any elevational sites. Therefore, in terms of diffusion resistance, the leaf trichomes of M. polymorpha can contribute to the adaptation to low‐temperature environments but not to dry environments.