Roles of lower-side leaf trichomes in diffusion resistance and gas-exchange characteristics across environmental gradients in Metrosideros polymorpha.

This article investigates the combined direct and indirect effects of lower-side non-glandular leaf trichomes on gas exchange, photosynthesis, and water-use efficiency (WUE) in Metrosideros polymorpha Gaud., a dominant Hawaiian tree species exhibiting wide variation in trichome mass across elevational and environmental gradients. Field measurements and simulation analyses across five elevation sites on Mauna Loa demonstrated that thicker trichome layers, especially in cold and dry alpine environments, increase leaf temperature by raising heat diffusion resistance, which can enhance photosynthetic rates under low-temperature conditions. However, the increased leaf temperature also leads to higher transpiration rates and consistently reduces daily WUE across all sites. The study concludes that while lower-side leaf trichomes in M. polymorpha may benefit carbon assimilation in cold alpine habitats, they do not improve drought tolerance via diffusion resistance, with their effects influenced by factors such as leaf size, stomatal behavior, and strong irradiance typical of Hawaiian environments.