Trichomes on the lower leaf surface are common in the wide range of plant taxa; however, the ecological significance of such leaf trichomes is still unclear. In theory, dense leaf trichomes can suppress gas fluxes through increased gas-diffusion resistance and indirectly enhance gas fluxes through increased leaf temperature due to increased heat diffusion resistance. To examine the effect of leaf trichomes on the gas-exchange rates, we built a model and conducted simulation analyses using various combinations of leaf morphological and physiological characteristics and environmental conditions. Our analyses demonstrate that leaf trichomes can enhance the photosynthetic rate only under low-temperature conditions. This positive effect of leaf trichomes tends to be larger for smaller leaves. Leaf trichomes do not increase water-use efficiency of gas exchanges especially for leaves with strong stomatal regulation or under strong-irradiance conditions. Therefore, in terms of diffusion resistance, leaf trichomes on the lower surface can contribute to carbon gain under low-temperature conditions but not contribute to water saving under most environmental conditions.