Conclusion

To our knowledge, this is a first study to quantify the combined direct and indirect effects of leaf‐trichome resistance on the gas-exchange rates across a wide range of environmental gradients. The large amount of leaf trichomes in M. polymorpha can increase diffusion resistance on the leaf surface and contribute to higher carbon gain through increasing leaf temperature in alpine conditions. In addition, this trichome effects are enhanced by smaller leaves which are typically found in alpine plants. On the other hand, leaf trichomes cannot increase the carbon gain in other lower sites partly because the photosynthetic rates are not sensitive to small increase in leaf temperature in warmer areas.
Leaf trichomes can have adverse effects on WUE at all study sites, primarily because leaf trichomes can greatly increase leaf temperature in M. polymorpha (strong isohydric species inhabiting strong-irradiance conditions) and higher leaf temperature results into increase in the transpiration rates more than increase in the photosynthetic rates. Our sensitivity analyses show that leaf trichomes could enhance WUE in more anisohydric leaves inhabiting low-temperature conditions or in leaves inhabiting low-irradiance conditions, but these conditions are not typical conditions where dense pubescent plants are found. Therefore, we do not support the hypothesis that leaf trichomes contribute to drought tolerance through increasing diffusion resistance.