Conclusion
To our knowledge, this is a first study to quantify the combined direct
and indirect effects of leafâtrichome 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.