Background/Question/Methods Trees absorb carbon at the cost of water as transpiration. Recent studies revealed that trees frequently face the risk of water stress in the vessels and the leaves, even in humid environments. Montane forests are rarely exposed to highly evaporative conditions but we do not have sufficient knowledge about the risk of water stress and stomatal closure as the means to prevent water loss. We installed sap flow sensors on Fagaceae species (Japanese oak and beech) and analyzed the dynamics in transpiration and stomatal conductance to detect their declines unpredictable from the models which do not take into consideration water stress factors.
Results/Conclusions Transpiration rates increased with the environmental forcing (solar radiation and vapor pressure deficit: vpd) but were saturated at relatively low solar radiation and vpd. Calculated stomatal conductance started to decrease at low levels (0.5 kPa). Model simulation with the leaf traits in close relationship to gas exchange showed increase in transpiration rates with saturation at higher vpd than the observed level. Results suggested that leaves had potential to achieve higher gas exchange and keep stomatal conductance high but other factors restricted them even in cool forests. Effects of snow depth and the duration of snow cover were, however, not clear in the comparison among sites of different snow depths.