Transpiration and plant hydraulics of Abies veitchii under fluctuating environmental conditions in cool montane forest

Abstract

In subalpine fir wave forests, strips of dead and weakened trees occur perpendicular to the slope next to strips of healthy trees. To reveal the transpiration by weakened Abies veitchii trees exposed to increased atmospheric evaporative demand, we investigated the ecophysiological traits closely related to the growth and transpiration, comparing them with those of the healthy trees and saplings in the fir wave of Mt. Shimagare in central Japan. The transpiration rate (E) was investigated using sap flux sensors to measure heat pulse velocity and compared with the surrogate for the needle water demand, which was computed using a multilayered gas and energy transfer model (modeled E, E_mod). Weakened trees exhibited smaller diameter growth and narrower sapwood than healthy trees, as well as lower heat pulse velocity compared with healthy saplings. However, needle-level traits did not differ significantly between weakened and healthy trees. Needle water potential at midday was as negative as the needle turgor loss point, and the measured heat pulse velocity increased linearly with E_mod but leveled off above a certain E_mod value in weakened trees and healthy saplings, suggesting that trees restricted E to balance the needle water budget. Heat pulse velocity of weakened trees leveled off at E_mod lower than that of healthy saplings, probably due to lower capacity for water supply to the needles. Restriction of E would occur less frequently but be necessary for both weakened and healthy A. veitchii on Mt. Shimagare to avoid hydraulic failure, sacrificing photosynthetic carbon assimilation.