Nighttime transpiration observed over a larch forest in Hokkaido, Japan

[1] Nighttime transpiration in a larch forest in northern Japan was investigated using concurrent measurements of the energy budget below and above the canopy and sap flow velocities. Upward latent heat flux (lE) above the canopy was observed on ∼20% of nights during the growing season. Nighttime lE was ∼7% of daily totals during the dry season (September–October) and averaged ∼3% over the entire growing season. A relatively low contribution of the understory to lE (<30%) and strong synchronicity between larch sap flow rates and lE on two warm windy nights indicated nighttime lE, occurring when warm dry air intruded from above, was largely due to transpiration from the larch overstory. High canopy conductance on some nights relative to reported maximum cuticular conductance and a strong correlation between canopy conductance and air humidity on these nights indicate the stomata of the larch trees did not close tightly at night.