Evapotranspiration from understory vegetation in an eastern Siberian boreal larch forest

We measured evapotranspiration in an eastern Siberian boreal forest, in which the understory was cowberry and the overstory was larch, during the entire growing seasons of 2005 and 2006. We compared evapotranspiration from the understory vegetation above the forest floor EU with evapotranspiration from the whole ecosystem above the overstory canopy EO. The EU/EO ratio had a seasonal trend with a flat-bottomed U-shape during the growing season (4 May–30 September). High-EU/EO ratios at the beginning and end of the growing season were observed because larch, one of the two sources of EO, was a deciduous tree, while the understory was the evergreen cowberry. The mean daily EU values during the foliated period of larch (1 June–31 August) were 0.8 and 0.9 mm day−1, or 51.4 and 51.8% of EO in 2005 and 2006, respectively. The understory vegetation was one of the most important components of the hydrologic cycle in this forest. A significant amount of EU was caused by plant physiological control, due to the aerodynamic conductance, which was much larger than the surface conductance, leading to a smaller decoupling coefficient. We found that 71% of EU was caused by the vapour pressure deficit above the forest floor.