Contribution of transpiration to the atmospheric moisture in eastern Siberia estimated with isotopic composition of water vapour Akihiro Ueta, 1† Atsuko Sugimoto, 1,2 * Yoshihiro Iijima, 3 Hironori Yabuki 3 and Trofim C. Maximov 4,5 1 Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan 2 Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan 3 Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan 4 Institute for Biological Problems of Cryolithozone, Siberian Branch of Russian Academy of Science, Yakutsk, Russia 5 International Center for BioGeoScience Educational and Scientific Training, North-Eastern Federal University, Yakutsk, Russia ABSTRACT Isotopic composition of atmospheric water vapour (d 18 O V ) was observed at a larch forest near Yakutsk in eastern Siberia during the late summer periods of 2006, 2007 and 2008. The d 18 O V [and deuterium excess (d-excess)] values observed in 2006 and 2008 positively (and negatively) correlated with mixing ratio of atmospheric water vapour, whereas, in 2007 when soil was extremely wet and resulted in limitation of plant transpiration, neither correlation was found between mixing ratio and d 18 O V nor d-excess. Observed results were also compared with components of atmospheric water balance calculated for a 500  500 km region; however, neither specific relationship between d 18 O V and horizontal advection (direction) nor evapotranspiration was observed. On the other hand, obviously low d 18 O V and high d-excess values were found with low mixing ratio after removal of water vapour from the atmosphere because of the process of rainout in 2006 and 2008. Assuming the d 18 O V under this condition to be a background, and also assuming the d 18 O of sap water in larch trees as transpired water vapour, contribution of transpiration to the atmospheric water vapour was calculated. Fraction of transpired water vapour to the atmospheric water vapour was nearly 08 in maximum when plant transpiration was active under warm condition. Our isotope data confirm the importance of recycling of water through transpiration of forest plants in taiga to the hydrologic cycle in eastern Siberia. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS eastern Siberia; taiga; stable isotope; atmospheric water vapour; transpiration Received 28 April 2013; Accepted 3 May 2013 INTRODUCTION Boreal forest covers about 25% of the total forest area in the world, and 76% of the forest exists in Russia (FAO, 2005). Because of its vast area, the forest should play an important role in material cycling, and it can greatly influence the cycles on global scale. It has been pointed out that terrestrial ecosystem in high latitude region is vulnerable (IPCC, 2007; McGuire et al., 2009), because not only temperature increase but also changes in hydrological processes of snow and frozen ground may affect the system. Taiga in eastern Siberian is characterized by domination of deciduous conifer (larch), continuous permafrost and continental arid climate with sparse annual precipitation ranging from 200 to 300 mm. Under such severe climate condition, taiga forest ecosystem has been maintained on the permafrost. Year to year variation in soil water storage is pronounced because of the existence of permafrost (Sugimoto et al., 2003), and the ice meltwater in the frozen ground works as an important source of water for plant during drought (Sugimoto et al., 2002). In addition, summer precipitation also shows large year to year variation (Ohta et al., 2008). However, hydrological processes described earlier results in stable evapotranspi- ration despite the large variation in precipitation (Kelliher et al., 1997; Sugimoto et al., 2003; Hamada et al., 2004; Ohta et al., 2008; Iida et al., 2009). Recently, extreme events such as severe drought, heavy rainfall and snowfall and extreme wet condition are frequently observed (Iijima et al., 2010), and especially, extreme wet condition affected vegetation (Iwasaki et al., 2010). Changes in vegetation and plant transpiration *Correspondence to: Atsuko Sugimoto, Faculty of Environmental Earth Science, Hokkaido University, Kita 10 Nishi 5, Kita-ku, Sapporo 060-0810, Japan. E-mail: atsukos@ees.hokudai.ac.jp † Current Address: Yokohama Joint Research Center, Isotope Research Institute, Yokohama, Japan ECOHYDROLOGY Ecohydrol. 7, 197–208 (2014) Published online 20 June 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1403 Copyright © 2013 John Wiley & Sons, Ltd.