Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 1 2 2004 10.5194/bg-1-133-2004 http://www.biogeosciences.net/1/133/2004/ http://www.biogeosciences.net/1/133/2004/bg-1-133-2004.html http://www.biogeosciences.net/1/133/2004/bg-1-133-2004.pdf 133 146 2004-12-23 Net ecosystem exchange of carbon dioxide and water of far eastern Siberian Larch (<I>Larix cajanderii</I>) on permafrost A. J. Dolman T. C. Maximov E. J. Moors A. P. Maximov J. A. Elbers A. V. Kononov M. J. Waterloo M. K. van der Molen Vrije Universiteit, Dept. Hydrology and Geo-Environmental Sciences, Faculty of Earth and Life Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Institute for Biological Problems of the Cryolithozone (IBPC), Yakutsk, Republic of Sakha (Yakutia), Russia Alterra, PO Box 47, 6700 AA Wageningen, The Netherlands Observations of the net ecosystem exchange of water and CO₂ were made during two seasons in 2000 and 2001 above a Larch forest in Far East Siberia (Yakutsk). The measurements were obtained by eddy correlation. There is a very sharply pronounced growing season of 100 days when the forest is leaved. Maximum half hourly uptake rates are 18 &micro;mol m^-2 s^-1; maximum respiration rates are 5 &micro;mol m^-2 s^-1. Net annual sequestration of carbon was estimated at 160 gCm^-2 in 2001. Applying no correction for low friction velocities added 60 g C m^-2. The net carbon exchange of the forest was extremely sensitive to small changes in weather that may switch the forest easily from a sink to a source, even in summer. June was the month with highest uptake in 2001. <P style="line-height: 20px;"> The average evaporation rate of the forest approached 1.46 mm day^-1 during the growing season, with peak values of 3 mm day^-1 with an estimated annual evaporation of 213 mm, closely approaching the average annual rainfall amount. 2001 was a drier year than 2000 and this is reflected in lower evaporation rates in 2001 than in 2000. <P style="line-height: 20px;"> The surface conductance of the forest shows a marked response to increasing atmospheric humidity deficits. This affects the CO₂ uptake and evaporation in a different manner, with the CO₂ uptake being more affected. There appears to be no change in the relation between surface conductance and net ecosystem uptake normalized by the atmospheric humidity deficit at the monthly time scale. The response to atmospheric humidity deficit is an efficient mechanism to prevent severe water loss during the short intense growing season. The associated cost to the sequestration of carbon may be another explanation for the slow growth of these forests in this environment.