Net ecosystem exchange of carbondioxide and water of far eastern Siberian Larch (Larix dahurica) on permafrost

International audienceObservations of the net ecosystem exchange of water and CO2were 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 daytime uptake rates are 18 ?mol m-2 s-1 maximum respiration rates are 5 ?mol m-2 s-1. Net annual sequestration of carbon (C) was estimated at 1.7 (±0.5) ton C ha-1 in 2001. 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. The average evaporation rate of the forest approached 1.46mm day-1 during the growing season, with peak values of 3mm day-1 with an estimated annual evaporation of 213mm, closely approaching the average annual rainfall amount. 2001 was a drier year than 2000 and this is reflected in the evaporation rates that show lower evaporation rates in 2001 than in 2000. 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 normalized net ecosystem uptake at the monthly time scale. The response to atmospheric humidity deficits 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