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We investigated the daily exchange of CO2 between undisturbed Larix gmelinii (Rupr.) Rupr. forest and the atmosphere at a remote Siberian site during July and August of 1993. Our goal was to measure and partition total CO2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage fluxes and then to determine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO2 uptake of the forest ecosystem was extremely low compared to the forests elsewhere, reaching a peak of only similar to 5 mu mol m(-2) s(-1) late in the morning. Net ecosystem CO2 uptake increased with increasing photosynthetically active photon flux density (PPFD) and decreased as the atmospheric water vapor saturation deficit (D) increased. Daytime ecosystem CO2 uptake increased immediately after rain and declined sharply after about six days of drought. Ecosystem respiration at night averaged similar to 2.4 mu mol m-2 s(-1) with about 40% of this coming from the forest floor (roots and heterotrophs). The relationship between the understory eddy flux and soil temperature at 5 cm followed an Arrhenius model, increasing exponentially with temperature (Q(10)similar to 2.3) so that on hot summer afternoons the ecosystem became a source of CO2. Tree canopy CO? exchange was calculated as the difference between above and below canopy eddy flux. Canopy uptake saturated at similar to 6 mu mol CO2 m-2 s(-1) for a PPFD above 500 mu mol m-2 s(-1) and decreased with increasing D. The optimal stomatal control model of Makela et al. (1996) was used as a ’big leaf’ canopy model with parameter values determined by the non-linear least squares. The model accurately simulated the response of the forest to light, saturation deficit and drought. The precision of the model was such that the daily pattern of residuals between modeled and measured forest exchange reproduced the component storage flux. The model and independent leaf-level measurements suggest that the marginal water cost of plant C gain in Larix gmelinii is more similar to values from deciduous or desert species than other boreal forests. During the middle of the summer, the L. gmelinii forest ecosystem is generally a net sink for CO2, storing similar to 0.75 g C m(-2) d(-1). Published by Elsevier Science B.V.