FOREST-ATMOSPHERE CARBON-DIOXIDE EXCHANGE IN EASTERN SIBERIA

We investigated the daily exchange of CO2 between undisturbed Larix gm elinii (Rupr.) Rupr. forest and the atmosphere at a remote Siberian si te during July and August of 1993. Our goal was to measure and partiti on total CO2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage fluxes and then to de termine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO2 uptake of the fo rest ecosystem was extremely low compared to the forests elsewhere, re aching a peak of only similar to 5 mu mol m(-2) s(-1) late in the morn ing. Net ecosystem CO2 uptake increased with increasing photosynthetic ally active photon flux density (PPFD) and decreased as the atmospheri c water vapor saturation deficit (D) increased. Daytime ecosystem CO2 uptake increased immediately after rain and declined sharply after abo ut six days of drought. Ecosystem respiration at night averaged simila r to 2.4 mu mol m-2 s(-1) with about 40% of this coming from the fores t floor (roots and heterotrophs). The relationship between the underst ory eddy flux and soil temperature at 5 cm followed an Arrhenius model , increasing exponentially with temperature (Q(10)similar to 2.3) so t hat on hot summer afternoons the ecosystem became a source of CO2. Tre e canopy CO? exchange was calculated as the difference between above a nd 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 de termined by the non-linear least squares. The model accurately simulat ed the response of the forest to light, saturation deficit and drought . The precision of the model was such that the daily pattern of residu als between modeled and measured forest exchange reproduced the compon ent storage flux. The model and independent leaf-level measurements su ggest that the marginal water cost of plant C gain in Larix gmelinii i s more similar to values from deciduous or desert species than other b oreal 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.