Net ecosystem–atmosphere exchange of CO2 (NEE) was measured in two boreal bogs during the snow‐free periods of 1998, 1999 and 2000. The two sites were located in European Russia (Fyodorovskoye), and in central Siberia (Zotino). Climate at both sites was generally continental but with more extreme summer–winter gradients in temperature at the more eastern site Zotino. The snow‐free period in Fyodorovskoye exceeded the snow‐free period at Zotino by several weeks. Marked seasonal and interannual differences in NEE were observed at both locations, with contrasting rates and patterns. Amongst the most important contrasts were: (1) Ecosystem respiration at a reference soil temperature was higher at Fyodorovskoye than at Zotino. (2) The diurnal amplitude of summer NEE was larger at Fyodorovskoye than at Zotino. (3) There was a modest tendency for maximum 24 h NEE during average rainfall years to be more negative at Zotino (−0.17 versus −0.15 mol m−2 d−1), suggesting a higher productivity during the summer months. (4) Cumulative net uptake of CO2 during the snow‐free period was strongly related to climatic differences between years. In Zotino the interannual variability in climate, and also in the CO2 balance during the snow‐free period, was small. However, at Fyodorovskoye the bog was a significant carbon sink in one season and a substantial source for CO2‐C in the next, which was below‐average dry. Total snow‐free uptake and annual estimates of net CO2‐C uptake are discussed, including associated uncertainties.

We investigated the daily exchange of CO 2 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 CO 2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage¯uxes and then to determine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO 2 uptake of the forest ecosystem was extremely low compared to the forests elsewhere, reaching a peak of only $5 mmol m À2 s À1 late in the morning. Net ecosystem CO 2 uptake increased with increasing photosynthetically active photon ux density (PPFD) and decreased as the atmospheric water vapor saturation de®cit (D) increased. Daytime ecosystem CO 2 uptake increased immediately after rain and declined sharply after about six days of drought. Ecosystem respiration at night averaged $2.4 mmol m À2 s À1 with about 40% of this coming from the forest¯oor (roots and heterotrophs). The relationship between the understory eddy¯ux and soil temperature at 5 cm followed an Arrhenius model, increasing exponentially with temperature (Q 10 $2.3) so that on hot summer afternoons the ecosystem became a source of CO 2 . Tree canopy CO 2 exchange was calculated as the difference between above and below canopy eddy¯ux. Canopy uptake saturated at $6 mmol CO 2 m À2 s À1 for a PPFD above 500 mmol m À2 s À1 and decreased with increasing D. The optimal stomatal control model of Ma Èkela È 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 de®cit and drought. The precision of the model was such that the daily pattern of residuals between modeled and measured forest exchange reproduced the component storagē ux. 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 CO 2 , storing $0.75 g C m À2 d À1. Published by Elsevier Science B.V.

No abstract

Energy and latent heat fluxes lambdaE were measured over ombrotrophic bogs in European Russia (Fyodorovskoye) and in central Siberia (Zotino) using the eddy covariance technique, as part of the EuroSiberian Carbonflux Project. The study covered most of the snowfree periods in 1998, 1999 and 2000; in addition some data were also collected under snow in early spring and late autumn 1999 and 2000. The snowfree period in Europian Russia exceeds the snowfree period in central Siberia by nearly 10 weeks. Marked seasonal and interannual differences in temperatures and precipitation, and hence energy partitioning, were observed at both sites. At both bogs latent heat fluxes (lambdaE) exceeded sensible heat fluxes (H) during most of the snowfree period: maximum lambdaE were between 10 and 12 MJ m(-2) d(-1) while maximum H were between 3 and 5 MJ m(-2) d(-1). There was a tendency towards higher Bowen ratios at Fyodorovskoye. Net radiation was the most influential variable that regulated daily evaporation rates, with no obvious effects due to surface dryness during years with exceptionally dry summers. Total snowfree evaporation at Fyodorovskoye (320 mm) exceeded totals at Zotino (280 mm) by 15%. At the former site, evaporation was equal to or less than precipitation, contrasting the Zotino observations, where summer evaporation was distinctly higher than precipitation. During the entire observation period evaporation rates were less than 50% of their potential rate. These data suggest a strong 'mulching' effect of a rapidly drying peat surface on total evaporation, despite the substantial area of free water surfaces during parts of the year. This effect of surface dryness was also observed as close atmospheric coupling

The exchange of carbon dioxide (CO2) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO2 uptake flux was −3 µmol m−2s−1, whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m−2s−1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* ≤ 0.08 ms−1). On average, 164 mmol CO2 m−2d−1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO2 m−2 (180 g CO2 m−2) released during the 3‐month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of −0.04 yr−1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.