Modeling carbon dynamics in two adjacent spruce forests with different soil conditions in Russia

International audienceNet ecosystem carbon exchange (NEE) were measured with eddy covariance method for two adjacent forests located at the southern boundary of European taiga in Russia in 1999?2004. The two spruce forests shared similar vegetation composition but differed in soil conditions. The wet spruce forest (WSF) possessed a thick peat layer (60 cm) with a high water table seasonally close to or above the soil surface. The dry spruce forest (DSF) had a relatively thin organic layer (5 cm) with a deep water table (>60 cm). The measured NEE fluxes (2000 and ?1440 kg C ha?1 yr?1 for WSF and DSF, respectively) indicated that WSF was a source while DSF a sink of atmospheric carbon dioxide during the experimental years. A process-based model, Forest-DNDC, was employed in the study to interpret the observations. The modeled NEE fluxes were 1800 and ?2200 kg C ha?1 yr?1 for WSF and DSF, respectively, which were comparable with the observations. The modeled data indicated that WSF and DSF had similar rates of photosynthesis and plant autotrophic respiration but differed in soil heterotrophic respiration. The simulations resulted in a hypothesis that the water table fluctuation at WSF could play a key role in determining the negative C balance in the ecosystem. A sensitivity test was conducted by running Forest-DNDC with varied water table scenarios for WSF. The results proved that the NEE fluxes from WSF were highly sensitive to the water table depth. When the water table dropped, the length of flooding season became shorter and more organic matter in the soil profile suffered from rapid decomposition that converted the ecosystem into a source atmospheric C. The conclusion from this modeling study could be applicable for a wide range of wetland and forest ecosystems that have accumulated soil organic C while face hydrological changes under certain climatic or land-use change scenarios

Carbon balance of a southern taiga spruce stand in European Russia

We present results from nearly three years of net ecosystem flux. measurements above a boreal spruce stand growing in European Russia. Fluxes were measured by eddy covariance using conventional techniques. In all years examined (1998-2000), the forest was a significant source of carbon to the atmosphere. However, the magnitude of this inferred source depended upon assumptions regarding the degree of "flux loss" under conditions of low turbulence, such as typically occur at night. When corrections were not made, the forest was calculated to be only a modest source of C to the atmosphere (3-5 mol C m(-2) yr(-1)). However, when the corrections were included, the apparent source was much larger (20-30 mol C m(-2) yr(-1)). Using a simple model to describe the temperature dependencies of ecosystem respiration on air and soil temperatures, about 80% of the night-time flux was inferred to be from soil respiration, with the remainder being attributable to foliage, branches and holes. We used reasonable assumptions to estimate the rate of ecosystem respiration during the day, allowing an estimation of canopy photosynthetic rates and hence the annual Gross Primary Productivity of the ecosystem. For the two full years examined (1999 and 2000), this was estimated at 122 and 130 mol C m(-2) yr(-1), respectively. This value is similar to estimates for boreal forests in Scandinavia, but substantially higher than has been reported for Canadian or Siberian boreal forests. There was a clear tendency for canopy photosynthetic rates to increase with both light and temperature, but the slope of the temperature response of photosynthesis was less steep that that of ecosystem respiration. Thus, on most warm days in summer the forest was a substantial source of carbon to the atmosphere; with the forest usually being a net sink only on high insolation days where the average daily air temperatures were below about 18 degreesC. These data, along with other studies on the current balance of boreal ecosystems, suggests that at the current time many boreal forests might be releasing substantial amounts of carbon dioxide to the atmosphere. This observed temperature sensitivity of this ecosystem suggests that this might be a consequence of substantially higher than average temperatures over recent years

Inter-annual and seasonal variations of energy and water vapour fluxes above a Pinus sylvestris forest in the Siberian middle taiga

Long-term eddy covariance measurements of energy and water fluxes and associated climatic parameters were carried out above a Scots pine (Pinus sylvestris) forest in the middle taiga zone of Central Siberia. Data from June 1998 through October 2000 are presented. With the exception of winter 1998/1999, data collection over this period were more or less continuous. A distinct seasonality in surface energy exchange characteristics was observed in all years. In early spring in the absence of physiological activity by the vegetation, about 80% of the net radiation was partitioned for sensible heat, resulting in Bowen ratios, beta, as high as 8. In the 1-2 wk period associated with onset of photosynthesis in spring, evaporation rates increased rapidly and beta rapidly dropped. However, even during summer months, sensible heat fluxes typically exceeded latent heat fluxes and beta remained above 2.0. Observed daily evaporation rates varied between 0.5-1.0 mm d(-1) in spring and autumn and 1.5-2 mm d(-1) in midsummer. The overall average for the three growing seasons examined was 1.25 mm d(-1). Precipitation was on average 230 mm for the growing period, with evaporation over the same time being about 190 mm for both 1999 and 2000. This represented only about 35% of the equilibrium evaporation rate. There was typically a positive hydrological balance of 40 mm for the growing season as a whole. However, in all three years examined, evaporation exceeded precipitation totals by 20-40 mm in at least one calendar month during summer. During the growing season, daily averaged surface conductances varied between 0.15 and 0.20 mol m(-2) s(-1) (3-4.5 mm s(-1)) in dry or cool months and 0.30- 0.35 mol m(-2) s(-1) (6.5-8 mm s(-1)) in moist and warm months. Despite a negative hydrological balance during midsummer, there was little evidence for reduced canopy conductances in response to soil water deficits. This may have been the consequence of roots accessing water from within or just above a perched water table, located at about 2 m depth

Comparative ecosystem-atmosphere exchange of energy and mass in a European Russian and a central Siberian bog I. Interseasonal and interannual variability of energy and latent heat fluxes during the snowfree period

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