ausblenden:
Schlagwörter:
Plant-growth responses; soil nutrient status; atmospheric CO2;
boreal forest; northern-hemisphere; eddy covariance; climatic-
change; dioxide uptake; rain-forest; snow cover
Zusammenfassung:
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.
