ausblenden:
Schlagwörter:
Carbon-dioxide; deciduous forest; exchange; CO2; evaporation
Zusammenfassung:
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.
