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Modeling dynamics of stable carbon isotopic exchange between a boreal forest ecosystem and the atmosphere
- CHEN, BAOZHANG, CHEN, JING M., HUANG, LIN, TANS, PIETER P.
- Global change biology 2006 v.12 no.10 pp. 1842-1867
- air, boreal forests, canopy, carbon, carbon dioxide, diurnal variation, ecosystem respiration, energy, growing season, leaves, mixing, model validation, models, remote sensing, stable isotopes, terrestrial ecosystems, vapor pressure, water use efficiency
- Stable isotopes of CO₂ contain unique information on the biological and physical processes that exchange CO₂ between terrestrial ecosystems and the atmosphere. In this study, we developed an integrated modeling system to simulate dynamics of stable carbon isotope of CO₂, as well as moisture, energy, and momentum, between a boreal forest ecosystem and the atmosphere, as well as their transport/mixing processes through the convective boundary layer (CBL), using remotely sensed surface parameters to characterize the surface heterogeneity. It has the following characteristics: (i) it accounts for the influences of the CBL turbulent mixing and entrainment of the air aloft; (ii) it scales individual leaf-level photosynthetic discrimination up to the whole canopy (Δcanopy) through the separation of sunlit and shaded leaf groups; (iii) it has the capacity to examine the detailed interrelationships among plant water-use efficiency, isotope discrimination, and vapor pressure deficit; and (iv) it has the potential to investigate how an ecosystem discriminates against ¹³C at various time and spatial scales. The monthly mean isotopic signatures of ecosystem respiration (i.e. δ¹³CR) used for isotope flux calculation are retrieved from the nighttime flask data from the intensive campaigns (1998-2000) at 20 m level on Fraserdale tower, and the data from the growing season in 1999 are used for model validation. Both the simulated CO₂ mixing ratio and δ¹³C of CO₂ at the 20 m level agreed with the measurements well in different phases of the growing season. On a diurnal basis, the greatest photosynthetic discrimination at canopy level (i.e. Δcanopy) occurred early morning and late afternoon with a varying range of 10-26[per thousand]. The diurnal variability of Δcanopy was also associated with the phases of growing season and meteorological variables. The annual mean Δcanopy in 1999 was computed to be 19.58[per thousand]. The monthly averages of Δcanopy varied between 18.55[per thousand] and 20.84[per thousand] with a seasonal peak during the middle growing season. Because of the strong opposing influences of respired and photosynthetic fluxes on forest air (both CO₂ and ¹³CO₂) on both the diurnal and seasonal time scales, CO₂ was consistently enriched with the heavier ¹³C isotope (less negative δ¹³C) from July to October and depleted during the remaining months, whereas on a diurnal basis, CO₂ was enriched with the heavier ¹³C in the late afternoon and depleted in early morning. For the year 1999, the model results reveal that the boreal ecosystem in the vicinity of Fraserdale tower was a small sink with net uptake of 29.07 g ¹²C m⁻² yr⁻¹ and 0.34 g ¹³C m⁻² yr⁻¹.