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Modeling dynamics of stable carbon isotopic exchange between a boreal forest ecosystem and the atmosphere

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.