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

Stable isotopes of CO 2 contain unique information on the biological and physical processes that exchange CO 2 between terrestrial ecosystems and the atmosphere. In this study, we developed an integrated modeling system to simulate dynamics of stable carbon isotope of CO 2 , 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 (Δ canopy ) 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 13 C at various time and spatial scales. The monthly mean isotopic signatures of ecosystem respiration (i.e. δ 13 C R ) 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 CO 2 mixing ratio and δ 13 C of CO 2 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. Δ canopy ) occurred early morning and late afternoon with a varying range of 10–26‰. The diurnal variability of Δ canopy was also associated with the phases of growing season and meteorological variables. The annual mean Δ canopy in 1999 was computed to be 19.58‰. The monthly averages of Δ canopy varied between 18.55‰ and 20.84‰ with a seasonal peak during the middle growing season. Because of the strong opposing influences of respired and photosynthetic fluxes on forest air (both CO 2 and 13 CO 2 ) on both the diurnal and seasonal time scales, CO 2 was consistently enriched with the heavier 13 C isotope (less negative δ 13 C) from July to October and depleted during the remaining months, whereas on a diurnal basis, CO 2 was enriched with the heavier 13 C 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  12 C m −2  yr −1 and 0.34 g  13 C m −2  yr −1 .