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In order to understand the factors influencing the isotopic composition of air above and within plant canopies, equations quantifying the effects of photosynthesis, respiration and turbulent transport on the isotopic composition of the surrounding CO2 are developed. These equations are then extended to the regional scale, allowing the average isotopic composition of CO2 within the convective boundary layer to be related to the isotopic composition of tropospheric CO2, and to isotopic fractionations during ecosystem carbon exchange. Equations presented have the potential to be inverted, allowing direct estimation of isotopic fractionations by vegetation at the local and regional scales. Equations allowing the estimation of the extent of refixation of respired CO2 ('recycling') at the regional scale are also presented. Using measurements of CO2 carbon isotopic composition in conjunction with ecosystem flux measurements, the theory is applied to a tropical rain forest in Amazonia and a boreal forest in Siberia. When examined on a ground area basis and over the course of a day it is observed that, by virtue of greater fluxes but similar isotopic fractionations, the tropical rainforest exerts much more influence over the isotopic composition of the surrounding air than does the boreal forest. Due to higher rates of ecosystem respiration, recycling of respired CO2 is modelled to be much greater for tropical rainforest, but values presented here are considerably lower than previously published estimates, the latter being based solely on the relationship between the isotopic composition and concentrations of CO2 within forest canopies. The reasons for these differences are examined.

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