VEGETATION EFFECTS ON THE ISOTOPIC COMPOSITION OF ATMOSPHERIC CO2 AT LOCAL AND REGIONAL SCALES - THEORETICAL ASPECTS AND A COMPARISON BETWEEN RAIN-FOREST IN AMAZONIA AND A BOREAL FOREST IN SIBERIA

In order to understand the factors influencing the, isotopic compositi on of air above and within plant canopies, equations quantifying the e ffects of photosynthesis, respiration and turbulent transport on the i sotopic composition of the surrounding CO2 are developed. These equati ons are then extended to the regional scale, allowing the average isot opic composition of CO2 within the convective boundary layer to be rel ated to the isotopic composition of tropospheric CO2 and to isotopic f ractionations during ecosystem carbon exchange. Equations presented ha ve the potential to be inverted, allowing direct estimation of isotopi c fractionations by vegetation at the local and regional scales. Equat ions allowing the estimation of the extent of refixation of respired C O2 ('recycling') at the regional scale are also presented. Using measu rements of CO2 carbon isotopic composition in conjunction with ecosyst em 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 r ainforest exerts much more influence over the isotopic composition of the surrounding air than does the boreal forest. Due to higher rates o f ecosystem respiration, recycling of respired CO2 is modelled to be m uch greater for tropical rainforest, but values presented here are con siderably 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 di fferences are examined.