The concentration and isotopic composition (13C, 18O) of atmospheric CO2 are key variables used in top-down analysis of the global carbon cycle. Isotopes in particular play a key role in distinguishing ocean from terrestrial sinks and recent studies indicate that on a regional terrestrial basis it should be possible to partition among landscape elements using isotope analyses. Our study will break new ground as a concerted modeling and measurement program focused on carbon and oxygen isotope exchange by terrestrial ecosystems. The proposed modeling and data acquisition efforts will be conducted primarily at the WLEF tall tower in Park Falls, Wisconsin. The objectives of our proposed work can be summarized as (a) to develop and test a model that can be used to define basis functions for the global

inversion of atmospheric measurements of CO2, δ13C of CO2 , and δ18O of CO2 and (b) to monitor the critical data on the isotopic composition of ecosystem components and on the CO2 emerging from ecosystems within the WLEF footprint that are essential for interpreting planetary boundary CO2 observations and also for constraining model predictions. The long-term objectives of the proposed research are the development of coordinated modeling and monitoring efforts to better predict CO2 dynamics between the biosphere and the atmosphere on regional and continental scales.