Carbon Isotope Discrimination of Terrestrial Ecosystems — How Well Do Observed and Modeled Results Match?

Abstract

Terrestrial ecosystems play an important role in the global carbon cycle. This chapter compiled a global dataset of measurements on ecosystem carbon isotope discrimination (Δe) and used this dataset to validate a global terrestrial biosphere model that simulates Δe (BIOME3.5). Measured Δe values (based on ecosystem measurements) averaged 18% globally, while the global modeled estimate (with BIOME3.5) averaged 15.6%. These differences between the measurements and the model may be due to site selection and lack of representative coverage of certain ecosystem types as well as to model parameterization. Field measurements in deserts, C3 and C4 grasslands, and savanna systems are very limited or do not exist yet. The model that does not incorporate information about land use simulates a mean Δe intermediate between those used in other modeling studies. The effects of land use may confound the global Δe signal. The model also shows that the ratio of ecosystem assimilation to canopy conductance is closely related to the ecosystem's Δe except in tropical savannas where roughly equal amounts of C3 and C4 vegetation coexist. Thus, carbon isotope discrimination is a useful tool for investigating the global carbon cycle as it provides information not only on isotopic fractionation during terrestrial CO2 exchange with the atmosphere but also ecophysiological information on the water status of the vegetation.