Abstract
Abstract. Measurements of the stable carbon isotope ratio (δ13C) on annual tree rings offer new opportunities to evaluate mechanisms of variations in photosynthesis and stomatal conductance under changing CO2 and climate conditions, especially in conjunction with process-based biogeochemical model simulations. The isotopic discrimination is indicative of the ratio between the CO2 partial pressure in the intercellular cavities and the atmosphere (ci∕ca) and of the ratio of assimilation to stomatal conductance, termed intrinsic water-use efficiency (iWUE). We performed isotope-enabled simulations over the industrial period with the land biosphere module (CLM4.5) of the Community Earth System Model and the Land Surface Processes and Exchanges (LPX-Bern) dynamic global vegetation model. Results for C3 tree species show good agreement with a global compilation of δ13C measurements on leaves, though modeled 13C discrimination by C3 trees is smaller in arid regions than measured. A compilation of 76 tree-ring records, mainly from Europe, boreal Asia, and western North America, suggests on average small 20th century changes in isotopic discrimination and in ci∕ca and an increase in iWUE of about 27 % since 1900. LPX-Bern results match these century-scale reconstructions, supporting the idea that the physiology of stomata has evolved to optimize trade-offs between carbon gain by assimilation and water loss by transpiration. In contrast, CLM4.5 simulates an increase in discrimination and in turn a change in iWUE that is almost twice as large as that revealed by the tree-ring data. Factorial simulations show that these changes are mainly in response to rising atmospheric CO2. The results suggest that the downregulation of ci∕ca and of photosynthesis by nitrogen limitation is possibly too strong in the standard setup of CLM4.5 or that there may be problems associated with the implementation of conductance, assimilation, and related adjustment processes on long-term environmental changes.
Highlights
Measurements of the stable isotope 13C : 12C ratio (δ13C) on samples from air and natural archives hold information on the carbon cycling in the Earth system
The goal of this study is to present the implementation of δ13C in the land component, Community Land Model version 4.5 (CLM4.5), of Community Earth System Model (CESM) and Land Surface Processes and Exchanges (LPX-Bern) and to discuss the model performance for δ13C on the global scale
The downregulation in ci/ca in CLM4.5 does not reflect a response to drought or worsening climatic conditions for CO2 assimilation. These findings suggest that the main reason for the model–model and model–tree-ring data difference in the trends of discrimination and intrinsic wateruse efficiency (iWUE) is rooted in the parameterization of the photosynthesis–conductance coupling
Summary
Measurements of the stable isotope 13C : 12C ratio (δ13C) on samples from air and natural archives hold information on the carbon cycling in the Earth system. A important area of isotopic research is the clarification of mechanisms controlling δ13C carbon assimilation and water transpiration by land plants (Farquhar et al, 1989; Saurer et al, 2014; Voelker et al, 2016) and their role in the global terrestrial carbon sink (Ciais et al, 2013). The δ13C data representing atmospheric air are used to quantify the global ocean and land carbon sources and sinks (Keeling et al, 1989; Joos and Bruno, 1998; Trudinger et al, 2002; Bauska et al, 2015). Δ13C observations allow identification of the imprint of fossil-fuel carbon in atmospheric air to quantify regional-tolocal-scale land carbon sources and sinks (Torn et al, 2011; Vardag et al, 2016), or to evaluate air–sea transfer velocity parameterizations (Krakauer et al, 2006). Paleo-δ13C data are used to trace water mass, circulation and biological productivity changes on glacial– interglacial timescales and during past abrupt events (Menviel et al, 2012; Schmittner and Somes, 2016), to disentangle processes of past glacial–interglacial carbon-cycle changes (Menviel and Joos, 2012; Schneider et al, 2013; Eggleston et al, 2016), and of ancient climate events (Kennett and Stott, 1991; Korte and Kozur, 2010)
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