Traits data target for calibration: ci:ca

[stineb]

Add ci:ca (leaf internal to ambient CO2 concentration) as a traits target data.
We can use this dataset: https://github.com/wcornwell/leaf13C

• Filter ps.type == "C3"
• Elevation should be obtained additionally
• Other forcing data to be prepared as workflow for Vcmax traits data
• In rsofun, introduce a non-structural C pool (NSC) with prescribed decay constant. Add GPP to NSC and treat isotopic signature explicitly (calculating d13C from simulated ci:ca, using Farquhar et al. (1989), see also Wang et al., 2017). NSC pool and d13C implementation @stineb
• Compare to Cornwell data .

References

Farquhar, G. D., Ehleringer, J. R. & Hubick, K. T. Carbon isotope discrimination and photosynthesis. Ann. Rev. Plant Biol. 40, 503–537 (1989).
Cornwell, William K., Ian J. Wright, Joel Turner, Vincent Maire, Margaret M. Barbour, Lucas A. Cernusak, Todd Dawson, et al. “Climate and Soils Together Regulate Photosynthetic Carbon Isotope Discrimination within C3 Plants Worldwide.” Global Ecology and Biogeography 27, no. 9 (2018): 1056–67. https://doi.org/10.1111/geb.12764.
Wang, Han, I. Colin Prentice, Trevor F. Keenan, Tyler W. Davis, Ian J. Wright, William K. Cornwell, Bradley J. Evans, and Changhui Peng. “Towards a Universal Model for Carbon Dioxide Uptake by Plants.” Nature Plants 3, no. 9 (September 2017): 734–41. https://doi.org/10.1038/s41477-017-0006-8.

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