Please find below some relevant picture of his slides and, just after the paper(s) he cited. Probably reading those papers can be a starting point to understand.
I. C. Prentice, X. Liang, B. E. Medlyn , and Y.-P. Wang, Reliable, robust and realistic: the three R’s of next-generation land-surface modelling, ACP, 2015Causes and implications of persistent atmospheric carbon dioxide biases in Earth System Models, J. Geophys. Res. Biogeosci., 119, 141–162, doi:10.1002/2013JG002381.
H. D. Graven, R. F. Keeling, S. C. Pipe, P. K. Patra, B. B. Stephens, S. C. Wofsy, L. R. Welp, C. Sweeney, P. P. Tans, J. J. Kelley, B. C. Daube, E. A. Kort, G. W. Santoni, J. D. Bent Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960, Science 2013
Thomas, R. T., et al. (2016), Increased light-use efficiency in northern terrestrial ecosystems indicated by CO2 and greening observations, Geophys. Res. Lett., 43, 11,339–11,349, doi:10.1002/2016GL070710.
Wenzel, S., P. M. Cox, V. Eyring, andP. Friedlingstein (2014), Emergent constraints on climate-carbon cycle feedbacks in the CMIP5 Earth system models, J. Geophys. Res. Biogeosci., 119,794–807, doi:10.1002/2013JG002591
Ainsworth EA1, Long SP, What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2., New Phytol. 2005 Feb;165(2):351-71.
Ning Dong, Iain Colin Prentice, Bradley J. Evans , Stefan Caddy-Retalic, Andrew J. Lowe, and Ian J. Wright, Leaf nitrogen from first principles: field evidence for adaptive variation with climate, Biogeosciences, 14, 481–495, 2017 doi:10.5194/bg-14-481-2017
Zaehle, S., Medlyn, B. E., De Kauwe, M. G., Walker, A. P., Dietze, M. C., Hickler, T., Luo, Y., Wang, Y.-P., El-Masri, B., Thornton, P., Jain, A., Wang, S., Warlind, D., Weng, E., Parton, W., Iversen, C. M., Gallet-Budynek, A., McCarthy, H., Finzi, A., Hanson, P. J., Prentice, I. C., Oren, R. and Norby, R. J. (2014), Evaluation of 11 terrestrial carbon–nitrogen cycle models against observations from two temperate Free-Air CO2 Enrichment studies. New Phytol, 202: 803–822. doi:10.1111/nph.12697
César Terrer, Sara Vicca,Bruce A. Hungate,Richard P. Phillips,I. Colin Prentice, Mycorrhizal association as a primary control of the CO2 fertilization effect, Science 2016