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Carbon-Nitrogen Cycle Models of Forest Responses to Elevated CO2
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 Phytologist 202: 803-822.

What was done
The authors employed eleven different ecosystem models to investigate the effects of nitrogen (N) availability on the effects of atmospheric CO2 enrichment on forest productivity and carbon (C) storage at two forest sites with similar temperate climates, comparable levels of nitrogen deposition, but contrasting vegetation types: the evergreen, needle-leaved Duke Forest studied by McCarthy et al. (2010) and the deciduous, broad-leaved Oak Ridge National Laboratory or ORNL Forest studied by Norby et al. (2010), both of which were home to major free-air CO2 enrichment (FACE) experiments.

What was learned
In the words of Zaehle et al., "most of the models reproduced the observed initial enhancement of net primary production (NPP) at both sites," but they say that none "was able to simulate both the sustained 10-year enhancement at Duke and the declining response at ORNL." In addition, they report that the models "generally showed signs of progressive N limitation." Yet the Duke Forest trees continued their CO2-induced increase in NPP over the entire time of the study that was conducted there, without any problem caused by lack of available N.

What it means
Noting that "most models failed to simulate the sustained NPP enhancement at the Duke FACE site," the 23 researchers - hailing from 7 different countries - conclude that "this tendency to underestimate the net transfer of N from soils to vegetation under elevated CO2 at Duke calls for a better representation of below-ground processes, in particular root allocation and microbial responses to enhanced rhizodeposition." We could not agree more; for something has clearly overpowered the Progressive Nitrogen Limitation Hypothesis in this and many other such studies, as is indicated in the reviews of numerous other analyses of the hypothesis that we have archived under this heading in our Subject Index.

McCarthy, H,.R., Oren, R., Johnsen, K.H., Gallet-Budynek, A., Pritchard, S.G., Cook, C.W., LaDeau, S.L., Jackson, R.B. and Finzi, A.C. 2010. Re-assessment of plant carbon dynamics at the Duke free-air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development. New Phytologist 185: 514-528.

Norby, R.J., Warren, J.M., Iversen, C.M., Medlyn, B.E. and McMurtrie, R.E. 2010. CO2 enhancement of forest productivity constrained by limited nitrogen availability. Proceedings of the National Academy of Sciences, USA 107: 19,368-19,373.

Reviewed 30 July 2014