Evaluation of 11 terrestrial carbon-nitrogen cycle models against observations from two temperate Free-Air CO2 Enrichment studies

Research output: Contribution to journalArticle


We analysed the responses of 11 ecosystem models to elevated atmospheric [CO2] (eCO(2)) at two temperate forest ecosystems (Duke and Oak Ridge National Laboratory (ORNL) Free-Air CO2 Enrichment (FACE) experiments) to test alternative representations of carbon (C)-nitrogen (N) cycle processes. We decomposed the model responses into component processes affecting the response to eCO(2) and confronted these with observations from the FACE experiments. Most of the models reproduced the observed initial enhancement of net primary production (NPP) at both sites, but none was able to simulate both the sustained 10-yr enhancement at Duke and the declining response at ORNL: models generally showed signs of progressive N limitation as a result of lower than observed plant N uptake. Nonetheless, many models showed qualitative agreement with observed component processes. The results suggest that improved representation of above-ground-below-ground interactions and better constraints on plant stoichiometry are important for a predictive understanding of eCO(2) effects. Improved accuracy of soil organic matter inventories is pivotal to reduce uncertainty in the observed C-N budgets. The two FACE experiments are insufficient to fully constrain terrestrial responses to eCO(2), given the complexity of factors leading to the observed diverging trends, and the consequential inability of the models to explain these trends. Nevertheless, the ecosystem models were able to capture important features of the experiments, lending some support to their projections.


  • Soenke Zaehle
  • Belinda E. Medlyn
  • Martin G. De Kauwe
  • Anthony P. Walker
  • Michael C. Dietze
  • Thomas Hickler
  • Yiqi Luo
  • Ying-Ping Wang
  • Bassil El-Masri
  • Peter Thornton
  • Atul Jain
  • Shusen Wang
  • Ensheng Weng
  • William Parton
  • Colleen M. Iversen
  • Anne Gallet-Budynek
  • Heather McCarthy
  • Adrien Finzi
  • Paul J. Hanson
  • I. Colin Prentice
  • Ram Oren
  • Richard J. Norby
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Physical Geography


  • carbon (C) storage, CO2 fertilization, ecosystem modelling, elevated, CO2, Free-Air CO2, Enrichment (FACE), model evaluation, nitrogen (N), limitation, plant physiology
Original languageEnglish
Pages (from-to)803-822
JournalNew Phytologist
Issue number3
Publication statusPublished - 2014
Publication categoryResearch