Functional responses of plants to elevated atmospheric CO₂ : do photosynthetic and productivity data from FACE experiments support early predictions?

Results from 16 free-air CO₂ enrichment (FACE) sites representing four different global vegetation types indicate that only some early predictions of the effects of increasing CO₂ concentration (elevated [CO₂]) on plant and ecosystem processes are well supported. Predictions for leaf CO₂ assimilation (Anet) generally fit our understanding of limitations to photosynthesis, and the FACE experiments indicate concurrent enhancement of photosynthesis and of partial downregulation. In addition, most herbaceous species had reduced leaf nitrogen (N)-content under elevated [CO₂] and thus only a modest enhancement of Anet, whereas most woody species had little change in leaf N with elevated [CO₂] but a larger enhancement of Anet. Early predictions for primary production are more mixed. Predictions that enhancement of productivity would be greater in drier ecosystems or in drier years has only limited support. Furthermore, differences in productivity enhancements among six plant functional types were not significant. By contrast, increases in productivity enhancements with increased N availability are well supported by the FACE results. Thus, neither a resource-based conceptual model nor a plant functional type conceptual model is exclusively supported by FACE results, but rather both species identity and resource availability are important factors influencing the response of ecosystems to elevated [CO₂]