Plant CO2 responses: an issue of definition, time and resource supply


Author for correspondence: Christian Körner Tel: +41612673510 Fax: +41612673504 Email:



  •  Summary 393

  • I. A traditionally scarce resource becomes abundant 394
  • II. Photosynthesis is not saturated at current CO2 concentrations  395
  • III. The fate of extra carbon 396
  • IV. Co drivers of plant growth responses to elevated CO2 397
  • V. Plant CO2 responses as a function of time 399
  • VI. Plant CO2 responses per unit land area, a matter of definition  401
  • VII. CO2 effects on biomass carbon stores depend on tree demography  402
  • VIII. Biomass responses to elevated CO2 in steady state  and expanding systems  403
  • IX. Conclusions 405
  •  Acknowledgements 406

  •  References 406


In this review I am drawing attention to some constraints and biases in CO2 enrichment experiments and the analysis of data in the literature. Conclusions drawn from experimental works differ when the data are grouped in a way such that the relative frequency of test conditions does not determine the emerging trends, for instance unrealistically strong CO2–‘fertilization’ effects, which are in conflict with some basic ecological principles. I suggest separating three test conditions: uncoupled systems (plants not depending in a natural nutrient cycle) (I); expanding systems, in which plants are given ample space and time to explore otherwise limited resources (II); and fully coupled systems in which the natural nutrient cycling governs growth at steady-state leaf area index (LAI) and fine root renewal (III). Data for 10 type III experiments yield rather moderate effects of elevated CO2 on plant biomass production, if any. In steady-state grassland, the effects are water-related; in closed tree stands, initial effects decline rapidly with time. Plant–soil coupling (soil conditions) deserves far greater attention than plant–atmosphere coupling (CO2 enrichment technology).