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

Abstract

Summary: In this review I am drawing attention to some constraints and biases in CO 2 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 CO 2–‘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 CO 2 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 (CO 2 enrichment technology). Contents Summary 393 I. A traditionally scarce resource becomes abundant 394 II. Photosynthesis is not saturated at current CO 2 concentrations 395 III. The fate of extra carbon 396 IV. Co drivers of plant growth responses to elevated CO 2 397 V. Plant CO 2 responses as a function of time 399 VI. Plant CO 2 responses per unit land area, a matter of definition 401 VII. CO 2 effects on biomass carbon stores depend on tree demography 402 VIII. Biomass responses to elevated CO 2 in steady state and expanding systems 403 IX. Conclusions 405 Acknowledgements 406 References 406