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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 …