Summary 1.We review the effects of root and shoot competition on plant biomass by meta-analysis of 38 published studies, each reporting on the factorial effects of root-competition and shoot-competition on herbaceous plants.2. There were significant differences in the overall effects of root, shoot and full competition. Root competition generally resulted in larger biomass reduction than shoot competition, particularly among the smaller of the two competitors. An interaction between root and shoot competition was observed in some experiments but was generally not significant. 3. While root competition was generally stronger than shoot competition at lower nutrient levels, there was no overall difference at higher nutrient levels due to much lower levels of root competition. In contrast, the overall effect of full competition increased considerably with nutrient level. 4. Root competition was generally more important when the neighbour was a grass as compared with a herb or a legume, and when domesticated plants competed with wild neighbours, suggesting that wild genotypes are stronger competitors for below-ground resources than are crop plants. 5. Effects of competition were generally stronger in experiments with additive designs than in those with substitutive designs. Also, experiments using single target individuals showed stronger effects of competition, root competition in particular, than did experiments using groups targets. 6. Synthesis. Despite large variation among experiments, some general patterns were supported: effects of root competition are generally stronger than shoot competition, particularly (i) for smaller competitors, (ii) at low fertility levels, (iii) when the competitor is a grass rather than an herb, (iv) when the competitor is a wild rather than a domesticated species, and (v) in additive design experiments. The effects of root and shoot competition appear to be additive under many conditions. Whereas root competition may often be the primary limitation on mean plant performance, shoot competition will influence the variation around this mean and may determine which individuals or species dominate.

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