Tragedy of the commons as a result of root competition

1 We develop and test a game-theoretic model for considering the effects of intra- and interplant competition on root proliferation and reproductive y ield. 2 We predict that if space and resources per individual are held constant, plants should produce more roots per individual and less reproductive yield per individual as the number of plants sharing the combined space increase s. 3 We tested the predictions using soybean plants (Glycine max) cultivated i n the glasshouse either as owners or as two individuals sharing twice the s pace and nutrients. 4 Sharing individuals produced 85% more root mass than owners. Owners, howe ver, produced 30% more reproductive yield per plant (dry mass of seeds), as a result of significantly more seed pods (8.70 vs. 7.66), more seeds per p od (1.87 vs. 1.72) and larger seeds (0.205 vs. 0.195 g seed(-1)), than did sharing individuals. 5 Total plant biomass did not differ between owners and sharing individuals , but owners had significantly higher shoot to root ratios, produced signif icantly more seeds per unit root mass, and allocated a significantly higher percentage of total biomass production to seeds. 6 Possession of an evolutionarily stable strategy (ESS) of root competition suggests that different roots and parts of a plant assess and respond to o pportunities in a manner that maximizes the good of the whole plant. Thus. plants may be more sophisticated and share more in common with animals in t heir non-cognitive behaviours than previously thought. A plant operating as a co-ordinated whole should, all else being equal, first proliferate roots in unoccupied soil, then in soil occupied by a conspecific competitor, and lastly in soil already occupied by its own roots.