Root competition can cause a decline in diversity with increased productivity

Summary 1 Plant community theory often invokes competition to explain why species diversity declines as productivity increases. Competition for all resources might become more intense and lead to greater competitive exclusion or, alternatively, competition for light only could become more intense and exclude poor light competitors. 2 To test these hypotheses, we constructed communities of seven old‐field species using combined monocultures. Constructs experienced no interspecific competition, only shoot competition or only root competition, with and without fertilizer. Diversity in these limited interaction communities was compared to diversity in unfertilized and fertilized mixtures of the seven species. 3 Diversity decreased with fertilization in mixtures and in communities with only root competition. Shoot competition had small effects on the community and did not contribute to changes in diversity with fertilization. 4 Root competition may strongly impact plant community structure in unproductive communities where light never becomes limiting, or under non‐equilibrium conditions following human disturbances.