Root competition in soils of different fertility: a paradox resolved?
Grubb, Peter J.
Observations on species richness of herbs and their cover in forests suggest that the impact of root competition is greater on nutrient-poor soils; experiments with root trenching and direct addition of nutrients tend to confirm this view. Paradoxically, experiments with target seedlings of herbs or trees in pots subjected to root competition from grasses (without appreciable shading) suggest that the impact of root competition is greater on soils richer in nutrients. It is proposed that the paradox can be resolved as a result of success in competition for nutrients being dependent on quite different plant properties in soil initially free of roots, and in soil with a relatively constant steady state mass of fine roots. In empty soil, by hypothesis, absolute growth rate and the ratio of root length to total plant mass will be most important, and a greater supply of nutrients in unit volume of soil will enable a plant with an inherent advantage to get further ahead in exploiting the newly available soil volume, and so have a greater inhibitory effect on the other plant. Where the soil has a fairly constant content of fine roots, the single most important factor is the ability of those roots to reduce the mean concentrations of nutrients in the soil solution; that ability is overcome when the rate of input of nutrients from mineralization etc is high, and thus enough nutrients are left over for potential invaders. The resolution of the paradox goes some way to resolving the conflicting views of J. P. Grime and D. Tilman on the relative impact of competition along gradients of nutrient availability. The final section considers the implications of the above argument for plants regenerating in forests, shrublands and grasslands.