ROOT PROLIFERATION AND MARGINAL PATCH VALUE

We describe and test a simple model of optimal root proliferation in a soil consisting of patches of various quality, with quality defined a s the rate of supply of limiting soil resource. Assuming that roots de plete resources in the patch, the model predicts that root biomass wil l be allocated such that the marginal gains from each patch are equili brated. With the further assumption that marginal returns are correlat ed with patch quality, the generally assumed positive relation between patch quality and root proliferation is predicted. This secondary pre diction is tested with the grass Sorghum vulgare grown in containers c onsisting of four patches of variable limiting N-supply. Mean fine roo t biomass showed a strong positive relation between patch quality and biomass consistent with the foraging hypothesis, especially when expre ssed on a per primary root basis. However, the high within-individual variation and the insensitivity of primary roots imply that much of th e root behavior remains unexplained by the model.