Intraspecific competition reduces foraging precision
Soil resource availability is spatially heterogeneous at scales both larger than and smaller than individual plant rooting zones. Plants respond to small-scale heterogeneity by proliferation of roots in resource-rich patches. Foraging precision, the degree to which root growth increases in patches, varies among species. Foraging precision is expected to provide a competitive advantage in heterogeneous soils, but foraging precision does not always successfully predict competitive outcomes. This disconnect may arise in part because precision is measured on isolated plants, but its expression may change in the presence of competitors. I measured foraging precision (ln(root length in patch/root length in non-patch)) of four old-field perennials in pots with single plants and in pots with four conspecific individuals, to test whether foraging precision changes in the presence of intraspecific competitors, and whether competition changes the hierarchy of foraging precision.
Competition reduced the size of individual plants (shoot mass and root length in non-patch), but pots with four plants produced greater total shoot mass and root length. Foraging precision, as measured on single plants, differed among the four species (Poa pratensis > Dactylis glomerata > Centaurea jacea > Plantago lanceolata). An increase in plant density led to decreased foraging precision for all four species, but the hierarchy of foraging precision did not change. This result suggests that foraging precision as measured on isolated individuals may be a reliable indicator of foraging precision in competition, at least when the competitors are conspecifics. Further work is needed to determine how foraging precision might change in the presence of heterospecific competitors.