Are competitive interactions influenced by spatial nutrient heterogeneity and root foraging behavior?

Nutrient heterogeneity, root foraging and competitive interactions were investigated for six species native to south-eastern USA. Monocultures, two- and six-species garden plots were fertilized to create spatially homogeneous or heterogeneous nutrient conditions. After 3.5 months, root proliferation in rich patches (precision), mean above-ground biomass per plant (scale) and influence of nutrient treatment on total plot biomass (sensitivity) in monocultures were measured. Competition (above-ground biomass) was assessed in two- and six-species plots. In monoculture plots, two species were relatively precise foragers, but no species showed significant sensitivity to nutrient treatment. Correlations between precision, scale and sensitivity were weak (-0.40 < r < 0.17), which contrasts with previous work showing a scale-precision trade-off. In two-species plots, competition was influenced by soil heterogeneity in two of six cases tested (ANOVA, P < 0.05), and precise foragers grew larger in heterogeneous than in homogeneous conditions. In six-species plots, nutrient treatment had no influence on growth or competition. In our study system, heterogeneity effects on competition are context specific, generally weak and potentially mediated by the degree of root foraging precision.