Spatial Distribution of Roots in Sweetgum and Loblolly Pine Monocultures and Relations with Above-Ground Biomass and Soil Nutrients

1. Models of plant-plant interactions often assume that root systems are symmetrically arranged around individual plants and, therefore, below-ground competition intensity is a function of inter-plant distance. To explore this assumption, we measured three-dimensional distributions (horizontal and vertical dimensions) of roots in one plot each of a 3-year-old Sweetgum Sprout (root-stock: 4 years) and 3-year-old Loblolly Pine monoculture. 2. In each plot, 0.5 m grids were set and all roots in each 0.25 m2section (horizontal dimensions) were washed from the soil in four horizons (vertical dimension) down to a total depth of 60 cm. 3. Fine-root biomass summed across the vertical dimension was not correlated with local above-ground biomass, an indication that distance-dependent patterns of root distribution were not strong, at least in the scale of this study. Three-dimensional fine root density was, however, correlated with levels of soil P and K but not soil N or organic matter. 4. Compared to a null model of symmetrical horizontal root distribution, actual root systems of individual plants appeared to avoid inter-plant overlap. 5. Compared with Loblolly Pine, Sweetgum root systems were less concentrated in the upper 10 cm and more symmetric overall. We propose that distance-dependent patterns of below-ground plant-plant interactions will be diminished by the tendency for roots to forage for nutrients and avoid overlap with roots of other plants, and by differences among species in root system behaviour.