Root and Shoot Competition Intensity Along a Soil Depth Gradient

1 Only a handful of studies have measured natural gradients of competition intensity, and only a fraction of these have separated above and below-ground competition. Yet such measurements are essential to distinguish among competing theories and models of plant competition. 2 We therefore examined the intensity of total, root, and shoot competition along a gradient of soil depth in a herbaceous vegetation system. Above-ground biomass was strongly correlated to soil depth $(r^2 = 0.65, P < 0.001)$ indicating that as soil resources increased, light decreased along the study gradient. Phytometers (plant indicators) were grown without neighbours, with neighbours' roots only, and with neighbours' roots and shoots. The final biomass of each (after one growing season) was compared in order to determine competition intensity. 3 Overall, the intensities of total and root competition were significantly greater than zero $(P < 0.05)$; that of shoot competition was not. This suggests that competition in this system was primarily below ground. 4 Competition intensity did not vary significantly along the soil depth gradient. 5 Results from this and other field studies of competition have produced apparently contradictory results. We suggest a graphical model that relates the various effects of competition and mutualism to biomass levels which may reconcile apparently contradictory field studies.