ROOT AND SHOOT COMPETITION INTENSITY ALONG A SOIL DEPTH GRADIENT

1 Only a handful of studies have measured natural gradients of competi tion intensity, and only a fraction of these have separated above and below-ground competition, Yet such measurements are essential to disti nguish 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. Abo ve-ground biomass was strongly correlated to soil depth (r(2) = 0.65, P < 0.001) indicating that as soil resources increased, light decrease d 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 great er than zero (P < 0.05); that of shoot competition was not. This sugge sts that competition in this system was primarily below ground. 4 Comp etition intensity did not vary significantly along the soil depth grad ient. 5 Results from this and other field studies of competition have produced apparently contradictory results. We suggest a graphical mode l that relates the various effects of competition and mutualism to bio mass levels which may reconcile apparently contradictory field studies .