An Experimental Test of Plant Strategy Theory

Two of the central hypotheses of the triangular model of primary plant strategies were tested by a novel technique involving seven grasses of contrasted ecology grown in pure stands and an additive mixture on an experimental matrix of crossed gradients of mineral nutrient stress and vegetation disturbance. The experimental design allowed reductions in vegetative and reproductive vigor resulting from interspecific competition to be distinguished from those arising from direct effects of nutrient stress and vegetation disturbance. It was also possible to determine the extent to which competitive suppression of each species was affected by stress and disturbance. In isolation, all species showed maximum vegetative and reproductive vigor at high soil fertility and low disturbance. In the mixture, absolute reductions in biomass and flowering due to competition were greatest at high soil fertility and low disturbance, and the species of most extreme strategy became restricted to areas of the matrix broadly consistent with those predicted by strategy classification. When standardized for differences in biomass in pure stands, the effect of competition remained relatively constant across the stress-disturbance matrix for all species except Poa annua, which was less restricted by competition at high intensities of stress. There were marked and consistent differences between species in their susceptibility to competition. At both high and low soil fertility, two species of natural occurrence on infertile soils (Festuca ovina, Bromus erectus) were poor competitors relative to Arrhenatherum elatius, a widespread dominant of productive grasslands. The effect of competition was least severe on flowering of annuals in low-stress portions of the matrix. If competition is assessed simply as the percentage of reduction in biomass between pure and mixed stands it appears that competition intensity is constant across different intensities of stress and disturbance. However, observations that maximum reductions in biomass coincided with low stress and low disturbance, that competition decreased in importance as a factor reducing yield and flowering (relative to stress and disturbance) as stress and disturbance intensities increased, and that there was a consistently inferior competitive ability of plants from infertile soils at all positions on the matrix all support the hypothesis that competition declines in importance as a vegetation determinant in the vegetation of infertile soils.