Competitive Effect and Response in Four Annual Plants

(1) Neighbourhood experiments were used to compare the magnitudes of competitive effect and competitive response using all pairwise combinations of Papaver rhoeas, Triticum aestivum and Avena sativa as neighbour species, and these three species plus Chenopodium album as target species. (2) Over 79% of the variance in target weight was explained by a simple hyperbolic function of neighbour weight or density for seven out of the twelve species combinations. (3) For all four target species, the two grass neighbour species always had statistically equivalent competitive effects that were generally greater than the effects of Papaver. For all three neighbour species, Papaver was the weakest competitor in terms of response and Triticum was the strongest competitor. With one exception, these hierarchies hold both for effects averaged over all densities of neighbours > 0 and when biomass of neighbours is restricted to a narrow range. (4) Species that attain larger size when grown with no competition and with larger seeds generally have a greater competitive effect (cause greater reductions in growth of target plants) and a lesser competitive response (growth is reduced less by the presence of neighbours), except that Chenopodium is a stronger response competitor than expected from its size with no competition and from its seed size. (5) Root: shoot ratio of the target plants was significantly affected by the presence of neighbours in nine of the twelve species combinations. The direction of charge in relative root allocation was often inconsistent among target species responding to the same neighbour species and among neighbour species affecting the same target species.