Plant strategies and trait trade-offs influence trends in competitive ability among gradients of soil fertility and disturbance

1 We present evidence of trade-offs in competitive ability among five perennial C4 grass species at different levels of soil fertility and disturbance in a garden experiment in KwaZulu-Natal, South Africa. 2 Our goals were to examine plant strategy theory, using grass species that differed widely in various traits and in their response to environmental influences. More specifically, we wished to examine whether the importance of competition was less in low fertility or regularly disturbed environments (stressed environments) than in more fertile or undisturbed conditions, and whether species made trade-offs in competitive ability along such stress gradients. 3 The trend in the competitive ability of two species in mixture along a soil fertility gradient tended to be inversely related if one had narrow and the other broad leaves or they differed in height, providing strong evidence for trait trade-offs. Thus, a tall broad-leaved species, Panicum maximum, was the most competitive species in fertile subplots, whereas a tall narrow-leaved species, Hyparrhenia hirta, was the most competitive species in infertile subplots, but no difference in competitive ability across a fertility gradient was observed when both species in a mixture had similar traits. 4 Regular cutting altered competitive interactions amongst species resulting in a short species, Themeda triandra, having a greater competitive effect on neighbours and a better response to neighbours in cut than in the less stressed uncut treatments, suggesting a trade-off for competitive ability in regularly disturbed habitats. However, cutting only improved the performance of T. triandra against P. maximum in infertile subplots. 5 We demonstrated that for some, but not all, mixtures of species, competitive hierarchies may reverse along gradients of soil fertility and disturbance. Moreover, soil fertility and disturbance may interact in their effect on competitive interactions. Thus, soil fertility and disturbance form the major axes of an ecological framework for understanding plant competitive interactions.