PLANT COMPETITION ALONG SOIL-MOISTURE GRADIENTS - A FIELD EXPERIMENT WITH THE DESERT ANNUAL STIPA-CAPENSIS

1 The interaction between natural and experimental gradients of productivity on competition intensity was tested by neighbour removal experiments. Water is the main limiting factor in this system and experimental gradients of productivity were obtained using a series of watering treatments. 2 Competition was determined as the total effect of neighbours on per-capita seed production of Stipa capensis. Both absolute and relative competition intensity were calculated for three types of habitats, during two successive years, and under different watering treatments in each habitat-year combination. 3 Both measures were positively correlated with productivity, but absolute competition intensity was more sensitive to changes in productivity than relative competition intensity. Natural gradients of productivity appeared to have stronger effects on competition intensity than experimental gradients, but this was largely due to their wider range. In those cases where the ranges of the two types of gradients were similar, experimental gradients had a stronger effect on competition intensity than natural gradients. 4 Patterns of spatial variation in competition intensity were correlated with standing crop under all watering conditions. However, slopes of the regression equations obtained for the various watering treatments were not homogeneous. This indicates that per-gram effects of standing crop on competition intensity may fluctuate from year to year, depending on rainfall conditions. Regression models constructed to test the relationships between standing crop and competition intensity-over different years, habitat types and watering treatments accounted for 88% of the variation in absolute competition intensity and 83% of the variation in relative competition intensity. 5 The overall results of this study are consistent with the hypothesis that plant competition increases along productivity gradients. The results also indicate that patterns of variation in competition intensity along productivity gradients may be influenced by the type of the gradient along which competition is measured (natural vs. experimental), its range, and the way competition intensity is defined.