The relative importance of competition for two dominant grass species as affected by environmental manipulations in the field

We examined how shade, drought and disturbance influenced the intensity and relative importance of competition experienced by Bromus erectus and Brachypodium pinnatum, the two dominant species of a calcareous grassland. Competition was intense in all treatments for both species, but its importance differed substantially. Competition was not important for Brachypodium when the drought was severe. Shade had a strong positive effect on Brachypodium by decreasing water stress. Bromus was more tolerant of drought and disturbance than Brachypodium, but was more inhibited by competition, which was important in all environmental conditions. Differences in the relative importance of competition for Bromus and Brachypodium, and variation in the effects of the environment, may explain the dominance of Bromus in conditions of high stress and disturbance and its exclusion in mesic grasslands. We suggest that the debate on how competition varies along productivity gradients may be due to the focus on competition intensity, which is highly dependant on particular target species and study systems, rather than on the relative importance of competition in different conditions.