Competition and Coexistence: Three Annual Grass Species

Abstract

Mechanisms of coexistence of 3 spp. (Avena fatua, Lolium multiflorum, Bromus mollis) common in California [USA] annual grassland vegetation were investigated. Rates and phasing of growth were similar among species throughout the active season. There was no evidence of selective predation or predation sufficiently heavy to significantly reduce the intensity of competition. Avena retains dominance by virtue of its large grains and consequent large seedlings, but is unable to reach sufficient densities at prevailing levels of soil N to exclude the other 2 spp. Both Lolium and Bromus had a greater capacity for growth under low nutrient conditions than Avena even when they were subdominant in the stand. The interaction between Lolium and Bromus depended on nutrient and temperature conditions.