The response of plants to elevated CO : II. Competitive interactions among annual plants under varying light and nutrients.

Communities, consisting of six co-occurring, disturbed site annuals, were subjected to CO unenriched (300 ppm) and to CO enriched (450 and 600 ppm) atmospheres at different levels of light and nutrient availability. In general, total community production increased with CO enrichment to 450 ppm, but a further increase in CO to 600 ppm had little or no effect. The response of community production to CO level was not affected by nutrient availability but was affected by light level.Of the six species, four display C metabolism. The proportion of total community production contributed by these species increased as a result of CO enrichment, and was dependent upon both light and nutrient availability. The relative success of some species, particularly in terms of reproduction (total seed biomass), was significantly altered by CO concentration depending on the level of nutrients. There were not only changes in reproductive success (seed biomass) and shoot biomass but also changes in the proportion of biomass allocated to seed.These experiments demonstrate that CO enrichment does affect annual plant communities both in terms of productivity and species composition and that the affect of CO on such system may depend upon other resources such as light and nutrients.