text: Elevated CO 2 did not significantly impact vegetative growth in three of these weedy species. For Spergula arvensis , however, atmospheric CO 2 enrichment increased maximum leaf length by an average of 15%, regardless of light treatment, and total dry weight by 20 and 68% at full and reduced light levels, respectively. Similarly, elevated CO 2 significantly enhanced reproductive success in only one of these species, by increasing the number of seeds in Poa annua by 50 and 26% at full and reduced light levels, respectively. Thus, CO 2 -induced increases in vegetative biomass do not necessarily lead to increases in reproductive biomass, and CO 2 -induced increases in reproductive biomass do not need to be preceded by increases in vegetative biomass. In contrast to the limited significant effects of elevated CO 2 on vegetative and reproductive growth, reduced light nearly always caused significant decreases in plant vegetative and reproductive success. Moreover, the negative effects of reduced light on these parameters were typically much stronger than any positive effect resulting from atmospheric CO 2 enrichment. Total plant dry weights, for example, decreased by 32 to 62%, depending upon species, when grown in reduced, rather than in full light, while atmospheric CO 2 enrichment only increased total plant dry weights by 2 to 30%. What it means