What was done
Even-aged monospecific stands of the summer annual Chenopodium album (a weed that is commonly found in open habitats, such as abandoned fields and flood plains) were established at ambient and twice-ambient atmospheric CO2 concentrations at low and high levels of soil nutrient availability in open-top chambers in the experimental garden of Tohoku University, Sendai, Japan, after which the growth of individual plants was monitored non-destructively every week until flowering.

What was learned
At the conclusion of the experiment, there was no significant effect of elevated CO2 on aboveground biomass in the low nutrient regime; but in the high nutrient regime, the extra CO2 increased aboveground biomass by 50%.  In addition, as time progressed throughout the study, the CO2-induced enhancement of growth in the high nutrient regime gradually waned and ultimately disappeared altogether in smaller subordinate individuals, but it continued in the larger dominants throughout the whole experiment.  Hence, in the words of the authors, "size inequality expressed as the coefficient of variation tended to increase at elevated CO2."

What it means
When conditions are such that elevated levels of atmospheric CO2 are able to benefit plants, they do not benefit them equally.  Individual plants that for whatever reason "get ahead" initially typically end up becoming proportionally bigger than the rest, thereby amplifying the differences in size that existed among them when they were both very young and very small.