What was done
Ten tropical tree species were grown in open-top chambers situated on the edge of a tropical forest in Panama for six months at ambient and twice-ambient CO2 concentrations to determine the effects of elevated CO2 on tropical forest communities.

What was learned
Even though tree biomass remained unaffected after six months exposure to elevated CO2, other important physiological changes had occurred.  The leaf area ratio of trees grown in elevated CO2, for example, was much lower than that of trees grown in ambient CO2, indicating that even with less resources invested in their leaves, trees grown in elevated CO2 still synthesized enough carbohydrate to support biomass production equivalent to that of ambiently-grown trees.  This phenomenon could only occur with a concomitant increase in photosynthetic efficiency, which was evidenced by higher net carbon assimilation rates in the trees grown in CO2-enriched air compared to those grown in ambient air.  Consequently, leaf starch concentrations for all ten trees were approximately doubled in the doubled CO2 environment.

What it means
As the CO2 content of the air rises, tropical forest trees can be expected to increase the efficiency at which they remove carbon from the air and convert it into carbohydrates, including starch.  With greater leaf carbohydrate concentrations, such trees will likely utilize them to ultimately increase their growth and biomass.  In the current paper, however, such biomass changes were not observed, possibly as a result of the short duration of the experiment.