What was done
Employing open-top chambers, the authors determined net ecosystem CO2 exchange (NEE) before, during, and after the severe Central Florida drought of 1998 in a scrub-oak ecosystem in ambient-CO2 (AC) air and in elevated-CO2 (EC) air that had been enriched with an extra 350 ppm of CO2 since May 1996, focusing on the ecosystem's dominant species (Quercus myrtifolia Willd.) for which they measured net photosynthetic rate (PN) throughout the daylight hours of several days.

What was learned
Li et al. report that elevated CO2 generally increased PN while drought decreased it. Under droughty conditions, therefore, PN peaked at around 0830 each day, after which it declined in a fairly steady fashion until solar noon, after which it typically remained at a relatively low level throughout the remainder of the daylight hours. Consequently, they assessed the interactive impacts of elevated CO2 and drought on tree PN by comparing the percentage reduction in PN from 0830 to 1230 in the two CO2 treatments. This approach revealed that in May of 1998 PN was reduced by 77% from 0830 to 1230 at AC but by only 48% at EC, while in July of 1998, when the drought had further intensified, PN was reduced by 82% at AC but by a lesser 69% at EC.

NEE responded in much the same way. In May and June of 1998, for example, its midday depression was 58% and 60% less at EC than at AC, while in July of 1999 it was 66% less. In addition, the researchers report that "the mitigation of the effects of water stress by EC was reflected in the aboveground biomass growth," such that "the relative effect of EC on biomass accumulation of the dominant species Q. myrtifolia was higher during the drought year (210% for 1998) compared to the non-drought years (67% for 1997)."

What it means
As Li et al. state in the title of their paper with respect to the specific ecosystem they studied, "elevated CO2 mitigates the adverse effects of drought on daytime net ecosystem CO2 exchange and photosynthesis in a Florida scrub-oak ecosystem." Or as they state more generally in the concluding sentence of their paper, "studies such as the one reported here strongly support the conclusion that the effects of rising atmospheric CO2 on plant and ecosystem processes [PN and NEE] are likely to be greater during drought."