What was done
The authors searched for natural CO2 springs that were free of toxic H2S and SO2 emissions in cool temperate regions of Japan, where "wild plants have been exposed to CO2 enrichment for hundreds of years." Three springs meeting the researchers' criteria were discovered: Ryuzin-numa and Yuno-kawa at the foot of Mount Hakkoda in Aomori Prefecture, and Nyuu at the foot of Mount Gassan in Yamagata Prefecture, about 300 km south of the other two springs (which are only separated from each other by 1 km). CO2-enriched and ambient-air plots were identified at the three sites. At Ryuzin-numa, these plots had mean high and low CO2 concentrations of 670 and 375 ppm, respectively, while at Yuno-kawa they had concentrations of 530 and 370 ppm, and at Nyuu they had values of 760 and 370 ppm. At the first two of these sites, basic physiological processes of Polygonum sachalinense were carefully measured in both the CO2-enriched and ambient-air plots, while at the last site similar measurements were made on Plantago asiatica.

What was learned
Percentage changes in the light saturated photosynthetic rate of the plants measured at the high compared to the low atmospheric CO2 concentrations at Ryuzin-numa, Yuno-kawa and Nyuu were, respectively, +45%, +21% and +58%. Corresponding CO2-induced changes in transpiration rate were -25%, -41% and -27%, while corresponding CO2-induced changes in water use efficiency were +93%, +90% and +105%, and in photosynthetic nitrogen use efficiency +59%, +40% and +96%. Linearly normalized to a common atmospheric CO2 concentration increase of 300 ppm, the first of these sets of results translates to percentage changes of +46%, +40% and +45%, the second set to -25%, -77% and -21%, the third set to +95%, +169% and +81%, and the fourth set to +60%, +75% and +74%.

What it means
Even though Onoda et al. observed there was a down-regulation of photosynthesis in the high CO2 plots, they found that "the photosynthetic rate at growth [our italics] CO2 concentration was [still] stimulated as a result of the direct effect of high CO2 levels," and significantly so, we might add. Likewise, transpiration was significantly reduced, and water and nitrogen use efficiencies were dramatically increased, demonstrating that the real-world of nature will likely be one of increasing plant growth and ever more efficient use of important natural resources, such as water and soil nutrients, as the air's CO2 content continues to climb.