Effect of elevated CO 2 levels on leaf starch, nitrogen and photosynthesis of plants growing at three natural CO 2 springs in Japan

Plant communities around natural CO 2 springs have been exposed to elevated CO 2 levels over many generations and give us a unique opportunity to investigate the effects of long‐term elevated CO 2 levels on wild plants. We searched for natural CO 2 springs in cool temperate climate regions in Japan and found three springs that were suitable for studying long‐term responses of plants to elevated levels of CO 2 : Ryuzin‐numa, Yuno‐kawa and Nyuu. At these CO 2 springs, the surrounding air was at high CO 2 concentration with no toxic gas emissions throughout the growth season, and there was natural vegetation around the springs. At each site, high‐CO 2 (HC) and low‐CO 2 (LC) plots were established, and three dominant species at the shrub layers were used for physiological analyses. Although the microenvironments were different among the springs, dicotyledonous species growing at the HC plots tended to have more starch and less nitrogen per unit dry mass in the leaves than those growing at the LC plots. In contrast, monocotyledonous species growing in the HC and LC plots had similar starch and nitrogen concentrations. Photosynthetic rates at the mean growth CO 2 concentration were higher in HC plants than LC plants, but photosynthetic rates at a common CO 2 concentration were lower in HC plants. Efficiency of water and nitrogen use of leaves at growth CO 2 concentration was greatly increased in HC plants. These results suggest that natural plants growing in elevated CO 2 levels under cool temperate climate conditions have down‐regulated their photosynthetic capacity but that they increased photosynthetic rates and resource use efficiencies due to the direct effect of elevated CO 2 concentration.