We studied photosynthetic responses of 3 birches (Betula ermanii, B. maximowicziana and B. platyphylla var. japonica) to future elevated CO₂ concentration with free air CO₂ enrichment (FACE). The tested birches are representative species in cool-temperate forest in Japan and have similar growth traits.

We set two levels of CO₂ treatment, i.e. ambient as control and elevated CO₂ (500 ppm). In each CO₂ treatment, there were two kinds of soil plot (brown forest soil and volcanic ash soil). The seedlings were planted in May 2010 and were grown for two growing seasons. Gas exchange rate of leaves was measured with an open gas exchange system (LI-6400, Li-Cor Inc., Lincoln, NE, USA) in summer of growing seasons. After the measurement, we determined leaf nitrogen (N) content by an NC analyzer (NC-900, SCAS, Osaka, Japan).

Decrease of photosynthetic activity was observed in all species under elevated CO₂. However, the timing was different between species, it observed in first growing season for B. maximowicziana and in second growing season for B. ermanii and B. platyphylla. Elevated CO₂-induced reduction in N content in leaves was considered as a main factor of decrease in photosynthetic activity.