Though birch and alder are the common pioneer tree species which dominate in northeast Asia, little is known about the effects of the predicted increase in atmospheric CO 2 concentrations ([CO 2 ]) upon their photosynthesis in field conditions. To investigate this, we grew 2-year-old saplings of three Betulaceae species (Betula platyphylla var. japonica Hara, Betula maximowicziana Regel, and Alnus hirsuta Turcz) for 2 years in a free air CO 2 enrichment system in northern Japan. Since the effect of high [CO 2 ] is known to depend on soil conditions, we evaluated the responses in two soils which are widely distributed in northern Japan: infertile and immature volcanic ash (VA) soil, and fertile brown forest (BF) soil. For B. platyphylla, photosynthetic down-regulation occurred in both soils, but for B. maximowicziana, down-regulation occurred only in VA soil. The explanation is reduced nitrogen and Rubisco content in the leaf. For A. hirsuta, down-regulation occurred only in BF soil because of the accumulation of starch in foliage, which restricts CO 2 diffusion inside the chloroplast. The higher photosynthetic rate of A. hirsuta in infertile VA soil could be due to the sink for photosynthates in the N 2 -fixing symbiont. These three species are all able to down-regulate at high [CO 2 ]. However, it is possible that A. hirsuta would dominate in VA soil and B. maximowicziana in BF soil in the early stages of forest succession in a CO 2 -enhanced world.
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