Prediction of the change and sustainable use of mixed conifer-broadleaf forests under changing environment based on FACE(Free Air CO_2 Enrichment)

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Since the effect of high [CO_2] is known to depend on soil conditions, we evaluated the response in two soils which are widely distributed in northern Japan: infertile and immature volcanic ash (VA) soil, and fertile brown forest (BF) soil. Except alder, most species showed photosynthetic down-regulation only in VA soil. The explanation is reduced nitrogen and Rubisco content in the leaf. For alder, the 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 alder in infertile VA soil could be due to he sink for photosynthates in the N_2-fixing symbiont. Water relations in woody species are intimately related to xylem hydraulic structures. Transpiration or stomatal conductance generally decreases at high CO_2 concentrations ([CO_2]). To determine the relation between water flow and hydraulic structure at high [CO_2], we investigated the single-leaf scale response of both sun and shade leaves of two species (diffuse-porous; birch and ring-porous; oak) grown on VA and BF. We used four-year-old saplings exposed to 500 μmolCO_2 mol^<1> for three years. For sun leaves, the water flow (I.e. stomatal conductance and leaf-specific hydraulic conductivity)and the total vessel area of the petiole consistently decreased at elevated [CO_2] despite differences in species and soil type. However, parameters in shade leaves did not show the significant change. Our result show that changes in water flow at elevated [CO_2] leads to acclimation in petiole hydraulic structure. These results help us to see how trees might accommodate themselves to the coming CO_2-enhanced environment. Based on our results, we can evaluate acclimation traits of broadleaf trees native to Japan to elevated CO_2 condition.