Soil moisture condition and growth of deciduous tree seedlings native to northern Japan grown under elevated CO2 with a FACE system

To better predict the soil moisture environment of cool-temperate forests, we studied the soil moisture for stands grown under elevated Co-2 concentration ([Co-2]) using Free Air Co-2 Enrichment (FACE) systems (50 Pa [CO2](e)) with controls at ambient levels (37 Pa [CO2](a))), over two years. Two distinct soil conditions, brown forest soil and volcanic ash soil, were prepared in each FACE. Two-year-old seedlings of six species were planted in May 2003. The soil moisture increased under [CO2](e) in both soil conditions. Since changes of soil moisture depend on evapotranspiration from a stand, we examined the fluctuation of soil moisture in relation to the transpiration (Tr) from leaves and evaporation (Ev) from the soil surface in the FACE. The Tr per leaf area decreased under [CO2](e) for both soil conditions, although the total Tr per seedling increased because the total leaf area per seedling increased under [CO2](e). Ev decreased under [CO2](e), because the soil surface temperature fell in both soils as a result of the increase of leaf area index (LAI), which blocked incident sunlight at the soil surface. These results show that the increase of soil moisture under [CO2](e) over two years is due probably to the change in Ev. Our results also indicate that the predicted rise in atmospheric [CO2] will lead to modification of the forest ecosystem accompanied by changing soil moisture conditions.