Morphological and physiological responses of Siebold's beech (Fagus crenata) seedlings grown under CO2 concentrations ranging from pre-industrial to expected future levels

Siebold's beech (Fagus crenata) is a common species in the cool temperate forests of Japan. As the natural regeneration of beech forests is expected to contribute to forest conservation in the future, we investigated the effects of different CO2 concentrations ([CO2]) on the growth of beech seedlings in relation to morphological and physiological changes. Acorns collected from beech forest in Minakami, central Japan were germinated and grown during a first growing season of 6 months under four [CO2] levels (200, 350, 550, and 750 mu L L-1). Stem mass increased with increasing [CO2]; however, root mass did not change significantly among the treatments. As [CO2] increased, net photosynthetic rate (P (n)) and leaf area increased, whereas transpiration (T (r)), stomatal conductance, leaf chlorophyll content, and leaf longevity decreased. Although water-use efficiency (WUE; i.e., P (n)/T (r)) improved with increasing [CO2], the density of stomata did not significantly change. Increases in the number of buds and the terminal bud length with increasing [CO2] indicated accelerated formation of additional branches and leaves in the next season. The enhanced WUE levels seen in beech saplings growing under the higher environmental [CO2] levels that are expected in the future may be advantageous for their survival, considering that beech saplings prefer mesic conditions.