Chapter 18 Leaf and Shoot Ecophysiological Properties and Their Role in Photosynthetic Carbon gain of Cool-Temperate Deciduous Forest Trees

Plant ecophysiological properties including photosynthetic responses to environmental conditions, canopy geometrical consequences with photosynthetic carbon gain and their seasonality have considerable responsibility to the forest ecosystem carbon fixation. This paper overviews these traits for canopy tree species in a cool-temperate deciduous broadleaved forest of Takayama AsiaFlux site located in central Japan. Two-year measurements of leaf CO₂ gas exchange characteristics for full growing season on canopy tree species, Betula ermanii and Quercus crispula, revealed that their photosynthetic capacity increases with leaf expansion from late spring (mid-May) to early summer (June) in 2004 or mid-summer (August) in 2003 and decreases in autumn (October). The delayed peak of photosynthetic capacity in 2003 was associated with a prolonged rainy season that year. In contrast, seasonal change of dark respiration was similar between the 2 years; the maximum was observed at the start of leaf expansion and decreased by June when the leaf area matured. Leaf- and shoot- (branch) level photosynthetic and geometrical consequences to canopy carbon gain were examined with a 3D structural-functional model, Y-plant. Simulations with shoots at the canopy top of Betula and Quercus revealed that steeper leaf angle and higher stomatal conductance of Betula contribute to maintaining a high photosynthetic activity by reducing high light stresses, such as heat load and photoinhibition. Comparison of sunlit (canopy top) and shaded (inner canopy) shoots of Quercus revealed that the leaf display with small self-shading of shade shoots is effective in receiving light incidence in the light-limited environment, but prolonged sun fleck limits photosynthesis of the shade leaves by increasing temperature and stomatal closure. These ecophysiological considerations of photosynthesis and plant structure would provide us a deeper insight into the mechanistic understanding of the forest ecosystem carbon gain in changing environments.