In order to quantify the effects of thinning on photosynthetic parameters and associated change in leaf nitrogen (N) contents, half of the trees in a 10-year-old Chamaecyparis obtusa (Sieb. et Zucc.) Endl. stand (36° 3'N, 140°7'E) were removed, giving a final density of 1 500 trees ha-1, in May 2004. Photosynthetic photon flux density (PPFD) and leaf N and carbon (C) contents in the lower (L), middle (M), and upper (U) crowns were monitored one, three, and five months after thinning in both the thinned stand and a non-thinned control stand. In addition, leaves' photosynthetic responses to CO2 concentration were simultaneously measured in situ to estimate the maximum rates of carboxylation (Vcmax) and electron transport (Jmax). Thinning increased PPFD in the L and M crowns but not in the U crown. Vcmax in both the L and M crowns of the thinned stand increased significantly in comparison with the same crown position of the control stand in the three and five months following thinning. In addition, the thinned stand exhibited an increase in N partitioned to ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) in the L and M crowns relative to the control stand three and five months after thinning, indicating that N had been redistributed within the photosynthetic machinery. Thinning did not affect N per unit area at any of the crown positions, but significantly increased the content of N as a fraction of the total leaf dry mass in the L and M crowns three and five months after thinning. This was a consequence of a decrease in leaf dry mass due to rapid shoot growth. Thus thinning did not cause a redistribution of N between leaves. Thinning improved irradiance in the L and M crowns of C. obtusa, leading to photosynthetic acclimation. Photosynthetic acclimation in the first year mainly occurred via redistribution of N within but not between leaves. and Q. Han, M. Araki, Y. Chiba.
Photosynthetic induction responses to a sudden increase in photosynthetic photon flux density (PPFD) from lower background PPFD (0, 25, 50, and 100 μmol m-2 s-1) to 1 000 μmol m-2 s-1 were measured in leaves of Fagus crenata, Acer rufinerve Siebold & Zucc., and Viburnum furcatum growing in a gap and understory of a F. crenata forest in the Naeba mountains. In the gap, A. rufinerve exhibited more than 1.2-fold higher maximum net photosynthetic rate (PNmax) than F. crenata and V. furcatum. Meanwhile, in the understory F. crenata exhibited the highest PNmax among the three species. The photosynthetic induction period required to reach PNmax was 3-41 min. The photosynthetic responses to increase in PPFD depended on the background PPFD before increase in PPFD. The induction period required to reach PNmax was 2.5-6.5-fold longer when PPFD increased from darkness than when PPFD increased from 100 μmol m-2 s-1. The induction period was correlated with initial PN and stomatal conductance (gs) relative to maximum values before increase in PPFD. The relationship was similar between the gap and the understory. As the background PPFD increased, the initial PN and gs increased, indicating that the degrees of biochemical and stomata limitations to dynamic photosynthetic performance decreased. Therefore, photosynthetic induction responses to increase in PPFD became faster with the increasing background PPFD. The differences in time required to reach induction between species, as well as between gap and understory, were mainly due to the varying of relative initial induction states in PN and gs at the same background PPFD. and M. Naramoto, Q. Han, Y. Kakubari.