Leaf photosynthetic responses and related nitrogen changes associated with crown reclosure after thinning in a young Chamaecyparis obtusa stand

In order to quantify the effects of thinning on biochemical photosynthesis parameters and changes in leaf nitrogen contents associated with the process of crown reclosure, the maximum rate of carboxylation (V (cmax)), the leaf nitrogen concentration per unit area (N (a)), and the photosynthetic photon flux density (PPFD) were measured at four crown heights in both thinned (1500 trees ha(-1)) and unthinned control (3000 trees ha(-1)) stands of ten-year-old Chamaecyparis obtusa (36A degrees 3'N, 140A degrees 7'E) trees during four consecutive growing seasons after thinning. Thinning increased V (cmax) in the lower and middle crowns in the first year after thinning, and leaves in the lower crown of the thinned stand maintained high V (cmax) for four years, whereas they abscised in the second year in the control stand. Significant increases in V (cmax) were detected even in the upper crowns of trees in the thinned stand in the second year. Thinning did not affect N (a) at any of the crown positions in the first year, but significantly increased N (a) in the middle crowns from the second year after thinning. Thus, the redistribution of nitrogen between leaves, driven by increases in light and nutrient availability due to the 50% thinning, appears to have enhanced photosynthetic rates in the thinned stand. Thinning also significantly affected the slope of the linear relationship between N (a) and V (cmax) initially after thinning, but its effect on this relationship was negligible after the second year. These quantitative results may be used to simplify the estimation of the likely effects of management practices on carbon fixation in forest canopies.