Spatial and temporal variations in the photosynthesis-nitrogen relationship in a Japanese cedar (Cryptomeria japonica D. Don) canopy

The original publication is available at www.springerlink.comSpatial and temporal variations in light-saturated photosynthetic capacity and needle nitrogen (N) content were investigated in one 8 m tall Japanese cedar (Cryptomeria japonica D. Don) canopy for a full year. The photosynthetic capacity and needle N content in various layers of the canopy were measured every month. Temporal variations in photosynthetic capacity and needle N content expressed on a projected-area basis (P-area, N-area) were similar. Furthermore, both P-area and N-area decreased with increasing depth from the top of the canopy on each sampling date. As a consequence, a significant correlation was observed between N-area and P-area. Temporal variations in photosynthetic capacity and needle N content expressed on a mass basis (P-mass, N-mass) were also similar. P-mass also decreased with increasing canopy depth. However, in contrast to N-area, there was only a slight decrease in N-mass with increasing canopy depth. Hence, the correlation between N-mass and P-mass was lower than the projected-area value. Because N-area was highly correlated with the needle mass per projected-area (NMA), the spatial variation in N-area (and therefore P-area) in the canopy is attributed to the variation in NMA, which decreases as the depth from the top of the canopy increases. Furthermore, the slope of the linear regression between N-area and NMA differed between sampling dates, indicating that the temporal variations in N-area (and therefore P-area) are strongly influenced by N-mass. For most of the sampling dates, a linear regression between N-area and P-area tends to converge into a single line segment. However, on several sampling dates, there was a pronounced decline in P-area below this line segment. This reduction in P-area, which does not accompany a reduction in Narea, seems to be attributable to stomatal limitations induced by the low soil temperature in winter and early spring.ArticlePHOTOSYNTHETICA. 48(2): 249-249(2010)journal articl