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We measured horizontal and vertical gradients of light (rPPFD) along four first-order branches of a  Pinus densiflora  Sieb. &amp; Zucc. crown, and compared variations in specific leaf area (SLA), needle nitrogen concentration ( N ), chlorophyll concentration (Chl) and photosynthetic capacity (i.e., maximum rate of carboxylation ( V  cmax )) along the two axes. The horizontal gradient of rPPFD along first-order branches was similar in magnitude to the vertical gradient of rPPFD from the upper to the lower crown. None of the measured parameters (i.e., SLA,  N , Chl and  V  cmax ) were strictly proportional to rPPFD, although they were more or less correlated with light when data obtained for all of the crown were pooled ( r  2  = 0.31–0.80). The slope of rPPFD against  N  on an area basis ( N  area ) for a branch in the middle of the crown orientated northward was significantly greater than the slope for a similar branch orientated southward. Horizontal variations were unrelated to age effects because measurements were all on 1-year-old needles. We conclude that factors other than light (i.e., orientation) may influence N allocation within branches. There was considerably less variation in the relationship of  V  cmax  to  N  area  ( r  2  = 0.58) than in the relationship of  V  cmax  to rPPFD ( r  2  = 0.41). Fractional N distribution among components of the photosynthetic machinery was constant within the crown. Together with the relationships between rPPFD and  N  on a mass basis ( r  2  = 0.80) and SLA and  V  cmax  ( r  2  = 0.60), these findings suggest that most light acclimation in  P. densiflora  occurs through changes in needle morphology (e.g., SLA) during development.

tree physiology

Horizontal and vertical variations in photosynthetic capacity in a  Pinus densiflora  crown in relation to leaf nitrogen allocation and acclimation to irradiance