Spatial and seasonal variability of temperature responses of biochemical photosynthesis parameters and leaf nitrogen content within a Pinus densiflora crown

We measured seasonal variation in area-based nitrogen concentration (N), maximum rate of carboxylation (V-cmax) and maximum rate of electron transport (J(max)) in I-year-old needles along four first-order branches within a Pinus densiflora Sieb. & Zucc. crown, and analyzed their relationships to growth irradiance and temperature. Each leaf light environment was expressed as a ratio of the monthly mean of daily integrated photosynthetically active irradiance (I-int) for the particular needle to I-int above the canopy (I-rel). Needle N decreased in the upper crown during the development of new needles, whereas it remained fairly constant in the lower crown, reflecting differences between upper and lower crown needles in their contribution to the nitrogen of new needles. Gradients of N within the crown were correlated with I-rel in all seasons (r(2) = 0.40-0.78). Seasonal variation in N was weakly correlated with mean daily air minimum temperatures. Both V-cmax and J(max) showed seasonal variation in all first-order branches, and decreased to their lowest values in winter. The gradients of V-cmax and J(max) within the crown were not correlated with I-rel in some seasons, but were correlated with changes in N in most months(r(2) = 0.33-0.75), except in the winter. Furthermore, the regression slope of the relationship between N and V-cmax and the temperature response of V-cmax and J(max) exhibited seasonal variation.