Seasonal development of leaf area in a young, widely spaced Pinus radiata D. Don stand.

Measurements of needle elongation and needle death were made at two-week intervals during a year on 250 branch units spread throughout the crowns of six trees (three high-pruned, three low-pruned) in a widely spaced, 6- to 7-year-old Pinus radiata D. Don plantation in New Zealand. The trees were well supplied with nutrients and water. During the year, mean tree height increased by 1.2 m and the cross-sectional area of stem below the green crown (used to predict leaf area) for the average tree increased from 8.7 x 10(3) to 13.9 x 10(3) mm(2). The increase in stem cross-sectional area occurred throughout the year except for two months in early winter (May and June). Elongation of Age 0 needles began in Spring (October), continued through summer, and the mean date for 95% completion of elongation was in autumn (early May), approximately 200 days after elongation began. Mean maximum needle length in the canopy decreased with increasing branch order and was 136 and 94 mm for Order 1 and Order 3 units, respectively. Needle elongation was related to thermal time, using growing degree days with a base temperature of 6 degrees C. The mean maximum rate of needle elongation in the canopy was 0.11 m ( degrees C day)(-1) and this occurred in early summer (mid-December), 47 days after elogation started. Maximum needle length and the rate of elongation increased, and the time taken to reach 95% elongation decreased with increasing height in the canopy. A smaller autumn flush of needles started in summer (January) and the needles elongated linearly at a mean rate of 0.07 mm ( degrees C day)(-1) until the end of the growing season when temperatures fell below the base value. At the end of the year, the mean length of needles from the autumn flush was 66 mm. The density of needles did not change with height in the canopy and there were no significant changes seasonally. The mean density values for Age 1 and Age 0 needles were 336 and 286 kg m(-3), respectively. Dry weight per unit length did not change seasonally for Age 1 needles, but the mean values for Order 1 and Order 2 needles were 0.32 and 0.23 mg mm(-1), respectively. Dry weight per unit length for Age 0 needles increased during the growing season (October to February). This was particularly apparent for Order 1 needles where the mean value increased from 0.12 to 0.25 mg mm(-1). Death of Age 1 needles started in midsummer (mean date was January 24) which coincided with the time of maximum elongation of Age 0 needles, but there were differences in timing among individual trees. The mean rate of death was 1.7% day(-1) and the mean duration was 59 days. Leaf area index for the stand increased from 3.2 to 5.3 (all-surfaces basis) during the year. At the end, 92% of the leaf area had grown during the year and 60% of it was on Order 2 branch units. The models for needle elongation and needle death were used to scale up to seasonal changes in canopy leaf area index. Leaf area index peaked with increasing Age 0 leaf area to 6.1 in summer (January), fell to 4.7 in March as Age 1 needles died, then increased slightly again. The seasonal dynamics are consistent with the hypothesis of Cannell (1989) that the timing of maximum leaf area index within a year is optimal for maximizing biomass production.