Seasonal shoot and needle growth of loblolly pine responds to thinning, fertilization, and crown position

The impacts of thinning, fertilization and crown position on seasonal growt h of current-year shoots and foliage were studied in a 13-year-old loblolly pine (Pinus taeda L.) plantation in the sixth post-treatment year (1994). Length of new flushes, and their needle length, leaf area, and oven-dry wei ght were measured in the upper and lower crown from March through November. Total shoot length was the cumulative length of all flushes on a,given sho ot and total leaf area per shoot was the sum of leaf areas of the flushes. By the end of June, first-flush foliage reached 70% of the November needle length (14.3 cm) and 65% of the final leaf area ( 15.0 cm(2)). Cumulative s hoot length of first- and second-flush shoots achieved 95% of the annual le ngth (30.3 cm), whereas total leaf area per shoot was 55% of the final valu e (75.3 dm(2)). Fertilization consistently stimulated fascicle needle lengt h, dry weight, and leaf area in the upper crown. Mean leaf area of upper-cr own shoots was increased by 64% six years after fertilization. A significan t thinning effect was found to decrease mean leaf area per shoot in the cro wn. For most of the growing season, the thinned-fertilized trees produced s ubstantially more leaf area per shoot throughout the crown than the thinned -nonfertilized trees. These thinned-fertilized trees also had,greater needl e length and dry weight, longer first flush shoots, and more leaf area per flush than trees in the thinned-nonfertilized plots. Needle length and leaf area of first Bush shoots between April and July were linearly related to previous-month canopy air temperature (T-a). Total shoot length strongly de pended on vertical light gradient (PPFD) within the canopy, whereas shoot l eaf area was a function of both PPFD and T-a. Thus, trees produced larger a nd heavier fascicles, more and longer flush shoots, and more leaf area per shoot in the upper crown than the lower crown. We conclude that thinning, f ertilization, and crown position regulate annual leaf area production of cu rrent-year shoots largely by affecting the expansion of first flush shoots and their foliage during the first half of the growing season. (C) 1999 Els evier Science B.V. All rights reserved.