TEMPORAL AND SPATIAL PATTERNS OF NET PHOTOSYNTHESIS IN 12-YEAR-OLD LOBLOLLY-PINE 5 GROWING SEASONS AFTER THINNING

Physiological parameters were measured under natural light conditions and needle orientation from towers and walkways erected in the canopy of a loblolly pine (Pinus taeda L.) plantation, Four silvicultural tre atments were randomly assigned to the twelve plots in the fall of 1988 . Plots were thinned to a density of 731 trees per hectare or left unt hinned, at a density of 2990 trees per hectare. The plots were left un fertilized or fertilized with 744 kg/ha of diammonium triple superphos phate was applied, During the fifth growing season (1993) following th inning and fertilization, needle level physiology was not different wi th respect to the thinning treatment for fertilized or unfertilized pl ots. In contrast, upper crown levels within the fertilized and unferti lized plots had significantly higher light levels and photosynthetic r ates than lower crown foliage. Light levels were greater in the thinne d, fertilized plots than in the unthinned, fertilized plots, In contra st, no effect of thinning on canopy light levels was found in the unfe rtilized plots. Within crown variation in photosynthesis was strongly dependent on canopy light levels. A strong interaction of canopy level with thinning was apparent for net photosynthesis, Loblolly pine, bei ng a shade intolerant species, showed only small physiological differe nces between needles from different parts of the crown. Because of the variability found in this study, more extensive sampling is needed to correctly describe the physiology of a forest canopy with adequate pr ecision. (C) 1997 Elsevier Science B.V.