Physiological Variation in Loblolly Pine (Pinus Taeda L.) as Related to Crown Position and Stand Density.

Physiological parameters were measured under natural light conditions and needle orientation from towers and walkways erected in the crowns of loblolly pine (Pinus taeda L.) trees. Photosynthetic light response curves, chlorophyll content, nutrient content and specific leaf area were determined for current-year foliage on detached branches. Two silvicultural treatments 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 unthinned, at a density of 2990 trees per hectare. The plots were left unfertilized or fertilized with 744 kg per hectare of diammonium triple superphosphate was applied. During the fifth growing season (1993) following thinning, light levels were greater in the thinned, fertilized plots than in the unthinned, fertilized plots. However, no effect of thinning on canopy light levels was found in the unfertilized plots. Needle level physiology was not different with respect to thinning treatment for fertilized or unfertilized plots. In contrast, upper crown levels within the fertilized and unfertilized plots, had significantly higher light levels and photosynthetic rates than lower crown foliage. Fertilization significantly increased foliar phosphorous, calcium and magnesium levels, but nitrogen levels were reduced. Thinning significantly decreased foliar phosphorous and potassium levels within the fertilized plots. Total chlorophyll content and specific leaf area were greater in the foliage from the lower canopy than that of the upper canopy due to lower light levels. The increase in nutrient availability accelerated leaf area development in the previously thinned 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. Additionally, southern exposed foliage within the thinned treatments had higher net photosynthesis rates than that of the northern exposed foliage in both the in situ and light saturated studies. Loblolly pine, being a shade intolerant species, showed rather small potential physiological differences between needles from different parts of the crown. Because of the variability found in physiological parameters in this study, more extensive sampling is needed to correctly describe the physiology of a forest canopy with adequate precision