Physiological and Growth Responses of Eight-Year-Old Loblolly Pine Stands to Thinning

Abstract

Abstract The influence of thinning at age 8 on the photosynthetic rate, needle conductance, xylem water potential, foliar nutrient content, and growth of loblolly pine trees was studied during their ninth and tenth growing seasons. At the end of the second post-thinning growing season, trees in thinned stands had 51% greater stem diameter growth, and 29% greater basal area growth than trees in unthinned stands. Two years after thinning, trees in thinned stands also had crown diameters over 78 cm larger than unthinned trees. Although not statistically significant, live crown ratios were beginning to decrease more rapidly in unthinned stands. Significant physiological changes due to thinning were generally observed only in the lower crowns where needle physiology was found to resemble that of upper crown needles. In thinned stands, lower and upper crown photosynthesis (3.98 vs. 4.07 μmol m-2 s-1, respectively) and needle conductance (94.2 vs. 95.3 mmol m-2 s-1, respectively) did not differ significantly. This is in contrast to the unthinned stands where both light saturated photosynthesis (3.22 vs. 4.17 μmol m-2; s-1, lower and upper crown respectively) and needle conductance (74.3 vs. 100.8 mmol m-2 s-1, lower and upper crown respectively) were substantially decreased in the lower crown. The increase in crown size and the ability (physiologically) of lower crown foliage to take advantage of the increased light following thinning are likely the major factors resulting in the increased growth of loblolly pine following thinning. For. Sci. 37(4):1030-1040.