Growth and Physiological Responses of Young Loblolly Pine Stands to Thinning

Abstract Although yield responses to thinning in loblolly pine have been well documented, short- and long-term morphological and physiological changes are largely unknown. To investigate these responses, three 8-yr-old, 0.2024 ha replicate stands of loblolly pine were thinned by 50% in May 1988, and bole growth and morphological changes in the crown were measured annually for the next 6 yr. In situ gas exchange measurements were monitored in the upper and lower third of the crowns monthly during the third through sixth growing seasons following thinning. First- and second-year physiological responses to thinning have been reported earlier (Ginn et al. 1991). Thinning slowed live crown recession and increased crown diameters resulting in increased photosynthetic surface area per tree. Over the 6 yr study period, the crown diameters of thinned trees increased 82% while the crown diameters of control trees increased only 20%. Larger bole diameters were observed for thinned trees four growing seasons after thinning. Photosynthetic rate and needle conductance did not increase overall, but were increased in the lower crown foliage of thinned trees. Needle dark respiration was higher in thinned stands in both the upper and lower crowns. Increased bole diameter growth is likely the result of the large increases in foliar biomass as well as initially higher physiological activity in the lower crowns of thinned stands. For. Sci. 43(4):529-534.