Thinning, Fertilization, and Crown Position Interact to Control Physiological Responses of Loblolly Pine
Summary To examine physiological responses to thinning, fertilization, and crown position, we measured net photosynthesis (Pn), transpiration (E), vapor pressure difference (VPD), stomatal conductance (gs), and xylem pressure potential (Yl) between 0930 and 1130 h under ambient conditions in the upper and lower crowns of a 13-year-old loblolly pine (Pinus taeda L.) plantation six years (1994) after the treatments were applied. Photosynthetic photon flux density (PPFD) and air temperature (Ta) within the canopy were also recorded. Needle Pn of thinned trees was significantly enhanced by 22-54% in the lower crown, because canopy PPFD increased by 28-52%. Lower crown foliage of thinned plots also had higher E and gs, than foliage of unthinned plots, but thinning had no effect on needle Yl and predawn xylem pressure potential (0430-0530 h; Ypd. Tree water status did not limit Pn, E and gs during the late-morning measurements. Fertilization significantly decreased within-canopy PPFD and Ta. Needle Yl was increased in fertilized stands, whereas Pn, E and gs, were not significantly altered. Upper crown foliage had significantly greater PPFD, Pn, VPD, gs, E, and more negative Yl than lower crown foliage. In both crown positions, needle Pn was closely related to gs, PPFD and Ta, (R2 = 0.77 for the upper crown and 0.82 for the lower crown). We conclude that (1) silvicultural manipulation causes microclimate changes within the crowns of large trees, and (2) needle physiology adjusts to the within-crown environmental conditions.