During the third growing season after thinning, eight-year-old loblolly pine gas exchange parameters were measured. Full sunlight and stand light conditions for upper (sun) and lower (shade) crown foliage were examined. Xylem water potential, litterfall dynamics and tree growth were monitored. Nutrient availability, chlorophyll content and chlorophyll a/b ratio were assessed.

At the end of the third post-thinning growing season (eleventh season since planting), thinned trees had larger stem and crown diameters, and live crown ratios. Thinned trees grew 11 cm more in height, and approximately 35% more in diameter, basal area and volume. Only crown diameter differences were statistically significant at p=0.05.

Upper crown foliage was found to have somewhat higher net photosynthesis, stomatal conductance and transpiration when measured under saturating light conditions. Lower crown needles displayed features usually associated with shade foliage: lower photosynthetic capacity, lower light compensation point and dark respiration rates. Observed physiological differences were not related to plant water status, chlorophyll content or chlorophyll a/b ratio. The higher light saturated gas exchange rates for upper crown foliage were paralleled by lower stomatal limitations to photosynthesis, possibly the effect of better potassium supply facilitating more efficient stomatal function.

Light conditions in thinned stands did not differ significantly between the upper and lower crowns, even though light levels in the lower crowns of unthinned stands were nearly a third of that in the upper crowns. This had an effect on in situ photosynthetic rates, which were similar in the upper and lower crowns of thinned trees. In situ photosynthetic rates in thinned stands were also similar to those measured under saturating light. In the unthinned stands, because of light differences, gas exchange rates were significantly reduced in the lower crowns, with only upper crown foliage fully utilizing its potential for photosynthesis. The physiological activity in the upper crowns of unthinned trees tended to be even higher than in the upper crowns of thinned trees.

Litterfall timing did not differ between the thinned and unthinned stands. Litterfall per hectare was higher in unthinned stands, but when expressed on a per tree or unit basal area basis it averaged higher in thinned stands. A larger photosynthetic apparatus together with full utilization of photosynthetic potential in both crown positions are likely the major reasons for better growth of thinned trees.