Relationship between intercepted radiation, net photosynthesis, respiration, and rate of stem volume growth of Pinus taeda and Pinus elliottii stands of different densities

Intercepted radiation, net photosynthesis, foliar respiration, stem respiration, and foliar nitrogen concentrations were measured to determine how changes in physiology that occur with increasing stand density affect the rate of stem volume growth. Intensively managed Pinus taeda L. (loblolly pine) and Pinus elliottii Engelm. (slash pine) stands in their third and fourth growing seasons planted at densities of 740, 2220, and 3700 trees ha^-1 on the lower coastal plain of the southeastern United States were sampled. During the third growing season, stem volume growth was not proportional to stocking density with smaller increases in stem volume growth occurring as stand density is increased. The proportion of radiation intercepted by the canopies during the period of maximum leaf area was linearly related to the rate of stem volume growth. Stem respiration was greater for the trees planted at 740 trees ha^-1 than for trees at the other densities on a stem surface area basis, but not on a stem volume basis. Foliar respiration and net photosynthesis were not different between the trees growing at different densities even though foliar nitrogen concentration decreased with increasing stocking density. One difference between the species was that the specific leaf area (m² kg^-1 of leaf) of P. elliottii was significantly lower than P. taeda, with the result that area-based comparisons of foliar gas exchange were similar between P. taeda and P. elliottii, but on a mass basis, P. elliottii gas exchange was lower than P. taeda. For both species, specific leaf area increased with increasing stocking density. These results indicate that differences in net photosynthesis or respiration rates among stocking densities, or between species, were minimal and did not explain differences in stand growth rates. Instead, growth was well correlated with intercepted radiation and trees in the different density treatments appeared to modify their leaf morphology to improve canopy light interception.