The initial physiological processes leading to enhanced growth of loblolly pine subsequent to fertilization are not clearly understood. Much of the debate revolves around the temporal response of photosynthesis (Pn) to fertilization or even if Pn increases at all due to enhanced nutrition. This study tracked loblolly pine light-saturated photosynthesis (Asat), dark respiration (Rd), volume, height, basal diameter, and leaf area responses in eight clones to fertilization (112 kg/ha N) over the course of a growing season in the field. Measurements were conducted intensively before and after fertilization in order to track the initial physiological changes prior to any changes in growth in the fertilized seedlings.

The results showed that fertilization does increase Pn rates although there was no significant effect on Rd rates during the study. The fertilized seedlings mean Asat rates were significantly higher on three sampling dates and remained higher throughout most of the sampling period. At the end of the growing season, the fertilized seedlings had a 30.5% higher projected crown area than the controls and 48% greater mean volumes. Physiological and growth responses were significantly different among clones with some showing large and others showing little or no response to fertilization. These results support the hypothesis from Gough et al. (2004b) that post-fertilization increases in Pn create extra photoassimilate used in building larger leaf areas. These larger leaf areas contribute to higher canopy photosynthesis levels, which leads to an increase in dry matter production.