The relationship between dominant height and age is the base of site index, the most widely used measure of site quality. In applying the site index concept, one typically assumes that height development is not affected by stand density or thinning treatment. This assumption has been challenged by recent studies on loblolly pine. A detailed data set with initial densities ranging from 6,730 to 750 trees/ha and covering ages 1 through 25 after plantation establishment was used to study and model the effect of initial spacing on height development of loblolly pine. Dominant height was found to be dependent on initial spacing. Height-age models are proposed that take into account the effect of spacing on average and dominant height. The differences among plantation densities are evident from age 6 and are consistent to the end of the 25-year period of study. Previous studies in other conifers have reported an early advantage in terms of height growth in denser stands that tend to disappear with age, producing a crossover of the growth trajectories. No evidence of this crossover effect in height was found.
