Development of size hierarchies prior to the onset of density-dependent mortality in irrigated and fertilized loblolly pine stands

Abstract

Nine years of growth and stand development were investigated in a 2 × 2 nutrient and water factorial experiment with four replications. The study was located on an infertile, excessively drained sandy site in Scotland County, North Carolina, U.S.A. The hypothesis tested was that increased growth following irrigation and fertilization would increase the rate at which size hierarchies develop. The hypothesis was investigated by comparing the coefficient of variation (CV) of stem volume over time and examining the stem volume relative growth rate (RGR) of trees of different initial size in control and irrigated + fertilized stands. Even though there were no statistically significant differences in CV among treatments, there was a tendency for increased CV over time in the control stands, whereas CV initially increased, then decreased, and became constant in the irrigated + fertilized plots. The lack of increase in CV in the irrigated + fertilized plots was explained by unusually low variation in RGR across tree size classes and negative relation of RGR and size. Therefore, the hypothesis that increased growth resulted in a more rapid development of size hierarchies was rejected. The high RGR of small trees in the irrigated + fertilized treatment could not be explained by differences in vertical distribution of needles compared with the control treatment. Small trees in the irrigated + fertilized plots were overtopped by neighboring trees to the same degree as small trees in the control plots.