Size heterogeneity, growth dominance development, and relationships with stand growth in unthinned and thinned loblolly pine plantations

Abstract

This study examined size inequality and size-growth relationships as essential aspects of stand structure in loblolly pine plantations (Pinus taeda L.), particularly focusing on a critical stage with strong competition from mid-rotation to rotation age and changes due to thinning. Data were from a loblolly pine thinning study of multiple sites in the western Gulf region, USA, which were thinned around age 14. Four treatments were evaluated: three thinning intensities (TIs: residual densities of 740, 555, and 370 trees·ha⁻¹) and an unthinned control. An operational thinning method was implemented, involving the removal of every fifth row and selective thinning from below to achieve the desired intensity. The Gini index (GI) for diameter at breast height (DBH), height (H), and volume was computed annually for the first five years since thinning and at year seven, while growth dominance (GD) of these traits was calculated by growth interval. The control exhibited increasing trends over the year in GIs and greater inequality in DBH compared to height (GI ​= ​0.10 vs. 0.04), and thinning reduced both GIs. Competition for DBH growth in the control was weakly asymmetric (GD ​= ​0.06), and thinning decreased GD, fostering weaker asymmetric competitive environments. The control displayed reverse asymmetric competition for height (GD ​= ​−0.10), and thinning strengthened reverse dominance. The thinning effects on reducing GIs and GD increased with TI. The observed patterns of competition appear to stem from adaptive resource allocation strategies rather than being influenced by neighboring tree sizes. The tree volume (VOL_T)-based GI and GD might overestimate size inequality for DBH or height, and misinterpret GD for height. Stand volume growth in the control is linearly related to GD, being negative for DBH and positive for height, indicating GD offers valuable insights into stand growth dynamics. Thinning did not change the relationships but slowed the rates of change. The decline in stand growth associated with reduced DBH GD by thinning suggests that GD itself may not be the primary causal factor behind growth changes by thinning.