Effects of management intensity, genetics and planting density on wood stiffness in a plantation of juvenile loblolly pine in the southeastern USA

Abstract

In a 6-year-old plantation of Pinus taeda in Northeast Florida, the affects on juvenile corewood stiffness of genotype, planting density and silviculture as well as their interactions, and some potential mechanisms for these effects were investigated. The stress wave method was used to determine the dynamic modulus of elasticity ( V 2) of the juvenile corewood in 4091 standing trees of seven full-sib families grown at two planting densities (1334 trees ha −1 versus 2990 trees ha −1) with two levels of silvicultural intensity (operational versus intensive). Planting density significantly influenced V 2 ( p < 0.0001) across all families and silvicultural intensities. At 2990 trees ha −1, V 2 averaged 7.11 and 5.44 km 2 s −2 for 1334 trees ha −1. Family and silvicultural intensity interacted significantly for V 2 ( p = 0.0003) with values ranging from 5.03 to 7.48 km 2 s −2. Under intensive silviculture, one family (L1) exhibited a large decrease in V 2. In contrast, another family (L4) with a high overall V 2 rank was not very sensitive to silvicultural treatment intensity. This lack of response under intensive silviculture occurred despite the fact that family L4 demonstrated the greatest increase in growth. Across all treatments, the ratio of height to diameter at breast height (HT:DBH) was strongly related to V 2 with more slender trees being the stiffest. After adjusting for this ratio, only the main effects of initial planting density and genotype were significant. Thus, HT:DBH correlated with variation in V 2 caused by silvicultural intensity. In a subset of trees within three selected families, variation in latewood percentage (LWP) did not correlate with variation in V 2 as predicted. Analysis of the treatments on LWP, showed a significant three-way interaction between genotype, planting density, and silvicultural intensity. Surprisingly, the LWP of a single family (L4) responded differently to silvicultural treatment under contrasting planting densities. Findings from this study indicate that considerable opportunity exists to improve corewood stiffness in juvenile loblolly pine plantations through the deployment of selected genotypes in combination with appropriate initial planting density and silvicultural intensity.