Loblolly pine growth response to herbaceous vegetation control at different planting densities

Growth of Pinus taeda L., with and without control of competing herbaceous vegetation, was evaluated over 15 years at planting densities ranging from 747 to 2990 trees/ha. A height response to vegetation control of 0.9 m at age 5 decreased to 0.3 m at age 15. Convergence in the cumulative height between treated and untreated stands did not translate into converging basal area and volume yield. For 1500 and 2200 trees/ha, volume response to early vegetation control increased to age 15. For 800 trees/ha volume gains increased to age 12 and were stable between ages 12 and 15. Higher densities apparently captured more of the site resources made available through vegetation control. Age-15 volume gains for 2200, 1500, and 800 trees/ha were 71 (23%), 48 (17%), and 19 m3·ha-1 (8%), respectively. A separate adjustment term in height and basal-area models captured the short-term positive effect of vegetation control on height growth and longer term basal-area responses. Previously published height and basal-area models were modified to account for planting-density effects.