Response of a young loblolly pine ( Pinus taeda) plantation to herbicide release treatments using hexazinone

Abstract

In March 1980, a study was initiated on a site in the Piedmont of Alabama to examine the effect of hexazinone rate and formulation in releasing newly planted and 1-year-old loblolly pine in a dense stand of hardwoods (4.6 m 2 ha −1 groundline basal area, 17 100 rootstocks ha −1). Treatments included a factorial combination of three hexazinone formulations (1 cc 10% a.i. pellet, 1 2 cc 10% a.i. pellet, 20% a.i. granules) at three rates (1.12, 1.68, and 2.24 kg a.i. ha −1), plus an untreated control (10 treatments). Hardwood density following treatment decreased with increasing herbicide rate. Reduction in hardwood density was similar for the two pelleted formulations and lower for the granules. High rainfall amounts soon after application (13.2 cm within 10 days) appeared to have washed some herbicide from the granule-treated plots prior to uptake by hardwoods, possibly causing lower reduction of hardwoods relative to the pellets. Effects of treatment on pine mortality differed between newly-planted (interplants) and 1-year-old (original) pines. This is likely due to differences in root system development and overall size between the two classes of pines at the time of treatment, and associated differences in both the ability to compete with hardwoods for site resources and the likelihood of uptake of a fatal dose of hexazinone. Five growing seasons after treatment, mean pine size (height, groundline diameter, and dbh) differed among treatments for both interplanted and original pines, with size increasing with increasing hexazinone rate. Mean tree volume of pines was more strongly correlated with hardwood groundline basal area than with any other measure of hardwood density ( r = −0.61 for interplanted pines ( P < 0.001), r = −0.51 for original pines ( P < 0.001)) five growing seasons after treatment.