Vegetation control and fertilization in midrotation Pinus taeda stands in the southeastern United States

Abstract

We quantified Pinus taeda L. plantation response to vegetation control (VC) applied using site specific methods including chemical (glyphosate, imazapyr, metsulfuron methyl, and triclopyr) and mechanical means and nitrogen and phosphorus fertilization on a variety of sites ranging in age from ten to twenty-two years old at treatment initiation in the Piedmont and coastal plain of the southeastern United States. We examined pine and hardwood (the primary competing vegetation) basal area and pine volume and foliar nutrient responses in a 2 x 2 factorial combination of a one time application of VC and fertilization in a randomized complete block design with three or four replications at each site. Vegetation control reduced hardwood vegetation at least 70% at all sites. On average, annual pine volume growth response was greatest on the combined treatment (6.1 and 11.0 m3 ha-1 yr-1) followed by fertilization alone (5.5 and 7.9 m3 ha-1 yr-1) and then VC alone (1.1 and 4.5 m3 ha-1 yr-1) for years one and two and years three and four, respectively. The range in pine volume response across all treatments for the sites examined here was -3 to 12 m3 ha-1 yr-1. There may be sites, not represented here, with greater water deficits, more competing vegetation, or where nitrogen and phosphorus are not the primary limiting factor that would be more responsive to VC. Fertilization alone did not significantly affect hardwood basal area at year two or four and the proportion of hardwood vegetation (as basal area) was about the same before treatment (12%) and up to four years after treatment (11%) in the check and fertilized treatments. We hypothesize that the combined treatment may provide the best pine response in later measurement periods as fertilization responses diminish because added nutrients are utilized and VC responses increase from increased nutrient and moisture availability to the pines provided the competing vegetation does not recover.