Response of Midrotation Pine Stands to Fertilizer and Herbicide Application in the Western Gulf Coastal Plain

Abstract

Application of nitrogen (N) and phosphorus (P) to midrotation loblolly pine (Pinus taeda L.) stands is a common silvicultural practice used to increase crop-tree volume production in the western Gulf Coastal Plain of Arkansas and Louisiana. Studies in other regions have shown that midrotation herbicide application, with or without an application of fertilizer, can also benefit crop-tree growth. We studied the impact of herbicide (16 oz of imazapyr with 3.2 oz of surfactant ac−1), fertilizer (200 lb of N and 35 lb of P ac−1), and a combined herbicide+fertilizer treatment on pine crop tree and competing woody vegetation growth in three, thinned midrotation stands for 5 years. The density and basal area of woody competition was significantly reduced (73–78%) with an application of imazapyr, but was not affected by fertilization. The application of imazapyr inhibited height growth of the pine crop trees and without an application of fertilizer had little impact on basal area or volume growth during the 5-year measurement period. Fertilizer application increased merchantable volume growth (21.5–25.1 ft3 ac−1 yr−1), but response was low compared to that reported for loblolly pine stands elsewhere in the southern United States. Fertilization increased mortality of smaller, less vigorous pine trees. The combined herbicide+fertilizer resulted in a 20.5% increase in chip-n-saw volume, suggesting that the larger, more vigorous trees in these stands respond most rapidly to the combined herbicide and fertilizer treatments.