Five-Year Volume Increment From Nitrogen Fertilization in Thinned Plantations of Pole-Size Loblolly Pine

Abstract Nitrogen was applied at 0, 112, 224, 448, and 672 kg/ha on two pole-size loblolly pine plantations in the Coastal Plain of South Carolina and at 0, 168, and 336 kg/ha on one in the Virginia Piedmont. The plantations had been thinned once and basal areas ranged from 19.9 to 23.8 m²/ha. The relation between increase in 5-year volume increment from fertilization and initial basal area (IBA) and N rate was estimated with a regression model which indicates that small variations in IBA can mask responses to fertilization. Estimated 5-year increments from 224 kg of N/ha were 14 m³ of wood for a Coastal Plain site low in total soil N, 17 m³ for a Piedmont site low in N, and 2.1 m³ on a Coastal Plain site high in N. The 672 kg/ha rate of N produced a smaller response than the 448 kg/ha rate on the Coastal Plain site low in N. On the Coastal Plain site high in N, applying 672 kg/ha N reduced volume increment. Results show that IBA must be carefully controlled in studies of stand response to N fertilization. Forest Sci. 22:85-90.