Nutrient concentrations and contents, and their relation to stem growth, of intensively managed Pinus taeda and Pinus elliottii stands of different planting densities

Foliar, stem, and fine root nitrogen (N), phosphorus (P), and potassium (K) concentrations were measured, and their contents calculated, to determine the relationship between nutrient acquisition and stem biomass increment on a stand basis in 4-yr-old pine plantations planted at different densities. The study examined stands of loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm.) in the lower Coastal Plain of the southeastern United States that were intensively managed (i.e., received bedded site preparation, a high level of fertilization, and nearly complete weed control). The stands were planted at densities of 740, 2,220, and 3,700 trees ha^-1 on three sites, each with a different soil type. Increases in stem biomass growth on a stand basis were not proportional to increases in stand density, indicating that competition for resources was limiting growth at the higher densities. Foliar N and P, stem wood N and K, and fine root N concentrations decreased with increased stand density. For loblolly pine, foliar N concentrations fell from 13.1 mg g^-1 at 740 trees ha^-1 to 10.9 mg g^-1 at 3700 trees ha^-1 (average of current-year and 1-yr-old foliage), the latter considered below a critical threshold concentration for maintaining high growth rates. Slash pine foliar N concentrations followed a similar pattern, decreasing from 11.2 to 9.1 mg g^-1. In both species, foliar P and K concentrations remained above critical concentrations at all planting densities. Overall, foliar N concentration was negatively correlated to stem biomass increment on a stand basis (r= -0.57) whereas folia r N content of 1-yr-old foliage was positively correlated with stem biomass increment (r = 0.59). However, biomass of 1-yr-old foliage was better correlated to total stem biomass growth (r = 0.76) suggesting that amount of foliage, rather than its nutrient content, was a better estimator of growth.