Effects of intensive fertilization, complete competition control and site quality on aboveground net primary production (ANPP) dynamics of loblolly pine plantations

Abstract

Changes in aboveground net primary production (ANPP) are relevant to forest management, ecosystem ecology and carbon budget. It is believed that ANPP reaches a maximum in young forest stands and then gradually declines as stands age. Intensive silvicultural treatments such as competing vegetation control and fertilization have been widely used in pine plantations in the southern US and have shown large early growth gains. The effects of site quality and silvicultural treatments on long-term patterns of ANPP in pine plantations remain to be studied. These effects were rigorously tested by examining their effects on the parameters of an ANPP-age model for loblolly pine, using 33-year data from a large field experiment containing a factorial arrangement of complete competition control (H) and intensive fertilization (annually until age 12 years and then biennially until age 28 years). Site quality and silvicultural treatments significantly affect the magnitude and timing of maximum ANPP; silvicultural treatments substantially alter the pattern of ANPP-age relationship, but site quality does not. Under better site conditions loblolly pine stands will reach a greater maximum ANPP at earlier ages. Non-fertilized stands achieve a smaller maximum ANPP and thereafter have no obvious decline in ANPP, while ANPP in intensively fertilized stands quickly reach a greater maximum and then sharply decline to a lower level than that of the non-fertilization stands. The early repeated fertilization enhances growth rates in young stands, but continuous application of fertilization at mid- or late-rotation reduces growth rates in older stands. Our results showed that nutrient limitations are not responsible for age-related declines in ANPP for loblolly pine plantations in the southern US.