Factors influencing production efficiency of intensively managed loblolly pine plantations in a 1- to 4-year-old chronosequence

Changes in biomass, nutrient accumulation and production efficiency (PE-total biomass increment per unit leaf area) were investigated in intensively managed plantations of loblolly pine (Pinus taeda L.) using an age sequence of replicated 1-, 2-, 3-, and 4-year-old stands (n = 13). All stands, located on sandy Spodosols in the Coastal Plain of southern Georgia, were managed using a similar prescription that included a common genetic source, fertilization (years 1 and 3) and understory competition control. Total biomass accumulation (above- and below-ground) ranged from about 13 Mg ha−1 at age 2 years to 49.7 Mg ha−1 at age 4 years. Dry matter distributions averaged 30%, 34%, 18%, 6%, 15%, 12% and 3% for foliage, stemwood, branches, bark, taproots, coarse roots and fine roots, respectively, at age 4 years. Intensive management and rapid growth rates markedly increased soil nutrient demands, being about 15-fold higher than that documented in extensively managed stands of comparable age. Nutrient accumulations in tree biomass at age 4 years averaged 195, 22, 86, 72 and 28 kg ha−1 for N, P, K, Ca and Mg, respectively. Large declines (∼50%) in PE (2.6 Mg ha−1 year−1/unit versus 1.3 Mg ha−1 year−1/unit LAI) were apparent among all sites between the second and third years, with a noted recovery occurring at age 4 years. Changes in branch and taproot PE were most apparent, whereas stemwood PE did not change appreciably between ages 2 and 3 years. Correlation analyses suggested that larger declines in PE were associated with decreasing foliar nutrient levels (dilution), although changes in growth dynamics (carbon allocation) associated with advancing stand development may have also contributed. Collectively, these results suggest that intensive management may induce multiple nutrient limitations on sandy Spodosols, and that a better understanding of nutrient requirements, including macro- and micronutrient supply, will be necessary to maintain and enhance soil quality and long-term site productivity on these soils.