Production efficiency of loblolly pine and sweetgum in response to four years of intensive management.

We tested the hypothesis that productivity of intensively managed loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.) stands is dependent not only on leaf area, but also on foliar photosynthetic rate. Effects of irrigation (irrigation treatment), irrigation with a fertilizer solution (fertigation treatment), and fertigation + pest control (loblolly pine only; fertigation + pest control treatment) on leaf physiology and growth were compared with control plots during the third and fourth growing seasons. Complete weed control was maintained on all plots. Aboveground net primary productivity of loblolly pine and sweetgum increased from 16.3 to 40.5 Mg ha(-1) and from 4.2 to 23.9 Mg ha(-1), respectively, in response to the most intensive treatment. Relative to the control treatment, neither fertigation of sweetgum nor fertigation + pest control of loblolly pine had a significant or consistent influence on foliar N concentration, quantum yield, carboxylation efficiency, net photosynthesis, stomatal conductance, or production efficiency (increment in woody biomass per unit leaf area). Irrigation increased predawn leaf water potential and photosynthesis of loblolly pine, but it had no effect on production efficiency. Leaf area was the predominant determinant of maximum productivity in these rapidly growing plantations.