INFLUENCE OF INTENSIVE CULTURE ON LEAF NET PHOTOSYNTHESIS AND GROWTH OF SWEETGUM AND LOBLOLLY-PINE SEEDLINGS

The influence of (1) complete weed control only, (2) weed control and irrigation, (3) weed control and irrigation with a fertilizer solution (fertigation), and (4) weed control and fertigation plus pest suppres sion (pine only) on midday light-saturated leaf net photosynthesis (P- n) of sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus t aeda L.) was examined monthly, two growing seasons after plantation es tablishment. Irrigation but not fertigation increased P-n on all dates and more than doubled height; and diameter of sweetgum after the firs t growing season. Greater P-n in sweetgum in response to irrigation wa s, in part, a result of reductions in stomatal limitation to P-n and d ecreased stomatal sensitivity to vapor pressure deficit. In loblolly p ine, P-n was increased only by irrigation and only on one date during the first growing season. No effect of treatment on P-n of either spec ies was observed for the second season. After two growing seasons, an eight-fold increase in aboveground biomass and six-fold increase in le af area were observed in sweetgum in response to fertigation relative to the control treatment. Fertigation plus pest control treatment elim inated tip moth damage to shoots of loblolly pine during the first gro wing season and resulted in a doubling of total biomass and tripling o f leaf area. Maximum biomass accumulation in sweetgum and loblolly pin e after two seasons of intensive culture was mainly a function of leaf area rather than P-n.