NEEDLEFALL, CANOPY LIGHT INTERCEPTION, AND PRODUCTIVITY OF YOUNG INTENSIVELY MANAGED SLASH AND LOBLOLLY-PINE STANDS

Canopy dynamics, light interception, and productivity of 6-yr-old slash (Pinus elliottii var. elliottii Engelm.) and loblolly pine (P. taeda L.) were investigated using a 2 x 2 x 2 factorial experiment (species, annual fertilization, sustained weed control) in north central Florida. The strong nutritional gradient imposed by the cultural treatments significantly accelerated canopy development. Needlefall (NF) mass for the weed control + fertilization treatments was increased over the checks by about 400% (0.8 to 4.0 Mg ha-1) for slash pine and 1050% (from 0.4 to 4.6 Mg ha-1) for loblolly pine. Levels of annual NF were strongly correlated (r2 > 0.90) with stand basal area, and cumulative NF through time was successfully modeled using a logistic function. Cultural treatments had no significant effect on needle longevity or temporal NF patterns; however, large treatment-related responses in leaf area index (LAI; all-sided) were apparent for both species (slash pine = 1.5 to 7.2 m2 m-2; loblolly pine = 1.0 to 10.6 m2 m-2). Mean annual light interception (photosynthetically active radiation) for the check and combination treatments paralleled LAI responses and ranged from 28 to 74% for slash pine and 22 to 81% for loblolly pine. Significant species differences in aboveground biomass production (loblolly pine = 3.1 to 16.0 Mg ha-1 yr-1; slash pine = 3.5 to 8.0 Mg ha-1 yr-1) were principally due to greater LAI (reflecting differences in specific leaf area and branch structure) and higher light use efficiency (0.81 vs. 0.47 g MJ-1) of loblolly pine.