Leaf area index was estimated in 6 yr-old slash (Pinus elliottii Engelm. var. elliottii) and loblolly pine (P. taeda) plantations, affected by intensive fertilization and weed control. Three indirect approaches were applied: two yr needlefall production, dimensional analysis and canopy light interception measurements. Similar results were obtained with all 3 methods, with leaf area index ranging from 2.2 to 5.6 for slash pine and 1.5 to 10.3 for loblolly pine. Estimates of monthly variation in leaf area, using the canopy interception technique, were not accurate due to the clumpiness of these pines foliage. Needlefall nitrogen turnover, used as index of soil nutrient condition, explained the 97% of the leaf area variation. Aboveground net primary production (needlefalls mass + annual biomass increment) was positively related to foliar surface. This relationship implied a higher efficiency of loblolly pine (i.e. greater net primary production for the same leaf area index) compared with slash pine.