Leaf area is one of the most important biophysical characteristics of a plant, as it determines how much photosynthetic radiation is intercepted so that the photosynthesis can occur. The presented research aim was to generate numerical indicators of leaf area and to study the behavior of black wattle (Acacia mearnsii De Wild.) throughout a plantation cycle (7 years), relating them with environmental variables. The study was conducted in commercial stands located in agroecological regions with high densities of black wattle plantations in the state of Rio Grande do Sul, Brazil. The leaf area was obtained by measuring leaf biomass and leaf area with an integrator (3000 Canopy Analyser, Li-Cor). Leaf area was found to be strongly related to leaf biomass, and is not influenced by planting site, but rather by stand age. Values ​​of individual leaf area vary depending on stand age and plantation area. The leaf area is related to soil properties and particularly with the phosphorus content. It is also related to meteorological conditions, most notably accumulated solar radiation. The obtained model, which involves global accumulated solar radiation, phosphorus and clay content, adequately explain variations in leaf area. Key words: Global solar radiation, phosphorus, leaf biomass, black wattle, plantation cycle.