The amount of photosynthetic radiation intercepted by a crop is a function of the incident solar radiation on the plants, the leaf area index(L_(AI)), and the light extinction coefficient(k). We quantified L_(AI) and k in stands of black wattle(Acacia mearnsii De Wild.) over a 7-year growth cycle at two locations in the state of Rio Grande do Sul, Brazil. Our study was conducted in commercial stands in agroecological regions with high densities of black wattle plantations. L_(AI)was calculated as the ratio between the leaf area of a tree and its planting space, and k was derived from Beer’s law. L_(AI)depends on the planting site and stand age. Between the two sites, the L_(AI) was similar over time, the amount of variation differed. Values of k depended only on stand age, with the highest average observed for stands up to 5 years old. The trend of k during the plantation cycle was inversely proportional to L_(AI)and was correlated with L_(AI), leaf area, leaf dry mass, canopy volume, height, branches dry mass, total dry mass, and crown diameter. The amount of photosynthetic radiation intercepted by a crop is a function of the incident solar radiation on the plants, the leaf area index (L_ (AI)), and the light extinction coefficient (k). We quantified L_ (AI) and k in stands of black wattle (Acacia mearnsii De Wild.) over a 7-year growth cycle at two locations in the state of Rio Grande do Sul, Brazil. Our study was conducted in commercial stands in agroecological regions with high densities of black wattle plantations. L_ (AI) was calculated as the ratio between the leaf area of ​​a tree and its planting space, and k was derived from Beer’s law. L_ (AI) depends on the planting site and stand age. Between the two sites, the L_ ( AI) was similar over time, the amount of variation differed. Values ​​of k depended only on stand age, with the highest average observed for stands up to 5 years old. The trend of k during the plantation cycle was inversely proportional to L_ (AI ) and was correlated with L_ (AI), leaf area, leaf dry mass, canopy volume, height, branches dry mass, total dry mass, and crown diameter.