Performance of three leguminous species (Acacia farnesiana, A. nilotica subspecies cupressiformis and Cassia siamea) was investigated at three planting densities (10,000, 20,000 and 30,000 plants ha(-1)) on a highly alkaline soil site (pH 8.6-10.5) in order to identify promising species and suitable plant spacing for optimum biomass harvest per unit area under shorter rotation harvests (3 year). The study revealed the differential behaviour of various species with respect to plant growth, survival and stand productivity in different population densities. Performance of A. farnesiana and C siamea in terms of plant height, stem diameter and plant establishment was marginally affected by population density. Stand basal area (2.4-6.4 m(2) ha(-1)) and biomass (4.45-13.5 t ha(-1)) in A. farnesiana increased markedly with increasing population density. Similar gains in biomass were observed in C siamea when planted at higher densities. Individual tree biomass also was not affected by increasing plant densities, suggesting that these two species respond well to high-density plantation. A. nilotica subspecies cupressiformis, on the other hand, showed a negative response when planted in high density. Its biomass and basal area decreased beyond 20,000 plants ha(-1) planting density, suggesting that planting density of 20,000 plants ha(-1) and above were supra-optimal. Plants spaced at 10,000 plants ha(-1) showed faster growth rate and higher productivity as compared to the same at 20,000 and 30,000 planting density. Competition for space also effected individual tree growth in higher densities. The concept of high-density plantation is not applicable in A. nilotica subspecies cupressiformis. However, this species has significantly greater potential since it has relatively high biomass production even at a low population density of 10,000 plants ha(-1). The study is useful in identifying productive species and optimum plantation density per unit area for maximizing gains in terms of biomass productivity in short rotation energy plantation programs on substandard soil sites. (C) 2004 Elsevier Ltd. All rights reserved.