The Three Treatment Hybrid Poplar biomass experimental plantation was established at Lajta-Hansagi State Farm (Mosonmagyarovar, Hungary) in 1981. The first treatment applied was fertilization by three doses of complex fertilizer and a non fertilized control. The second treatment was four European hybrid poplar clones, and the third, three narrow spacing. The design randomization was split-split-plot blocks with two exact replications. Height and Breast Height Diameter (DBH) were measured annually, and sample trees were cut. Some blocks were clear cut at age four, and height and DBH were measured in trees in remaining blocks until age nine. There were no significant differences among the treatments of fertilizers until age four. It was because of permanent water deficiency in the soil. There were significant differences among biomass production of the clones and also the spacings. Always the narrowest spacings gave the biggest amount of biomass at every clone until age four, except the ''Blanc du Poitou'' French clone at age four. However, spacing was not constant by time in the experiment, because the mortality (self-thinning) reduced the number of trees. As data were saved in matrixes by the situation of trees in the blocks, the actual growing space was calculated (space potentially available) of every tree at every age. The asymptotic Baule-Mitscherlich function was found to fit well on biomass data of single trees at every clone, although its asymptotic value changed in time. The Chapman-Richards function was found to be suitable for computing asymptotic values in Baule-Mitscherlich function. In this way a two independent variable three-dimensional function model, as a 3D response surface, was developed for modeling biomass production of single trees in hybrid poplar energy plantations. The single tree response surface multiplied by frequency or distribution of trees by growing space and age is a new basis for computing stand biomass production. Copyright (C) 1996 Elsevier Science Ltd.