A high density, intensively managed held trial with 17 different poplar (Populus) clones was established in April 1996 in Boom (province of Antwerpen, Belgium) on a former waste disposal site. All plants were cut back at the end of the establishment year and the above ground biomass production, as well as carbon concentration were measured. Significant clonal variation was observed in overall mortality rates and length of the growing season. Mortality rates ranged from 2 to 55% due to difficult soil conditions. Based on incoming available photosynthetically active radiation, length of growing season and above ground biomass production, conversion efficiency ratios were calculated. These were rather low, ranging from 0.002 to 0.041 g MJ(-1), with the highest values calculated for the poplar clone Hoogvorst. The carbon concentration of the wood ranged from 46 to 47%, with 3.2 to 5.5% remaining in the ash. Net photosynthesis measurements were made in the field during mid-summer and revealed significant clonal differences in maximum photosynthetic rate and carboxylation efficiency. Clonal differences in net photosynthetic rate, length of growing season, mortality and biomass production rate are relevant to overall carbon uptake rates of short rotation biomass plantations.