Below-ground characteristics of five Populus clones, belonging to different species and parentages, were studied during the second growing season of the third rotation of a high-density coppice culture. Size (length, area and volume), biomass, nitrogen and carbon concentrations of three classes of fine roots (diameter classes of 0-1, 1-2 and 2-5 mm) were determined for four different soil layers. Fine root biomass varied significantly among clones and among soil layers. Clone Primo (Populus deltoides x Populus nigra) had the highest root biomass and the longest fine roots, while clone Hazendans (Populus trichocarpa x P. deltoides) had the lowest root biomass and shortest fine roots. The topsoil layer (0-5 cm) was very rich in fine roots; the fine root biomass and distribution of all clones decreased with increasing soil depth. Fine root area index (diameter classes of 0-1 and 1-2mm) varied among clones, with higher values for clones Wolterson and Primo (3.6 and 3.7, respectively), while clones Hazendans and Columbia River had lower values (1.7 and 2.2, respectively). The absence of a significant correlation between fine root traits and above-ground biomass leads to the conclusion that above-ground biomass was not a reliable indicator of below-ground biomass in poplar, probably because of the age of the plantation in our study (stump age of 10 years). Fine root area index was positively correlated with leaf area index for all clones and at all soil depths, i.e., clones with a high fine root area index also had a high leaf area index. We conclude that leaf area index can be an indicator of root area. (C) 2007 Elsevier Ltd. All rights reserved.