Aims Forest-steppe and sub-taiga, two main biomes of southwestern Siberia, have been predicted to shift and spread northward with global change. However, ecological projections are still lacking a description of belowground processes in which fine roots play a significant role. We characterized regional fine root patterns in terms of length and mass comparing: 1) sites and 2) vegetation covers. Methods We assessed fine root length and mass down to one meter in aspen (Populus tremula) and in grassland stands on six sites located in the forest-steppe and sub-taiga zones and presenting contrasting climate and soil conditions. We distinguished fine roots over diameter classes and also between aspen and understorey in forest. Vertical fine root exploration, fine root densities and total length and mass were computed for all species. Morphological parameters were computed for aspen. Results In both forest and grassland, exploration was deeper and total length and mass were higher in forest-steppe than in sub-taiga. Exploration tended to be deeper in forest than in grassland and for trees than for understorey vegetation within forest stands. Conclusions The differences in rooting strategies are related with both pedo-climatic conditions and vegetation cover. Further investigations on nutrient and water availability and on fine root dynamics should permit a better understanding of these patterns and help predicting their future with global changes.