Description: This paper quantifies roots down to 1 m depth in two study-sites of a short-fallow multi-cycle shifting-cultivation system in eastern Amazonia. Root biomass was 38.7 Mg ha −1 in the 3-year-old fallow-site and 12.7 Mg ha −1 in the 1.5-year-old cultivation-site, corresponding to 55.9 % (fallow-site) and 65.1 % (cultivation-site) of total ecosystem biomass. Root distribution was shallow, with 33–41 % of all roots and 47–55 % of fine roots concentrated in the top 10 cm. Cultivation caused large reduction of topsoil fine-root biomass. We specially focus on roots of the ruderal babassu palm, very widespread and increasingly dominant in degraded lands throughout deforested Amazonia. Whereas babassu-shares in total root biomass (14.9–25.2 %) were similar to aboveground biomass-shares, fine-root shares were substantially higher (29.0–44.1 %), pointing to great competitive strength of this palm. Fine roots of babassu and of all other vegetation were closely correlated, suggesting that both occupy the same space within the study sites and throughout the soil profile, contrary to hypotheses of babassu vertical resource complementarity or deepsoil nutrient-pumping. Babassu coarse-roots were markedly deeper than those of remaining vegetation, a serious obstacle for density-control efforts. This study indicates that intensification of shifting-cultivation strongly increases the role of roots, with high coarse root biomass and root:shoot-ratios, and strong plasticity of the fine-root component to environmental changes associated with slash & burn, cultivation- and fallow-phases. Roots are key for the environmental success of ruderal babassu. We find (i) a high biomass-share in fine roots which guarantees high competitivity in soil resource acquisition, and (ii) significant coarse root stocks in the subsoil, well protected reserves which guarantee vigorous resprouting in multi-cycle slash & burn.