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Root and shoot biomasses and their ratio (R:S) were determined for three stages of forest regeneration (18, 40 and > 60 years.), and for open and dense vegetation, in four soil classes in the semi-arid region of Northeast Brazil.

Shoot biomasses were estimated by allometry and roots were collected in 0.7 x 0.7 x 1 m trenches.

Root and shoot biomasses and the R:S ratio were over double in the > 60 year-old vegetation (R:S = 0.67) when compared to more recent regenerated areas (0.32). In dense vegetation the biomass of roots and shoots were also more than the double of those in open vegetation but the R:S ratios were not significantly different (0.51 and 0.49). Litholic Neosols had lower ratio (0.22) than the other soil classes (0.53 to 0.63) and dense and open vegetation did not differ. In all areas except in deep sandy Quartzarenic Neosols most of the roots (> 90 %) were in the upper 40-cm layer of the soil profile, and consisted of coarse roots.

Root biomass accumulates more slowly than aboveground biomass and it takes several decades to stabilize in shallower soils. The R:S ratios are higher when compared to other dry land forests, probably due to low water availability.