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In Brazil, the semi-arid tropical zone comprises approximately 12% of its territory. In this region the Caatinga is the only exclusively Brazilian biome. Nonetheless, there is a lack of knowledge about its structure and functioning. This study aimed at assessing the effective depth of the root system of the preserved Castings biome, as well as its temporal and spatial variation. To this end, 72 soil profiles were evaluated, both in the dry and rainy seasons, including three soils (Acrisol, Luvisol and Regosol) and four profiles per trench, with three replications. The results showed that the effective depth of the Caatinga roots range from 0.60 m to 0.78 m in non-restrictive deep soils, three times smaller than values commonly used in hydrological models. Vegetation adapted to shallow soils, presenting effective root depth as low as 0.36 m. The effective depth of the roots showed spatial homogeneity, for each soil-vegetation association, regardless of the proximity of large trees. It was also observed that the root depth in the dry season is, on average, 10 cm shorter than that of the rainy season. This can indicate an adaptation strategy, generating soil secondary porosity to enhance infiltration in the root zone. (C) 2012 Elsevier Ltd. All rights reserved.