Root construction and maintenance costs were estimated in four evergreen and three deciduous Quercus species that are typical in the landscape of southern Spain. The cost quantification was based on analysis of the growthrespiration ratio. Values observed for both construction cost (ranging from 1·17 to 1·29 g glucose g − − − − 1 dry weight) and maintenance cost (ranging from 6·22 to 11·71 mg glucose g − − − − 1 dry weight d − − − − 1 ) were generally lower than those reported in other studies. The results showed non-significant differences between deciduous and evergreen species. The lack of significant differences between species appeared to be due to the homogeneity of growth conditions. Hydroponic culture, with unrestricted nutrient and water supply, would lead to low tissue carbon content and low respiration rates, leading to the low costs observed. Furthermore, the fact that root organs are clearly importers of organic molecules inevitably entails some underestimation of the respiration associated with growth and, to a lesser extent, with maintenance respiration. This leads in turn to underestimation of the corresponding construction and maintenance costs. All this raises doubts as to the suitability of this method for studying root systems.