The construction cost of fine roots was studied in 23 woody species and two grassland communities, growing under natural conditions in southern Spain. Calculation of the energy (glucose) required for their synthesis was based on the quantification of chemical components present in tissues. Despite considerable differences in the chemical composition of the three life forms studied (trees, shrubs and herbaceous), detected differences in construction cost were non-significant (mean value: 1·64 ± ± ± ± 0·13 g glucose g− 1 ). However, shrubs and herbaceous plants growing in more fertile habitats expended significantly less energy on root synthesis (1·58 ± ± ± ± 0·06 and 1·41 ± ± ± ± 0·05 g glucose g− 1 , respectively) than those growing in less fertile areas (1·80 ± ± ± ± 0·06 and 1·57 ± ± ± ± 0·1 g glucose g− 1 , respectively), because they contained smaller amounts of either waxes (shrubs) or lignins (herbaceous), both expensive to synthesize, and, proportionately, more cellulose; which is inexpensive to synthesize. Deciduous and evergreen tree species also differed mainly with regard to wax and cellulose contents, giving rise to a significantly higher construction cost in evergreens (1·57 ± ± ± ± 0·07 g glucose g− 1 versus 1·78 ± ± ± ± 0·02 g glucose g − − − − 1 ). The differences observed in construction cost appeared to be due more to habitat-induced differences in chemical composition than to any intrinsic difference between the species studied.