Belowground structure and annual production in an open Mediterranean scrub were studied on a sandy substrate which had not been harvested or exposed to fire during the last 30–40 years. Estimated belowground biomass (1328 ± 93.4 g m−2) and production (548 ± 246.9 g m−2 yr−1) were lower than in other Mediterranean scrubs. However, the energy investment in belowground structures was high (root biomass/ shoot biomass = 2.7; annual belowground production/ annual aboveground production = 7.1), which is associated with a high density of fine roots in the top soil (the average distance between the fine roots = 0.76 cm). A very simple model based on nutrient diffusion was considered to analyse the resource constrains of the community. The results underline the importance of nutrients (and more specifically phosphorus) rather than water, as possible determinants of the structure and dynamics of the root system, as well as for the primary production of the community during the vegetative period.