Extensive research has been devoted to understanding the role of biodiversity as a driver of ecosystem functioning. However, no previous study has evaluated the relative contribution of complementarity and selection to productivity in shrublands. We have attempted to do this for a Mediterranean shrubland dominated by Quercus coccifera, Cistus albidus, Ulex parviflorus and Rosmarinus officinalis. We found a highly significant and linear positive relationship between productivity and species richness. No selection effect was apparent, but both the complementarity and net effects were highly significant. The magnitude of these effects increased from two to three species, but became non-significant in the four-species mixtures. Analysis of paitwise interactions revealed that legumes did not promote overyielding. Complementarity was mostly driven by Cistus, which always performed better when growing with other species than when growing with conspecifics. Our results are an addition to the still scarce literature dealing with diversity-productivity relationships in communities dominated by woody species, and show that methodologies commonly used to assess complementarity may not provide a precise estimation when a given species has negative effects on its conspecifics.