Hidden diversity and productivity patterns in mixed Mediterranean grasslands

A detailed study of the variation in productivity across a diversity gradie nt in an experimental Mediterranean grassland examines the effects of a dom inant perennial grass species upon the overall diversity-productivity relat ionship. The experiment took place at the Greek site of the European-wide B IODEPTH programme. The experimental design is characterized by the use of a number of communities containing annuals and perennials within the total s et of manipulated plots. The main results are: 1) a log-linear relationship between diversity and productivity exists in Mediterranean grasslands synt hesized by annuals only, 2) in mixed communities where multiple growth form s coexist, the performance of a dominant or keystone species may reverse or hide the diversity-productivity pattern of a functional or growth form gro up of species taken separately, and 3) the introduction of the dominant gra ss in the low-diversity mixtures creates an 'inverted' sampling effect whic h can produce as an artefact a constant productivity response across the di versity gradient.