1. Introduction
In natural ecosystems, increased plant species diversity has been shown to increase net primary productivity (Tilman et al. 1996). Two possible explanations have been proposed to explain this; the sampling effect hypothesis and the complementarity effect hypothesis. Multispecies systems may include highly productive species that dominate the community (Hector, 1998), leading to what is known as the “sampling effect”. The likelihood of including species that contribute disproportionately to overall community productivity increases as the number of species in the community increases (Loreau, 1998; Gastine et al., 2003). Therefore, an increase in total community productivity may be due to one or few dominant species rather than the biological interactions underlying the complementary effect hypothesis.