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Background: Discrepancies in the shape of the productivity-diversity relationship may arise from differences in spatial scale. We hypothesised that there is a grain size effect on the productivity-diversity relationship.Aims: To determine the effect of three sampling grain sizes on the productivity-diversity relationship.Methods: We applied generalised linear mixed effect models on community data from 735 vegetation plots in the Taleghan rangelands, Iran, sampled at three grain sizes (0.25, 1 and 2m(2)) to ascertain plant productivity-diversity patterns, while accounting for the effects of site, plant community type, disturbance, and life form.Results: Overall, relationships between biomass and plant species richness were unimodal at grain sizes of 0.25 and 1m(2), and asymptotical at 2m(2). The spurious occurrence of a single large shrub may overwhelm a small-sized sampling unit, resulting in a high estimate of the sample’s biomass relative to species richness. However, the relationship between biomass and species richness at larger grain sizes is more likely to reach an asymptote.Conclusions: Shrubs are partly responsible for driving the relationship between plant biomass and species richness. Given that the frequency of shrubs is highly variable between small plots but not so in large plots, their presence may result in unimodal productivity-diversity relationships at small but not at large grain sizes.