Environmental and biotic factors drive species richness patterns, but the nature of this relationship can vary with sampling grain. Understanding the scale-dependent effects of these factors is crucial for interpreting species richness patterns in ecosystems experiencing rapid environmental change. We investigated the effects of local environmental drivers on plant species richness at small (1 x 1 m) and large (3 x 3 m) sampling grains, and the factors correlated with differences in richness between the two grains, on a sub-Antarctic island. Broadly, richness was higher in warmer (i.e., lower altitude, north-facing) and wetter (i.e., higher topographic wetness index, lower distance from drainage line) sites, and in more topographically heterogenous (i.e., steeper slopes) sites. Additionally, there was some evidence of competition with a keystone plant limiting species richness, though this was only evident at low elevations. However, the effects of several drivers on richness depended on spatial grain. Differences in species richness between large and small grain sizes were more pronounced at low elevations, indicating that there is more compositional heterogeneity at low altitudes at both grains. Richness was positively related to northness at large grain size but not at small grain size, suggesting that higher northness increases local turnover at a grain size > 1 m2. On the other hand, TWI boosted richness at small but not large grains, implying that competition limited coexistence at low TWI, and that higher TWI only resulted in more species coexisting at a grain of 1 m2, while having no effect on richness at large grains. Our study therefore highlights that drivers of plant species richness can vary with sampling grain, suggesting that environmental effects on local species turnover affect richness patterns at different grains. Assessing how the influence of such drivers differ with grain size provides insight into local patterns of species assemblage.