Establishment is critical for community assembly, but mechanisms regulating establishment can be obscured by covarying influences of incoming seed diversity and fine-scale spatial environmental heterogeneity (microhabitat heterogeneity). Both can maximize establishment, depending on whether species differences or environmental variability more fundamentally structures plant communities.2. We experimentally assembled limestone-pavement herb communities to examine the relative effects of seed diversity and microhabitat heterogeneity on establishment. This included testing (i) whether effects of seed diversity strengthen with heterogeneity, as would be expected if potential niche differences are more strongly expressed in more heterogeneous environments and (ii) whether a greater number of incoming species can establish in more heterogeneous environments due to environmental filtering. Species interaction theory predicts that increased facilitation and niche complementarity with realized diversity has the potential to increase overall community density.3. Heterogeneity operated independently of seed biodiversity and explained establishment even when spatially averaged microhabitat conditions were accounted for. Homogeneous plots sown with six species supported establishment of plant density that could be increased c. 1% by doubling added seed diversity (while holding heterogeneity constant), but increased c. 4% by holding seed diversity constant and maximizing heterogeneity.4. Ordinations revealed that species establishment was sorted by gradients in soil pH and surface cover by moss, litter and open bedrock. Regressions indicated more species established in plots featuring a greater diversity of surface cover types and/or greater soil depth variability. Community density increased with established richness, and heterogeneity ceased to explain variance in density once established richness was included as an explanatory variable. All but one of the sown species exhibited increased population density with increased plot richness.5. Synthesis: Community density in a high-stress environment increased with both fine-scale spatial heterogeneity and added seed diversity. However, these effects were independent of one another, and impacts of heterogeneity were stronger than those of seed diversity. Our results suggest heterogeneity promotes density indirectly, through downstream effects of enhanced establishment diversity such as facilitation. These findings confirm establishment-stage interrelationships among biodiversity, density and heterogeneity as overlooked determinants of community structure by providing important field support for ideas primarily tested in the greenhouse.