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Questions To what extent do fine‐scale substrate variations affect the structure and diversity of rock outcrop vascular plant communities? How can we define the appropriate scale to measure fine‐scale substrate heterogeneity in rocky systems? Location Nine rock outcrops in the Iron Quadrangle, a priority area for biodiversity conservation in southeast Brazil. Methods Geomorphology and rock mechanics methods were adapted to determine the appropriate scale to measure rock surface heterogeneity on granitoid, quartzitic and ironstone (canga) outcrops. Then, a roughness index was calculated from 60 microtopographic profiles in each lithotype to investigate the response of the plant community to this scale. The relation between surface heterogeneity and plant community richness, abundance, total plant cover and dominant species cover was quantified through linear regressions. A cluster analysis compared the percentage cover of functional groups in each outcrop, and diversity profiles were built based on Rényi's alpha. PERMANOVA was used to test for significant difference in the relative cover among the functional groups. Results The roughness index, scaled at 1 cm, had the best power to diagnose the presence of vascular plants. High surface heterogeneity was caused by high frequency of fissures, cracks and rock fragments. Linear regression models indicated that the community parameters are strongly related to variations in surface roughness. The community patterns revealed by the cluster analysis and the diversity profiles matched those of the roughness analysis. Dominant functional groups differed strongly between ‘smooth’ and ‘coarse’ microtopography. Desiccation‐tolerant species prevailed in smooth outcrops, while sclerophytes and graminoids were better represented in coarse outcrops. Conclusions The composition of plant functional groups was affected by rock microrelief, and roughness was a significant predictor of plant community parameters. Functional group proportions reflected different adaptive plant strategies to two main stressful bedrock features: water shortage and mechanical resistance to root growth.

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