Intraspecific trait variation (ITV), based on available genetic diversity, is one of the major means plant populations can respond to environmental variability. The study of functional trait variation and diversity has become popular in ecological research, e.g. as a proxy for plant performance influencing fitness. Up to now, it is unclear which aspects of intraspecific functional trait variation (iFDCV) can be attributed to the environment or genetics under natural conditions. Here, we examined 260 individuals from 13 locations of the rare (semi-)dry calcareous grassland species Trifolium montanum L. in terms of iFDCV, within-habitat heterogeneity, and genetic diversity. The iFDCV was assessed by measuring functional traits (releasing height, biomass, leaf area, specific leaf area, leaf dry matter content, Fv/Fm, performance index, stomatal pore surface, and stomatal pore area index). Abiotic within-habitat heterogeneity was derived from altitude, slope exposure, slope, leaf area index, soil depth, and further soil factors. Based on microsatellites, we calculated expected heterozygosity (He) because it best-explained, among other indices, iFDCV. We performed multiple linear regression models quantifying relationships among iFDCV, abiotic within-habitat heterogeneity and genetic diversity, and also between separate functional traits and abiotic within-habitat heterogeneity or genetic diversity. We found that abiotic within-habitat heterogeneity influenced iFDCV twice as strong compared to genetic diversity. Both aspects together explained 77% of variation in iFDCV (Radj² = 0.77, F2, 10 = 21.66, p < 0.001). The majority of functional traits (releasing height, biomass, specific leaf area, leaf dry matter content, Fv/Fm and performance index) were related to abiotic habitat conditions indicating responses to environmental heterogeneity. In contrast, morphology-related functional traits (releasing height, biomass and leaf area) were only related to genetics. Our results suggest that both within-habitat heterogeneity and genetic diversity affect iFDCV, and are thus crucial to consider when aiming to understand or predict changes of plant species performance under changing environmental conditions.