Understanding the determinants of forest understory diversity provides a theoretical basis for sustainable management practices. Our study aimed to identify the dominant factors determining understory diversity in temperate forests among three potentially important factors: management type (managed vs unmanaged), stand attributes (basal area by species and by diameter class, shrub layer cover) and soil micro-environment (soil compaction and soil moisture). Understory diversity was represented by the richness and abundance of plant ecological groups based on life form crossed with successional status, light and moisture requirements. We selected 50 stands in five French national forests, half of which were managed and half unmanaged. After model comparison, our results showed that, depending on the ecological groups studied, basal area and micro-environment rather than management type were the best explanatory models for understory diversity. Our magnitude analyses showed that understory diversity did not differ much between managed and unmanaged forests. Basal area, which had ecologically important effects, negatively correlated with understory diversity. For two ecological groups (mature-forest and shade-tolerant woody species), diversity was better explained by basal area in interaction with management type than by the sole effect of basal area. Soil micro-environment, which had ecologically important effects, mostly showed positive relationships with understory diversity; e.g. hygrophilous and intermediate-light herbaceous species positively correlated to soil moisture, while heliophilous herbaceous species were positively correlated to soil compaction. Our study indicated that forest management abandonment after 20-40 years did not induce higher understory diversity than in managed forests; abandonment's role in understory diversity was weak compared to basal area or soil environment. Whether such results persist after longer periods of abandonment remains unknown.