Stand attributes or soil micro-environment exert greater influence than management type on understory plant diversity in even-aged oak high forests

•Soil compaction and moisture did not differ between managed and unmanaged stands.•Tree basal area or soil micro-environment best indicated floristic group diversity.•When strong, the relationships between basal area and biodiversity were negative.•When strong, the relationships involving soil micro-environment were positive.•Effect of the best indicators on biodiversity sometimes depended on management type.