Dark diversity refers to species that belong to a particular species pool, although they are locally absent from the given sites. To understand the effects of forest coverage and fragmentation on plant species diversity, we examined whether coverage and fragmentation influence the dark diversity of plant communities in forests under different management practices. We compiled a comprehensive database to quantify the observed diversity, dark diversity, and community completeness of managed forests. We used regression models with forest management practice type as a random factor to determine the relationships among observed diversity, dark diversity, and community completeness with forest coverage and fragmentation. We detected significant associations between forest coverage and fragmentation and the observed diversity, dark diversity, and community completeness of forests, although these associations tended to be weak on a global scale. Forest management practices play a crucial role in determining the effects of forest coverage and fragmentation on dark diversity, offering valuable insights into biodiversity conservation and ecological restoration management. Increased forest coverage initially declined and subsequently increased the observed diversity and community completeness in tropical plantation forests (rotation <= 15 years) in both 2000 and 2020. Conversely, in both assessed years, the increasing forest fragmentation tended to result in initially increasing and then decreasing observed diversity, dark diversity and community completeness in tropical plantation forests (rotation <= 15 years). Forest fragmentation had a positive effect on the dark diversity in temperate plantation forests (rotation >15 years). The contrasting effects of forest fragmentation on dark diversity between tropical plantation forests (with rotation <= 15 years) and temperate planted forests (with rotation >15 years) have different management implications for the conservation and restoration of plant diversity. Consequently, dark diversity and community completeness can be used to predict changing trends in species diversity of plant communities, using forest coverage and fragmentation as direct indicators and measures for biodiversity conservation and environmental management, depending on the type of biome and rotation time.