It is well documented that forest amount in the surrounding landscape increases understory plant species diversity in a forested site. However, the extent to which fragmentation and structural connectivity (wooded corridors linking patches) also influence understory plant diversity remains largely unknown due to repeated conflation with forest amount. Here, we test the independent effects of these three landscape variables on understory plant species diversity at 70 forested sites in Ontario. Forest amount had large positive effects on richness and negative effects on species assemblage uniqueness. Fragmentation reduced species richness and evenness, through negative effects on short-distance dispersers. Both had their maximum effects within 5 km of sites. Connectivity did not affect species richness but reduced both evenness and species assemblage uniqueness. The results demonstrate that maximizing forest amount is of primary importance for conserving forest plants, and increasing structural connectivity is not a viable strategy for maintaining forest plant communities.