Proximity to forests, disturbance and plantation traits influence understory species richness but not phylogenetic diversity in African mahogany plantations

More than a quarter of the world's biodiversity is conserved outside of protected areas. Human-modified ecosystems such as plantations, previously considered as green deserts, can serve as stepping-stones or corridors for the migration of species between source and sink populations thus maintaining metapopulations. Understanding community assembly in the understory of plantations and how this is mediated by neutral versus niche processes can shed light on their role in biodiversity conservation. In practice, how plantation size and isolation affect understory plant diversity can help decide the location and size of plantations that optimize biodiversity conservation and plantation yield. We predicted that understory plant species diversity in plantations is influenced by plantation size, age, and distance to the nearest native forests. Disturbance can outweigh the influence of distance by limiting the successful establishment of species dispersing from nearby forests. We conducted biodiversity surveys in African mahogany plantations in Benin. We developed a piecewise structural equation model to test the effects of plantation size, age, proximity to forests, and the mediating role of disturbance on understory plant species richness and phylogenetic diversity. Species richness and phylogenetic diversity increased with plantation size but only species richness was positively associated with plantation age. Species richness decreased with increasing plantation isolation and disturbance. However, phylogenetic diversity was not associated with disturbance or plantation proximity to forests. Our study suggests that small plantations and those far from forest reserves are more vulnerable to biodiversity filters and need more attention from conservation planners. We further showed that plantations can harbor important species richness in their understory if disturbance is limited.