African Mahogany (Khaya anthotheca) negative distance-dependent recruitment in a Ugandan rainforest and implications for restoration

Abstract

Negative distance-dependent recruitment of seedlings is supposedly widespread in tropical tree communities. However, data from African rainforests is scarce, and it is still poorly understood how proximity to conspecific adults affects successful transitioning from seedlings to saplings of species of African mahogany that are of conservation concern. Such basic ecological information is of critical importance to enrich logged Afrotropical forests. We studied offspring dispersion relative to 37 isolated parent trees of African mahogany (Khaya anthotheca) in a Ugandan semi-deciduous rainforest to provide information for ecological restoration programs. Khaya recruitment is supposedly positively density-dependent as the abundance of regeneration increases with the density of adult conspecifics. Contrary to this, we found offspring recruitment patterns consistent with the Janzen-Connell hypothesis. Mid-sized seedlings were located farther away from the parent tree than small-sized seedlings, while saplings were farther away than mid-sized seedlings. We found a significant positive effect of distance on saplings, suggesting a distance-dependent process drove higher seedling survival farther away. Regardless of the process responsible for seedling mortality, our results show that K.anthotheca recruitment in our study site is negatively distance-dependent, with proximity to reproductive adult trees reducing the survival of seedlings during the transition to saplings. Our results suggest enrichment planting of K.anthotheca seedlings away (> 15 m) from remnant conspecific adult trees may promote greater seedling survival, thereby contributing to the restoration and conservation of this valuable tree species.