Continuous degradation of forest structure and composition causes a shift in phorophytes and population genetics in aphyllous Vanilla species

Background and Research Aims: The natural habitats of aphyllous vanillas in Madagascar have been extensively degraded by human activities. The implications of this degradation are complex given their intricate interactions with other organisms. This study highlights possible approaches to the conservation of aphyllous vanillas after examining the impact of forest degradation on phorophyte diversity, reproductive modes, and genetic diversity. Methods: We conducted comparative studies of conserved and degraded forests in two contrasting bioclimatic zones. We characterized the changes in horizontal and vertical forest structures and assessed the ecological importance of the tree species. We identified the preferred phorophytes of two aphyllous Vanilla species, Vanilla madagascariensis and Vanilla decaryana, and investigated their population genetics. Results and Conclusions: Our findings confirmed a declining trend in tree diversity, number of trees, and cover rate in degraded forests. Canopy openness is associated with an increase in the number of phorophytes and the asexual reproductive mode of Vanilla species. Dominant tree species were more likely to become phorophytes, and there was a positive correlation between phorophyte diameter and number of hosted vanilla individuals. Significant deviations from the Hardy-Weinberg equilibrium were observed in each population, particularly in the dry forest and degraded sites, compared with the conserved sites. Additionally, slight differentiation was observed between vanilla populations in conserved and degraded forests, with an alarming effective population size (Ne) that did not exceed 13 individuals. Conservation Implications: We recommend the introduction of new genotypes to promote genetic enrichment, the ban on logging potential phorophytes, and the inclusion of some degraded forests comprising Vanilla species in conservation.