Topographical gradient of the structure and diversity of a woody plant community in a seasonally dry tropical forest in northwestern Madagascar

Few studies have evaluated the structure and dynamics of forests in Madagascar with high levels of endemism. Ankarafantsika National Park holds the largest of the remaining primary dry forests in northwestern Madagascar, where most of the forests have been lost or degraded by fire and other human activities. In this primary forest, we established a 15-ha forest dynamics monitoring plot and mapped and identified all woody stems with a diameter at breast height (DBH) ≥5 cm. The forest stand was characterized by small-sized individuals (75% of stems with DBH <10 cm, with 99% of trees with height <15 m), encompassing 35,758 tree and 513 liana individuals belonging to 160 species in 53 families (including 20 species of lianas accounting for 1.4% of the stems). Most species-rich families were Rubiaceae, Fabaceae, Lamiaceae, and Malvaceae, which together encompassed 43 tree species and 23% of all tree individuals. The plot was located on a hill consisting of white sand at elevations from 150 to 200 m, with a gentle slope down from southwest to northeast. Over this elevation gradient, stem density and species diversity increased with elevation, while tree height and diameter decreased. The most abundant two species, Drypetes perrieri (Putranjivaceae) and Noronhia alleizettei (Oleaceae) are evergreen, suggesting the importance of a nutrient conservation strategy. These results provide the foundational knowledge necessary for the conservation and restoration of natural semi-deciduous dry forests that used to cover large areas in northwestern Madagascar until recently.