Tree Species Diversity in a Naturally Regenerated Secondary Forest in the Ruhande Arboretum, Rwanda

This work studied the vegetation in a seven-hectare self-regenerated and protected forest about nine decades-old located in a previously cultivated site in the Ruhande Arboretum to identify woody species and their diversity. Ten parallel transects were established at 34 m intervals, leaving 25 m on either side to avoid an edge effect. Along transects, circular 16 m diameter plots spaced 20 m apart were established, making a total of 56 plots. In each plot, woody species were recorded and those with heights >2 m had their diameter at breast height measured. Phytosociological data including basal area, density, and frequency and their respective relative values were computed and used to determine species and family importance value indices within each plot. Across all plots, twenty-eight genera in 17 families were identified and 844 plants were recorded, including 755 trees and 89 shrubs, with most trees found in smaller diameter classes. Across all plots, only one Markhamia lutea tree was in the 50–60 cm diameter class and one Polyscias fulva was in the >90 cm diameter class. Of all woody species, Polyscias fulva was the most dominant since it had individuals with the biggest diameter. The number of individuals per family across all plots ranged from one for Cupressaceae, Dracaenaceae, Moraceae, and Solanaceae to 414 for Bignoniaceae. Across all plots, the diameter at breast height ranged from 1.8–97 cm. The species importance value index ranged from 0.3–41.8 for Nicotiana tabacum and P. fulva, respectively, while the family importance value index ranged from 0.2 for Annonaceae, Cupressaceae, Dracaenaceae, and Solanaceae to 41.6 for Araliaceae. Shannon and Simpson’s diversity indices were 1.772 and 0.707, respectively, while the evenness was 0.532, signifying that the forest was reasonably diverse. It is recommended that this forest can be conserved owing to its rich vegetation and to monitor its successional development.