Woody Species Diversity, Structure and Distribution of Sterculia setigera Del. in Togo (West Africa)

Aims: This study aims to contribute to the sustainable management of S. setigera, a native gum and multipurpose tree in Togo. Especially, it aims to: (i) Assess the diversity of woody species within S. setigera stands, and (ii) Analyze the distribution, abundance and structure of S. setigera trees, according to contrasting environmental factors and human activities within the different eco-floristic zones of Togo.
Place and Duration of Study: Fieldworks were done from 10 October to 15 December 2013 throughout Togo.
Methodology: Dendrometric and ecological data of Sterculia setigera stands were recorded in 350 plots. In each plot, total height and girth of all woody species with diameter at breast height (DBH) ≥ 10 cm were measured and all human activities were recorded. In addition, seedlings and suckers of S. setigera species (DBH < 10 cm) were counted. The spatial distribution analysis of the species was done using Geographic Information System (GIS).
Results: Ninety six (96) woody species distributed among 71 genera and 31 families were identified. The highest species richness were recorded in the eco-floristic zone I (82 species), followed by zone III and zone II (41 and 40 respectively). Within these zones, the most representative woody species apart from S. setigera are Vitellaria paradoxa and Parkia biglobosa. In contrast, the most represented woody species in zone V were Cussonia arborea and Odinaacida. Statistic tests showed a significant difference of S. setigera densities following eco-floristic zones (p=0.00); especially between zone I and zone V (30.45 stems/ha and 7.57 stems/ha respectively). The overall regeneration rate was weak with a total absence within zone V. This regeneration status could justify the relatively few small stems proportion described by the coefficient of skewness values (g>0). Green Index values (GI, nearest 0) showed a random distribution of the species. The main human activities threatening S. setigera stands were bushfire, woodcutting, pasture and agriculture. According to the level of human activities Fisher test showed a significant difference (p=0.00), particularly between zone I with highest activities level and zone V with lowest activities (mean, 2.62±1.12 and 1.07±0.26).
Conclusion: The species richness within S. setigera stands and the density of S. setigera decrease from north to south (respectively from eco-floristic zone I to V). Both density and regeneration of S. setigera were shaped by human activities and environmental factors. Weak regeneration highlights that the future existence of the species is jeopardized.