Influence of topography on the natural stand structure of Boswellia dalzielii Hutch. in a semi-arid environment: Insight into hill ecosystems conservation in Burkina Faso

As the dominant woody species of some hills, the conservation of Boswellia dalzielii Hutch. could contribute to preserve vegetation of these azonal ecosystems, but also to restore degraded ones. However, given the topographical variation in hills, understanding the effects of such variation on the stand structure of B. dalzielii can assist the sustainable management of the species and therefore that of the hills. This paper compared the floristic diversity and the population characteristics of B. dalzielii natural stands across four topographical units: eastern and western slopes, hilltops, and plains. Data were collected from 80 plots of 50 m × 30 m equally distributed among the four topographical units in natural stands of B. dalzielii across the Sudano-Sahelian zone of Burkina Faso. The topographical units were compared based on woody species diversity. Additionally, B. dalzielii trees density, stem diameter, slenderness coefficient and tree diameter and height size class distribution were compared among the topographical units. A total of 3481 trees representing 74 species distributed into 51 genera and 25 families were recorded. Woody species richness varied slightly but was higher on the plains (62 species) than on slopes (west: 50 and east: 53 species) and hilltops (55 species). The dissimilarity between the four units in terms of species composition was low (R = 0.161, p = 0.001). Trees density of B. dalzielii was two to eight times higher on eastern slopes (374.3 ± 255.5 trees/ha) than on hilltops (181.3 ± 118.7), western slopes (138.3 ± 132.2) and plains (45.0 ± 22.5). The largest trees were found on the western slopes (dbh = 27.4 ± 10.3 cm) while the smallest were found on the plains (dbh = 6.5 ± 1.4 cm). About all trees found in the study area, the slenderness coefficient was < 75%, indicating low risk or susceptibility to windthrow. The stem diameter distribution exhibited a clear stable population of B. dalzielii in all units, but with recruitment difficulties. Our findings suggest that assisted natural regeneration actions are needed, especially in the plains and hilltops to assist the restoration of hills vegetation and to create refuge environments for biodiversity.