Assessment of floral composition, structure and natural regeneration of the Tano Offin globally significant biodiversity area

Tano Offin Globally Significant Biodiversity Area (GSBA - 178.34 km²) is a reserve within the Tano Offin Forest Reserve in the Ashanti Region of Ghana. The reserve which experiences occasional wildfires has been logged and mined for bauxite in the past. Current menace of forest degradation presents the need to generate information on the current state of the floristic patterns of the forest. To address conservational needs of the reserve, a study was carried out in the GSBA to assess the floristic composition, structure, natural regeneration, canopy closure as well as to evaluate the influence of elevation and invasive species on the aforementioned. Ten 50 m x 50 m plots were demarcated for the assessment of diameter and height of all trees, lianas and shrubs with dbh ≥ 10 cm as well as the identification of other plant life forms. These individual plots varied in invasiveness and altitude: ranging from 264 m – 623 m. A 10 m x 10 m plot was located within each of the 50 m x 50 m plots where diameter of trees and other plant life forms of dbh < 10 cm were identified and measured. Quadrats (1 m x 1 m) were laid at the corners of the 50 m x 50 m plots and its centre for canopy closure and natural regeneration assessments. Plant species (240) belonging to 59 families were identified. These comprised of 171 trees, 41 lianas, 11 shrubs, 7 herbs, 7 herbaceous climbers, 1 epiphyte, 1 grass and 1 fern. Fabaceae was the predominant family in terms of species richness. Species diversity (H´) of the tree layer, shrub and herb layers were 2.55, 2.54 and 2.48 respectively. Trees and other plant life forms of dbh ≥ 10 cm were grouped into six diameter classes and four height classes; the number of plants in these groups was decreasing as the group size increased so that the highest-size group had the least number of plants. Basal area of the GSBA was 28.36 m²/ha and average tree height of the emergent layer was 46.19 m. Floristic diversity did not differ significantly (P > 0.05) in the tree, shrub and herb layers of all three forest types namely, lowland, transition and highland forests. Celtis mildbraedii was both the most significant species among trees and other plant life forms with dbh ≥ 10 cm in general, and in all three forests types. Rinorea welwitschii was also both the most significant species among trees and other plant life forms with dbh < 10 cm, and at the shrub layer of highland and lowland forests. Hymenostegia afzelii was the most significant species for the shrub layer at the transition area though Rinorea welwitschii was completely absent from this zone. Basal area for trees and other plant life forms with dbh ≥ 10 cm increased with increasing elevation. Generally, there was increase in the number of taller trees with increase in elevation. Areas with invasive species recorded lower mean basal area at both the tree and shrub layers. Invasive species were present only at the transition area which had the lowest percentage of shade bearers and non-pioneer light demanders but the highest with pioneers. A total of 75 plant species were found regenerating as saplings and seedlings. Eight species of the regeneration flora were absent from the adult community. For the lowland forests, 50.94 % of the adult tree population were regenerating while 46.74 % were regenerating in the transition area and 42.86 % at the highlands. Pioneer saplings were absent from lowland forests. Invasiveness had significant influence (P < 0.05) on the species diversity of seedlings. There was a decline in canopy closure with respect to decreasing altitude, measuring 89.06 %, 87.84 % and 84.9 % for the highlands, transition area and the lowland forests respectively. It is revealing that the GSBA is under exploitation especially in lowland forests. The information generated on species composition, structure and regeneration should be useful in designing conservation measures for the Tano Offin GSBA.

Source: KNUST