Tree composition, diversity, and structural complexity of the Cryptosepalum forests of North-Western Zambia: Implications for conservation and sustainable management

Résumé fourni par la source

• Tree composition, diversity and structure were assessed in the C ryptosepalum forest (CF), across sites and across canopy types in Zambia. • The CF is dominated by the endemic Cryptosepalum exfoliatum species. • Co-existence of the Cryptosepalum exfoliatum with the Guibourtia coleosperma (Rosewood) threatens biodiversity of the CF. • Beta diversity was mainly driven by nestedness across forest canopy types. • Structural complexity varied across sites, reflecting disturbance and succession. • Forest canopy type shaped species diversity and evenness more than soil nutrients and elevation. A lack of information on the ecological status of forest ecosystems, makes them vulnerable to exploitation, and consequently threatens their existence. This study explores the ecological status of the unique and endemic Cryptosepalum forest, mainly to understand its forest’s structure, tree species richness, diversity, as well as the environmental factors that potentially influence these attributes. The study was conducted in the Manyinga and Zambezi Districts of North-western, Zambia. A systematic sampling design was adopted and a total of 128 circular plots of 20m radius were selected and assessed. All tree species of diameter at breast height (Dbh)≥5cm were identified scientifically, and their height, and Dbh established. A total of 6732 individual trees were recorded in 128 plots, belonging to 31 species, 25 genera, and 13 families. The Dbh size class distribution of all species combined revealed an inverse J-shape, typical of a regenerating forest. The overall stem density and basal area were 418.55 stems ha -1 and 14.59 m 2 ha -1 , respectively. The forest exhibited moderate diversity (H' = 2.46, D = 0.87). The study confirms dominance and endemism of Cryptosepalum exfoliatum (with IVI=30.21%) in this forest type, whose existence is, however, threatened by timber harvesting practices associated with one of its co-dominant species- Guibourtia coleosperma (Rosewood). Variations in species richness, diversity and evenness were observed between study sites. Alpha species diversity indices were significantly higher in the open forest canopy class (1.95±0.31) than in the dense forests (1.79±0.28). Beta diversity was mainly driven by nestedness (84%) across forest canopy types. Forest structural attributes such as stem density and tree abundance also significantly varied across study sites and forest canopy classes. Results of the linear mixed-effect models revealed that elevation and soil fertility had little influence on species diversity and evenness, with p=0.76 and p=0.73 respectively. The study provides useful baseline information for sustainable management and conservation of this unique forest ecosystem.

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