Floristic Structure and Diversity of a Tropical Sub-Montane Evergreen Forest, In the Mbam Minkom Massif (Western Yaoundé)

Abstract

The aim of this work was to study the floristic structure and diversity of the sub-montane forest of the Mbam minkom massif of Yaoundé region, in order to establish its structure and its features in relation to the other submontane forests of Africa and Madagascar, and Neotropical zones.

We listed all plant species of evergreen sub-montane forests on the upper summits of 12 hills (1000 to 1295m) of the Mbam minkom massif. An area of0.5 hawas inventoried per hill, using randomly 5 surfaces 25 × 40m (0.1 ha) plots, of a total area of6 ha. Ligneous plants (≤1 cm diameter at breast height) were inventoried:

- 324 vascular species, 65 genera and 42 families were recorded;

- 150 herbaceous species;

- 122 shrubs 1≤ dbh < 10cm;

- 146 ligneous species 10cm ≤ dbh.

The species-accumulating curves showed a high diversity in the category 10cm ≤ dbh than those of shrubs 1 < dbh >10 cm. The minimal area was reached at5.5 hainventoried.

For the category 10 cm≤ dbh, stem density was 544 stems ha^-1 and basal area 52.72 m² ha^-1.

Species diversity as measured by Shannon diversity index was 4.95. Most species (67, at least 77%) were common (densities > 1 stem ha^-1), and widely distributed among hill forests (36, at least 41%). Gacinia lucida is the species having the highest IVI value (58.52) and Clusiaceae the family having the highest FIV value (60.01), is mesothermal. The forest is also marked by the abundance of Sterculiaceae, Burseraceae, Myristicaceae, Meliaceae, Oleaceae, Leguminosae, Annonaceae, Apocynaceae and Olacaceae.  These are megathermal families (or hydromegathermal) that present their maximum occurrence at the basal and average altitudes.

The Mbam minkom submontane forest presents the features of an ecotone. The hydromegathermal families such as Leguminosae present relative density and basal area indices that decrease with altitude. The hydromesothermal families as Clusiaceae find there their ecological preferendum with relative density and basal area indicesthat increase upto 1295m. The hydro-oligothermal families as Rubiaceae show relative density and basal area indices that increase with altitude. This increase goes along with increasing cold weather. The family with 15 species comes behind the Orchidaceae (20 species) when the herbaceous species are taken into consideration in denumbering of the diversity. This unique forest massif is not yet endangered but needs to be conserved as a priority.