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Plant diversity in a Central African rain forest: Implications for biodiversity conservation in Cameroon
Caesalpinioideae, the submontane forest on hilltops, to the mixed evergreen and
semi-deciduous forest in the drier Ma’an area. This results in an increase in species
richness from the coast to the hilltops and a gradual variation in dominant species,
from coastal indicator species along the coast to a forest rich in Caesalpinioideae
and a mixed forest rich in semi-deciduous elements.
In Chapter 3, we studied the diversity patterns in the flora of the Campo-Ma’an rain
forest. We tested out whether there is a correlation between tree species diversity
and diversity of other growth forms such as shrubs, herbs, and lianas in order to
understand if in the context of African tropical rain forest, tree species diversity
mirrors the diversity of other life forms or strata. Are forests that are rich in tree
species also rich in other life forms? To answer these questions, we analysed the
family and species level floristic richness and diversity of the various growth forms
and forest strata within 145 plots recorded in the various vegetation types. A
comparison of the diversity and species richness within forest layers and within
growth forms was done using General Linear Models followed by multiple
comparison tests. The results showed that tree species accounted for 46% of the total
number of vascular plant species with DBH ≥ 1 cm, shrubs/small trees 39%,
climbers 13% and herbs less than 1%. Only 22% of the diversity of shrubs and
lianas could be explained by the diversity of large and medium sized trees, and less
than 1% of herb diversity was explained by the tree diversity. The shrub layer was
by far the most species rich in the different plots and vegetation types. It was
significantly more diverse and species-rich than the tree and herbaceous layers.
More than 82% of tree species, 90% of shrubs, 78% of lianas and 70% of
herbaceous species were recorded in the shrub layer. Moreover, shrubs contributed
for 38% of the 114 strict and narrow endemic plant species recorded in the area,
herbs 29%, trees only 20% and climbers 11%. These results indicated that the
diversity of trees does not always reflect the overall diversity of the forest in the
Campo-Ma’an area, and therefore it may not be a good indicator for the diversity of
shrubs and herbaceous species. Furthermore, this suggests that biodiversity surveys
based solely on large and medium sized tree species (DBH ≥ 10 cm) is not an
adequate method for the assessment of plant diversity because other growth form
such as shrubs, climbers and herbs are under-represented. Therefore, inventory
design based on small plots of 0.1 ha, in which all vascular plants with DBH ≥ 1 cm
are recorded, is a more appropriate sampling method for biodiversity assessments
than surveys based solely on large and medium sized tree species.
Indicator species are usually defined as species whose status and ecology provide
information on the overall condition of the ecosystem, and that reflects the quality
and changes in environmental conditions. In the tropics, some of these indicator
species are used to trace the location and extend of tropical rain forest refuges
because of their limited dispersal and colonisation abilities. In Chapter 4, we studied
the distribution patterns of 178 sensitive bio-indicator forest species such as strict
and narrow endemics, as well as other well-known slow dispersal species, to find
out whether the entire Campo-Ma’an rain forest was part of a late Pleistocene rain
forest refuge or not. The distributions patterns obtained corroborate the view of
many authors who argue that during glacial times, forests were restricted to the
upper slopes of hills near the top, high altitude lowland forests, or along riverbanks,
where there was enough humidity for their survival. Our findings, therefore, do
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