Forest reserves are expected to host a wide array of biodiversity and provide refuge for rare species that may be threatened in nearby forest landscapes. While this is the guiding protocol for most reserves across the tropics, such as Nigeria, the extent to which they host biodiversity and act as potential stores for carbon are quite uncertain. This study used a four hectare randomly selected forest plots to verify the biodiversity of the reserve, its stand structure and potentials for carbon storage. Species importance value was used to summarize the composition of the landscape. Both the diversity (mean diversity = 0.85) and species richness (eleven species) were low. Biodiversity in the area was quite poor and was mostly composed of Elaeis guineensis and Gmelina arborea, which had relative densities of 74.6% and 11.96%, respectively. Over exploitation and preference for fast-growing exotic species explained the poor stand structure and composition of the landscape, respectively. Very few tree stands were found in the mature structural class, and its capacity to facilitate regeneration and resilience seemed low. Its ability to store carbon in its biomass is equally low; since the forest landscape was much degraded. Maximizing the vast land of the reserve for targeted carbon storage (through mass tree planting) is a potential step that could forestall carbon sequestration across the region, especially because, such vast and available (forest) land cannot be guaranteed in most other forest landscapes.
Tropical forests are host to a wide array biodiversity and important for global climatic processes. Yet its rate of forest loss and conversions are so alarming that up to 13 million forests are felled or razed each year across the tropics [
Even though Nigeria occupies a large land mass, only about 10 percent of its land area are occupied by forests [
These reserves (which are mostly extensive in size) are managed at the state levels by the different State Forestry Departments whose emphasis or target is (varied) dependent on the ecological location or peculiarities. Hence, while the ones in southern Nigeria (where most of the forests are found) are mostly particular with log harvesting, those in the northern part focus mostly on tree establishment for environmental protection, fuelwood and livestock production [
The study region is an extensive sedimentary basin that is found in the lower reaches of the Niger basin (
activities, the dry lands are used predominantly for farming activities and for settlement purposes.
Vegetation surveys were conducted in the forest reserve (
Importance values were used to characterize the sites and summarize the abundance of each site as follows:
Species importance values (SIV) = Relative density + Relative frequency + Relative dominance [
The diversity of the ecosystem was calculated [
Shannon-Wiener index:
where
Species richness was calculated as the number of species in occurrence.
The sites varied in their species composition, frequency and abundance. A total number of eleven species were found in the reserve. Mean diversity for the reserve was 0.85. The most abundant species in the reserve were: Elaeis guineensis, Gmelina arborea and Tectona grandis, with SIV values of 182.03, 24.97 and 16.36, respectively (
Species | Family | Relative density | Relative frequency | Relative dominance | Species importance value |
---|---|---|---|---|---|
Anthocleista djalonensis | Gentianaceae | 0.478469 | 11.11111 | 0.0060222 | 11.5956 |
Brachystegia eurycoma | Leguminosae | 0.239234 | 5.555556 | 0.0019372 | 5.796727 |
Ceiba pentandra | Malvaceae | 0.478469 | 11.11111 | 0.0074459 | 11.59703 |
Elaeis guineensis | Arecaceae | 74.64115 | 11.11111 | 96.277904 | 182.0302 |
Eucalyptus spp | Myrtaceae | 0.239234 | 5.555556 | 0.003912 | 5.798702 |
Ficus capensis | Moraceae | 0.717703 | 11.11111 | 0.0123637 | 11.84118 |
Gmelina aborea | Lamiaceae | 11.96172 | 11.11111 | 1.9141603 | 24.98699 |
Mangifera indica | Meliaceae | 0.717703 | 11.11111 | 0.0180519 | 11.84687 |
Pentaclethra macrophylla | Leguminosae | 1.196172 | 11.11111 | 0.0415558 | 12.34884 |
Strombosia pusulata | Olacaceae | 0.239234 | 5.555556 | 0.0005916 | 5.795382 |
Tectona grandis | Lamiaceae | 9.090909 | 5.555556 | 1.716055 | 16.36252 |
like Gmelina arborea, Tectona grandis and Eucalyptus spp are common features of the reserve’s biodiversity (
Even though the reserves have the potential to produce wood (timber), they however stand the chance of being affected easily by diseases and pests. This is mainly because, since the landscapes are composed of only few species, their ability to withstand disease and pest infestations are much reduced, unlike other forest landscapes with higher species diversity. Since the forest reserve are equally characterised by the presence of other (few indigenous) tree species, efforts should be targeted at promoting the coexistence of both indigenous and (planted) exotic species; as such could help in promoting the resilience of such landscapes as well as their functional traits.
The plots were generally characterized by a higher concentration or proportion of trees in the lower structural class (10 - 40 cm;
Gmelina arborea; as both had the highest relative densities (74.64 and 11.96, respectively;
Understanding the stand structure of forest ecosystems are vital measures or steps towards providing guidelines for the effective management of forest ecosystems. However, since this is not being considered in most occasions, there is usually a wide gap between the number of tree stems in the lower and higher stratum. Since much of the trees are normally logged before they reach their emergent phase, the ability of such landscapes to have sufficient seed bank and provide other vital ecosystem functions become much reduced. This affects both the biodiversity potentials of such landscapes and their conservation prospects as well. On the other hand, the abundance and dominance of Elaeis guineensis across the landscape reduces its capacity to be characterized by indigenous species and in turn, their resilience.
The use of forest reserves for other purposes apart from the original aim for establishing them have become common features across most reserves in Nigeria. In most occasions, it has suffered from degradation and loss due to encroachments from farming activities, illegal logging, firewood gathering, settlement expansion and road constructions [
The reserve were generally degraded (
Though forest reserves were originally gazetted across Nigeria to facilitate biodiversity conservation, it is far from being achieved as most reserves (as in this study) are degraded and characterized by low species richness. Relying on other forest landscapes (outside the reserve) to achieve biodiversity conservation may not equally be a better option since their owners easily permit logging and degradation of the forests for economic reasons [
Even though forest reserves have the potential to host a wide range of biodiversity across tropical forest landscapes, this is not mostly achieved. This study showed that though the reserve had such potentials, it was characterized by low diversity and a low relative density that concentrated only on two species. Degradation and its consequent poor stand structure were seen to reduce the capacity to which the landscape could reach stable/climax population and suitably provide its functions and services. Utilizing the reserve’s large expanse of land effectively will help to facilitate biodiversity conservation and also provide a suitable landscape that could enhance more carbon storage capacities for the region.
Igu, N.I., Nzoiwu, C.P. and Anyaeze, E.U. (2017) Biodiversity and Carbon Potentials of a Nigerian Forest Reserve: Insights from the Niger Basin. Jour- nal of Environmental Protection, 8, 914-922. https://doi.org/10.4236/jep.2017.88057