Plant Diversity and Carbon Storage Potential Across Different Land Use Types in Infrastructure Development Landscapes in Cameroon: Implications for no Net Loss of Biodiversity

Study Site

The study was conducted within the proposed Dibombe Hydropower Project (HPP) area in the Littoral Region of Cameroon Figure 1. The proposed Dibombe River catchment hydropower plant is a Run-of-River Hydropower Plant in the Littoral Region-Cameroon. It covers the Loum and Djombe-Penja sub-divisions of the Moungo division and the Yabassi sub-division of the Nkam division. The closest communities to the study site are Mabombe, Lamba and Sole to the left bank of the Dibombe river where the HPP infrastructure will be established while Njanga, Boneko and Penja are communities to the right bank of the Dibome River within the area affected by the 11km long transmission line corridor (Figure 1). The project area encompasses diverse land use types, including riparian forests, cocoa plantations, fallow land, and croplands, providing a representative sample of the region’s land use systems. The vegetation around the study area characteristically falls within the lowland evergreen Atlantic or coastal rainforests zone of the Congo Basin. The forest type belongs to the Lower Guinea region of endemism [4] and is abundant in leguminous trees (Fabaceae). Agriculture is the main source of subsistence for the local population. Palms, cocoa, coffee, white paper and banana plantation are the major agricultural activities practiced within the study site. The study area, located within the Littoral Region of Cameroon, experiences a hot and humid climate with high and constant temperatures averaging 27.4°C annually. Rainfall is intense, peaking at 700 mm in August and averaging 3664.2 mm from 1998 to 2007. The region’s geology includes basalts, syntectonic granites, porphyroid granites, and gneisses, contributing to its high tectonic instability. Soils in the area are primarily loam, sandy, and lateritic. Hydrologically, the Douala Basin is significant due to its sandy formations which serve as primary aquifers, recharged by local precipitation and surface water infiltration. The Wouri hydrographic basin features a dendritic drainage pattern, characterized by falls, rapids, and pools, particularly in the Dibombe River.

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Data Collection

Botanical surveys were carried out across different land use types within the project area using the line transect sampling design inspired by several authors [5, 6]. Line transects have been proven to be robust and more accurate plant sampling techniques in both homogenous and heterogenous landscapes as they provide higher chances of fully capturing species diversity and land use change at the same time compared to plots. Ngansop, et al. [6, 7] employed this sampling technique based on its accuracy and reliability to assess natural regeneration of non-timber forest products across different land use types in southeastern Cameroon. A base line of about 6 km long and 20 m wide was cut that runs from T1 to T6 and is oriented along the geographic north. The equidistance between each transect was 1 km and all transects were numbered and named from T1-T2- T3-T4-T5-T6. Transects T1-T2-T3-T4-T5-T6 were established around the proposed HPP project footprint area while transects T7-T8-T9 were established upstream around the potential reservoir area. The start and end of each transect was numbered and named. Along each transect, small quadrats of 5 m × 5 m were established to collect data for understorey plants. The transect lines RS1, RS2 and RS3 were aligned along the left bank of the Dibombe river along the impoundment area. These transect total about 3km and were linked to each other. A total of nine transects were established with approximately 1650 ha of the proposed project-affected area were sampled (Figure 2). Along each transect, all trees with a diameter ≥5 cm were measured at 1.3 m breast height. For trees with early ramification (e.g. Cocoa plant), their diameter were measured at 30 cm above the ground. Palms and Raphia trees were not measured for their diameter, rather their total height were estimated. The later steps were followed by taxonomic identification of the species. After these, the land use type was determined based on GIS imagery and confirmed by ground truthing.

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Data Analysis

Assessment of Species Diversity: Species diversity was evaluated using the number of species, number of individuals and diversity indexes Simpson index of diversity (SID) and was expressed per transect and per land use type. The number of species was simply represented as the count of each species per sampling unit (transect or land use type). Species abundance was determined as the number of individuals for a given species. The Simpson diversity index combines species richness and evenness (relative abundance of the different species) across a the transect or land use type considered. The important value index (IVI) was calculated for key tree species to assess their ecological dominance. Assessing conservation status: The conservation status of each species was determined based on the IUCN redlist for plant species. Complementary endangered status was obtained to conform species vulnerability at the national level using Onana [8]. Additionally, the distribution of threatened species was analyzed by computing their relative abundances to determine conservation priorities within the project area.

Computation of Aboveground Biomass (AGB): Aboveground Biomass (AGB) was computed independently for woody plants and non-woody plants (e.g. Palms and Raphia). For woody plants, equation 5 of Chave, et al. [9] was used to compute the AGB of the trees. This equation uses the diameter of the trees, their wood density and the environmental stress index (E). The E of Chave, et al. [9] captures variability of the environment and represents acts as a proxy for tree total height. The later index was calculated from precipitation and temperature data of the locality. For this, the GPS coordinate of each transect was used to retrieve the precipitation and temperature data used to calculate the stress index. The wood density for each species was obtained from the Dryad global wood density database [10]. For species without wood density values in the Dryad global wood density database, a mean wood density value of the transect was attributed to them. For Palms and Raphia, the equation 8 of Migolet, et al. [11] developed for Palms in the Congo Basin was used. This formular uses the total height of the palm tree as shown thus: AGB=a+b log(HT) × where AGB is aboveground biomass in kg (in ton when divided by 1000), a and b are the coefficients of the intercept and slope of the model and HT is the total height of the palm or Raphia plant.

Land Use Types

Seven land use types were identified across the study area along the Debombe river area from GIS imagery and confirmed by walking the nine transects (Figure 3). In order of importance (number of transects and surface area covered) the land use types include cocoa agroforests (eight transects, 16-ha), forest (six transects, 12-ha), fallow and palm plantation (four transects, 8-ha each), cropland and riparian forest (three transects, 6-ha each) and Rubber plantation (1 transect, 2-ha).

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For the HPP infrastructure site, six land types were identified with the absence of the riparian forest. The forest and cocoa agroforests were the most important land use types around the infrastructure site covering 6 transects and 12-ha each. Palm plantations spread along four transects (8- ha), the fallow and cropland each occupied 4-ha along two transects while rubber plantation covered 2-ha along one transect. Four land use types were identified around the Reservoir area including riparian forest (6-ha), fallow and cocoa agroforests (4-ha each) and Cropland (2-ha). For the transmission line, six major land use types were identified including cocoa agroforests, palm plantation, cropland, fallow, riparian forest and old forest. Cocoa agroforests which represented the most important land use type along the transmission line is a type of agriculture systems wherein the cacao plant (Theobroma cacao) is the predominant cash crop. The plant is grown at a regular distance of about 4-5 m interval and can grow 5-10 m high. Few other trees species are purposefully allowed in the cacao farms for shade, fruits or as non-timber forest products (Medicine, spice, xxx). Few of these trees include Irvingia gabonensis (bush mango), Ricinodendron heudelotii (spice), Cola lateritia (monkey kola), Garcinia kola (bitter kola). Other food crops also intercropped in the Cocoa agroforests include Musa spp (banana and plantain), Xanthosoma sagittifolium (Cocoyam), Manihot esculenta (cassava).

Like the Cocoa agroforests, oil palm plantations equally cover an important portion along the transmission like. The palm plant (Elaeis guineensis) is planted as a cash crop at an interval of 7-9 m. Unlike the cocoa agroforests, rarely are other tree species found among mature palm plantations. Singleton timber trees in the palm plantation were recorded, for instance, Pterocarpus soyauxii (red padouk), Milicia excelsa (iroko). The land use type identified as cropland characterizes areas where all other natural occurring vegetation was removed and replaced by active agriculture. The type of farming in this land use type is mainly for subsistence and food crops are the main plants. Some crops identified were Zea mays (corn), Arachis hypogaea (groundnut), banana and plantain. However, some trees spotted are Albizia zygia, Alchornea cordifolia and Alstonia boonei.

The land use type identified as fallow defines those areas where the original forests were cut for the purpose of agriculture, cultivated for a certain period and later abandoned. Unlike the later three land use types, fallows were densely covered with mostly liana species and other plants of secondary growth (e.g. Anthocleista Schweinfurthii). Relics of the previously cultivated plants were identified in the area such as palms, cocoa, cocoyam and cassava. Species such as Adenia spp, Hypselodelphys danckelmaniana and Haumania spp are the major climbers (lianas) recorded.

Plant Species Diversity Across Different Land Use Types

Overall, 328 species were counted across the seven different land use types (Annex 1). The overall Simpson index of diversity of 0.87 was determined Table 1. Among the 205 genera and 52 families recorded and Fabaceae with 42 species were the most abundant family while Cola with 11 species represents the most genera. Plant species diversity assessment within the entire project affected area revealed a rich diversity of plant species across different land use types. Species diversity measured using the Simpson Diversity Index, showed high values across all land use types, indicating well-balanced ecosystems Table 1.

The results indicate that forests harboured the highest number of species (159) and the highest Simpson Diversity Index (0.92), highlighting their importance as biodiversity hotspots. Cocoa plantations also showed significant diversity, with 136 species and a diversity index of 0.89, suggesting that these agroforestry systems may play a role in biodiversity conservation. Riparian forests, despite having fewer species than cocoa plantations, maintained a high diversity index (0.87), underscoring their ecological significance. Fallow lands and other land use types (cropland and rubber plantations) exhibited lower species numbers but still maintained relatively high diversity indices.

Occurrence of Threatened Species Across Different Land use Types

The verification of the IUCN status of all plant species revealed the presence of 2 endangered species (EN), Guibourtia tesmannii and Microberlinia bisulcata all belonging to the Fabaceae family, 23 vulnerable species (VU), and 2 near-threatened (NT) species. Most of the species fell under the category of least concern (LC) while a few were not evaluated by the IUCN. A cross-examination of the species in the different land use types revealed the presence of most of the vulnerable species in forests, cocoa agroforests and riparian forests. Interestingly, some of these species were found in cropland while plantations recorded the least abundance of threatened species Table 2.

Carbon Storage Potential of the Different Land use Types

The assessment of aboveground biomass across different land use types provided insights into the carbon storage potential of the ecosystem (Table 1). The results show that riparian forests stored the highest amount of aboveground biomass (100 tons/ha), highlighting their crucial role in carbon sequestration and climate regulation. Fallow lands and cocoa plantations also demonstrated significant carbon storage potential, with 33 tons/ha and 29 tons/ha, respectively. These findings suggest that allowing natural regeneration in fallow lands and maintaining diverse agroforestry systems like cocoa plantations could contribute substantially to carbon sequestration efforts. The total AGB for the HPP site (1327.8 tons) and the reservoir area (850.2 tons) indicates the significant carbon storage capacity of the entire project area, emphasizing the importance of careful land use planning and management to minimize carbon losses (Table 3).

Land use Types, Species Diversity and Carbon Storage and Threatened Species

Six major land use types were observed occupying important surface areas across the different transects surveyed. The highest proportions were observed for cocoa agroforests, forests and fallows and palm plantations, respectively. The predominance of these land use types is evidence of the high anthropogenic action on the landscape. These findings are in line with the findings of Biah, et al. [12] which highlighted that anthropogenic pressure causes regressive conversion of forests to croplands, fallows and other land use types. The high species diversity observed across different land use types, particularly in forests and cocoa plantations, aligns with previous studies highlighting the biodiversity richness of tropical ecosystems [13]. The significant diversity in cocoa plantations (136 species, Simpson Index 0.89) suggests that agroforestry systems can contribute to biodiversity conservation, supporting the findings of Strassburg, et al_. [14] in the Brazilian Atlantic Forest. The aboveground biomass results highlight the carbon storage potential of different land use types. General decrease in carbon stocks as one move from forest to plantations. This could be attributed to the high level of anthropogenic factors driving the conversion of forests to other land use types. Basuki, et al. [15] observed that land forest conversion to other land types significantly influences on carbon sinks. This underscores the importance of preserving and restoring these ecosystems for climate change mitigation, as emphasized by Strassburg, et al._ [14] Riparian forests stored the highest amount (100 tons/ha). The significant AGB in fallow lands (33 tons/ha) suggests that natural regeneration in these areas could provide substantial benefits for carbon sequestration and habitat restoration.

The identification of threatened species within the project area, including two endangered species (Guibourtia tessmannii, Microberlinia bisulcata) and several vulnerable species, emphasizes the conservation value of the ecosystem. This aligns with the challenges highlighted by Jones, et al. [1] regarding the implementation of NNL policies in biodiversity-rich tropical regions. The presence of these species across different parts of the project area supports the need for a landscape-level approach to biodiversity offsetting, as suggested by Wachter, et al. [3] Achieving true “no net loss” of biodiversity remains challenging, as noted by Weissgerber. et al, the high biodiversity and presence of threatened species in the project area suggest that finding equivalent offset sites with similar ecological value may be difficult. This underscores the importance of prioritizing avoidance and minimization of impacts before considering offsetting measures.

Implications of Biodiversity Spread and Carbon Storage of Different Land use Types for Achieving no Net Loss Within Large Infrastructure Projects

The study revealed a dynamic infrastructure development landscape in which different land use types are observed with a considerable change in biodiversity across the different types. These findings underscore the important role that forest play in conserving biodiversity. The study also highlights the place of cocoa agroforest in minimizing biodiversity loss after the conversion of forests, with important carbon stocks that play a vital role in maintaining the ecological balance of the ecosystem in the absence of forest as it mimics the forest in its vegetation structure and plant diversity [16]. The predominance of cocoa agroforests in the study site could therefore be used as a measure of reducing biodiversity loss in infrastructure development landscapes where they are abundant. The analysis of the different land use types also revealed the abundance and spread of the major land use types on the landscape. This implies that minimizing damage during the construction works could considerably limit biodiversity loss as what may be lost on the construction site could possibly be compensated for in similar land use types elsewhere on the same landscape. This favours the conservation of threatened species on a landscape scale. This possibility is supported by the fact that majority of the threatened species were found in three land use types (forest, cocoa agroforests and riparian forests).

Infrastructure development in tropical regions often overlaps with critical, natural, and modified habitats, necessitating biodiversity offsets to ensure no net loss of biodiversity values and a net gain of critical habitat in accordance with internationally recognised standards. Net gains refer to additional conservation outcomes for the biodiversity values that designated the critical habitat and include conservation actions benefiting biodiversity value. In Cameroon, two primary classes of biodiversity offsets are applicable for critically endangered fauna: restoration offsets and averted offsets. Restoration offsets involve protecting or restoring habitats for populations of critically endangered/endangered fauna that have suffered significant non-Project related impacts in the past (i.e., populations at greatly reduced densities compared to previous levels), thereby allowing them to recover. Restoration offsets offer the advantage of delivering tangible gains and mitigating issues of predicting potential future losses. Averted offsets enhance protection of existing but threatened biodiversity values, preventing predicted future losses unrelated to Project activities. This involves implementing conservation actions at sites where large populations of the biodiversity value face threats from hunting, capture, and agricultural habitat clearance, aiming to reduce these threats as much as possible. The gains made represent the ‘averted loss,’ i.e., the difference between predicted loss under a counterfactual scenario and actual loss under an offset scenario.

Case studies in Cameroon, such as the Chad-Cameroon pipeline project, exemplify the focus on averted loss, leading to the establishment of the Campo Ma’an National Park to protect its rich wildlife (chimpanzees, elephants, and gorillas) and flora, and the Mbam-Djerem National Park to safeguard its wildlife (hippopotamus, elephants, chimpanzees, pangolins, etc.) from poaching. The construction of the Lom Panger Dam further financed the continuous support of the Mbam- Djerem National Park. Challenges in Cameroon include long-term planning and integrating developmental projects into biodiversity conservation strategies. Many national parks are downsized or reduced due to major infrastructure developments. Additional pressure was exerted on the Campo Ma’an National Park by the Memvele Dam and Hydropower Project, and similarly, the Mbam-Djerem National Park was impacted by the Lom Panger Dam Project. Furthermore, measuring the results of biodiversity offsets to determine no net loss (NNL) and net gain (NG) poses challenges, requiring consistent management of protected sites and sound inventories over 20 years to generate solid data supporting sustained NNL/NG justification. Demonstrating net gain, while difficult, is considered essentially achieved when the population of biodiversity values (e.g., chimpanzees, gorillas, elephants) is stable or preferably increasing. To ensure net gain, it is recommended to undertake both offset and restoration of project sites, as well as onsite set-asides, to maximise opportunities for biodiversity conservation [17, 18, 19].