Standing Biomass and Litter-Fall in Reforested Mangroves of Douala-Edea National Park, Cameroon
Ecological restoration is considered like the practical and sustainable management option for degraded mangroves. The ecological status of reforested mangrove areas (RMAs) in Douala-Edea National Park (DENP) is not well understood, despite extensive restoration projects. This study aims to estimate standing biomass and litterfall production in three RMAs located in Bolondo and Yoyo II. In each RMA and it natural vegetation, five 10m × 10m permanent sampling plots (PSPs) were established. Thirty PSPs were established equally in RMAs and natural vegetation. Height and diameter measurements were recorded, and allometric equations were used to estimate above-ground biomass (AGB) and below-ground biomass (BGB). In addition, 150 litter traps (1m × 1m) were evenly distributed across the PSPs to collect monthly litterfall, which was dried, sorted, and weighed. Mean abundances, diameters, and heights were: 4000±200 ind./ha, 1.2±0.5 cm, and 1.8±0.3 m; 3280±238.74 ind./ha, 2.58±0.85 cm, and 5.64±1.87 m; 2160±240.83 ind./ha, 2.93±1.4 cm, and 3.34±1.26 m for 3-year, 6-year and 11-year RMAs respectively. AGB, BGB, and annual litterfall biomass were: 11.98±0.76 kg/ha, 13.88±1.3 kg/ha, and 40.78±7.42 g/m²/year for 3-year RMAs; 61.18±2.16 kg/ha, 55.19±1.92 kg/ha, and 397.75±75.79 g/m²/year for 6-year RMAs; and 55.25±2.93 kg/ha, 47.8±1.31 kg/ha, and 576.23±106.75 g/m²/year for 11-year RMAs, respectively. These values correspond to approximately 12.31±2.19, 52.08±1.6, and 46.26±1.1 kgC/ha of total carbon sink and 18.27±3.48, 179.09±4.44, and 259.3±8.89 kgC/ha/year of total annual litterfall carbon sink. Although 6-year RMAs showed higher AGB and BGB than natural vegetation, the values remained lower overall. In contrast, 11-year RMAs exhibited higher annual litterfall production, indicating progress towards ecological balance.
Globally, mangroves cover an area of 137,000 km² (Spalding and Leal, 2021). These tropical coastal forest ecosystems are renowned for their high biomass productivity and carbon sequestration capacity (Alongi, 2011; Chowdhury et al., 2023). On average, a minimally disturbed mangrove sequesters 1087±584 MgC/ha (Sasmito et al., 2020). The conservation of mangroves is crucial for the wellbeing of local human communities and the fight against global warming (Din et al., 2016; Kathiresan et al., 2021).
However, mangroves are under threat from both natural and human-induced pressures, leading to a reduction in vegetation cover (Golberg et al., 2020; Emanè et al., 2021). Between 1996 and 2016, the rate of deforestation of mangroves was estimated at 4.3 % (Spalding and Leal, 2021). During the period of 2000- 2012, almost 1646 km² of the world's mangrove area disappeared. Along its western Atlantic coast, Cameroon has 1113 km² of mangroves (Hamilton and Casey, 2016). Over the last two decades, these mangroves have been subject to increased anthropisation, which has led to a considerable decrease in vegetation cover (Din et al., 2017). The mangroves in Douala-Edea National Park (DENP) are also affected by this issue. According to Ajonina and Usongo (2001), the annual regression rate of these mangroves was estimated to be 53 ha before 2000. Similarly, Findi and Wantim (2022) observed a regression rate of 58.38 % between 2011 and 2015 using satellite image analysis. For at least three decades, these mangroves have been the primary source of firewood used for smoking fish in various fishing camps and surrounding households (Ajonina, 2008).
In response to this ecological disaster, various options for mangrove conservation have been implemented worldwide, including restoration. Mangrove restoration refers to a set of human actions aimed at re-establishing ecological processes that accelerate the recovery of forest structure, ecological functioning, and biodiversity to levels typical of climax forest (Elliott et al., 2013). According to Worthington and Spalding (2018), over 190,147 km² of mangroves were restored globally by 2018. In Central Africa, Ajonina et al. (2016) reported that over 500 ha of degraded mangroves were reforested before 2017. Reforestation of degraded mangroves in Cameroon officially began in 2009 and continues to this day. In 2023, Planète-Urgence’s NGO implemented the planting of 40,000 seedlings in the mangroves of Bolondo fishing camp village located in DENP, with the involvement of governmental organizations and other partners (Planète-Urgence, 2023).
Mangrove restoration projects face numerous obstacles worldwide, often resulting in partial or complete failure (López-Portillo et al., 2017; Worthington and Spalding, 2018; Ellison et al., 2020; Lhosupasirirat et al., 2023). These failures not only cause ecological damage but also result in significant financial losses, with the average cost of restoring one hectare in Central Africa estimated at US$ 3200 (Ajonina et al., 2016). Lee et al. (2019) identified the absence of a comprehensive database for monitoring and assessment restoration projects as a major obstacle to mangrove restoration efforts.
There is a lack of information on the structure and functioning of restored mangrove areas along the African Atlantic coast (Zabbey and Tanee, 2016). To our knowledge, in Cameroon, there is only one specific study on mangrove restoration which is limited to determining the main abiotic factors that influence the growth of seedlings in nurseries (Boubakary et al., 2019). No published scientific study has yet been conducted on the monitoring and assessment of standing biomass and litterfall production of reforested mangrove areas in the DENP. However, quantifying the biomass of reforested areas would be crucial for safeguarding mangroves in the DENP and for improving climate change mitigation strategies (Malik et al., 2020). Furthermore, it is essential to incorporate the monitoring of objectively verifiable indicators of ecosystem functioning, such as litterfall production into any restoration project (Dahdouh-Guebas and Cannicci, 2021). To contribute improvement of mangrove restoration projects in DENP, this study aims to assess the impact of mangrove reforestation through the estimation of both standing biomass carbon stock and litterfall production of three reforested mangrove areas with reference to their respective adjacent good natural stands.
Source : Assessment of standing biomass and litter-fall production in reforested mangrove stands within Douala-Edea National Park (Cameroon)
