1. Introduction
⌅Mangroves are forests that grow in the tidal area of intertropical regions. They are an association of halophytic woody species, generally dominated by mangroves, which are more resistant to salinity and can live on anaerobic soils characterized by maximum humidity (Folega et al., 2017Folega, F., Rakotondrasoa, M.A., Wala, K., Woegan, Y.A., Kanda, M., Pereki, H., Polo-Akpisso, A., & Batawila, K.A.K., 2017. Écologie et dynamique spatio-temporelle des mangroves au Togo. VertigO - la revue électronique en sciences de l'environnement. 17(3): 1-24. 10.4000/vertigo.18791). Mangroves are found along shorelines, estuaries, and deltas (White & Edwards, 2001White, L. & Edwards, A., (eds.) 2001. Conservation des forêts pluviales Africaines: manuel des méthodes de recherche. New York, USA: Wildlife Conservation Society. 10.4000/vertigo.5176). Mangroves play crucial biological, ecological and social roles for the benefit of humanity. Proisy (1999Proisy, C., (eds), 1999. Apport des données radar à synthèse d’ouverture pour l’étude de la dynamique des écosystèmes forestiers. Phd ThesisUniversité Paul Sabatier Toulouse III, France.) found that mangroves produce an estimated 10 tons per hectare per year of excess dry plant matter.
Mangrove forests form an important barrier against natural disasters. They protect surrounding populations from damage caused by cyclones, hurricanes, and tsunamis (Murdiyarso et al., 2015Murdiyarso, D., Purbopuspito, J., Kauffman, J.B., Warren, M.W., Sasmito, S.D., Donato, D.C., Manuri, S., Krisnawati, H., Taberina, S., & Kurnianto, S., 2015. The potential of Indonesian mangrove forests for global climate change mitigation. Nature Climate Change, 5(7) : 1089-1092. 10.1038/nclimate2734; Nfotabong, 2011Nfotabong, A., 2011. Impacts des activités anthropiques sur la structure de la végétation des mangroves de Kribi, de l’embouchure du fleuve Nyong et de l’estuaire du Cameroun. Phd ThesisUniversité de Douala, Cameroum.). Mangroves limit coastal erosion and help land advance towards the ocean, while providing a buffer area in regions prone to storms and cyclones (Hervieu, 1969Hervieu, J., 1969. Contribution à l'étude du milieu fluvio-marin sur la côte occidentale de Madagascar. Revue de géographie de Madagascar3470(8): 11-66.). Mangroves play an important role in controlling water pollution. The tangled roots of mangroves help filter estuarine water, trapping coarse debris and sediments (Folega et al., 2017Folega, F., Rakotondrasoa, M.A., Wala, K., Woegan, Y.A., Kanda, M., Pereki, H., Polo-Akpisso, A., & Batawila, K.A.K., 2017. Écologie et dynamique spatio-temporelle des mangroves au Togo. VertigO - la revue électronique en sciences de l'environnement. 17(3): 1-24. 10.4000/vertigo.18791; Fromard, 2002Fromard, F., 2002. Les mangroves du Vietnam du Sud: Histoire récente, dynamique actuelle et perspectives. Bois et forêts des tropiques, 273(3): 31-42. .https://revues.cirad.fr/index.php/BFT/article/view/20135).
Like all forest ecosystems, mangroves sequester atmospheric CO2 and thus help reduce global warming. They are an ecosystem of great importance in the global carbon cycle, due to their high productivity, global distribution, and position at the interface between land and ocean. Mangroves have a dual role as a sink for atmospheric CO2 and as a source of organic and inorganic carbon for coastal zones (Sharma et al., 2020Sharma, S., MacKenzie, R.A., Tieng, T., Soben, K., Tulyasuwan, N., & Resanond, A., Blate, G., & Litton, C.M., 2020. The impacts of degradation, deforestation, and restoration on mangrove ecosystem carbon stocks across Cambodia. Science of the Total Environment, 706(1): 135-416. 10.1016/j.scitotenv.2019.135416; Ajonina, 2013Ajonina, G.N., 2013. Etablissement du bilan carbone des mangroves des zones humides du Complexe ouest du sud-Bénin en vue de la préparation d’un projet MDP. Technical report. https://www.ecobenin.org/wp-content/uploads/2014/06/pdf_etude_de_bilan_de_sequestration_de_carbone_des_mangroves_des_zones_humides_du_complexe_ouest_du_sud_final_1_.pdf.; Nfotabong, 2011Nfotabong, A., 2011. Impacts des activités anthropiques sur la structure de la végétation des mangroves de Kribi, de l’embouchure du fleuve Nyong et de l’estuaire du Cameroun. Phd ThesisUniversité de Douala, Cameroum.). Mangroves play a significant role in climate stability: according to recent estimates, around 11% of the total mass of organic carbon at the land-ocean interface is stored by mangroves (Nfotabong, 2011Nfotabong, A., 2011. Impacts des activités anthropiques sur la structure de la végétation des mangroves de Kribi, de l’embouchure du fleuve Nyong et de l’estuaire du Cameroun. Phd ThesisUniversité de Douala, Cameroum.).
Mangrove forests are home to a variety of vertebrates (monkeys, snakes, birds, etc.) and invertebrates (crabs, gastropods, insects, etc.). Mangroves are an important habitat for maintaining aquatic biodiversity; they are the breeding, spawning, egg-hatching, and nursery grounds for young fish and crustaceans. (Folega et al., 2017Folega, F., Rakotondrasoa, M.A., Wala, K., Woegan, Y.A., Kanda, M., Pereki, H., Polo-Akpisso, A., & Batawila, K.A.K., 2017. Écologie et dynamique spatio-temporelle des mangroves au Togo. VertigO - la revue électronique en sciences de l'environnement. 17(3): 1-24. 10.4000/vertigo.18791). On the one hand, these intertidal forests are a potential recreational environment for the development of ecotourism activities. On the other hand, they constitute a reserve of goods and services for the population living in and around them (Cormier-Sale, 1994Cormier-Sale, M.C., 1994. Dynamique et usages de la mangrove dans les pays des rivières du sud (du Sénégal à la Sierra Leone), Ed. ORSTOM, Paris, 357 p. https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_6/colloques2/41056.pdf (Page consulted on October 07, 2022)).
However, recent studies on mangrove dynamics point to an increasing regression in the areas occupied by woody mangrove vegetation due to anthropogenic pressure: urbanization, energy wood extraction, land conversion for aquaculture and agriculture, dyke construction, etc. (Marjolaine et al., 2022Marjolaine, O.G., Rogombe, L.G., Ondo, AE., Lembe, B.A., Akendengue, A.I., & Mombo, J.B., 2022. Les moteurs de la déforestation des mangroves urbaines du Grand Libreville (Gabon). VertigO - la revue électronique en sciences de l’environnement. 22(1): 1-35. https://journals.openedition.org/vertigo/35668 (Page consulted on July 07, 2022); Bhomia et al., 2016Bhomia, R.K., MacKenzie, R.A., Murdiyarso, D., Sasmito, S.D., & Purbopuspito, J., 2016. Impacts of Land Use on Indian Mangrove Forest Carbon Stocks: Implications for Conservation and Management. Ecological Applications, 26(5): 1396–1408. 10.1890/15-2143; Webber et al., 2016Webber, M.H., Calumpong, B., Ferreira, E., Granek, S., Green, R., Ruwa, & Soares, M., 2016. Mangroves. New York, USA: World Ocean Assessment, United Nations. https://www.un.org/depts/los/global_reporting/WOA_RPROC/Chapter_48.pdf; Mpoyi et al., 2013Mpoyi, A.M., Nyamwoga, F.B., Kabamba, F.M., & Assembe-Mvondo, S., 2013. Le contexte de la REDD+ en République Démocratique du Congo : Causes, agents et institutions. Document Occasionnel, CIFOR, Bogor, Indonésie, 84 p.; CormierSale, 1994Cormier-Sale, M.C., 1994. Dynamique et usages de la mangrove dans les pays des rivières du sud (du Sénégal à la Sierra Leone), Ed. ORSTOM, Paris, 357 p. https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_6/colloques2/41056.pdf (Page consulted on October 07, 2022)).
Most coastal villagers (81 %) use resources from mangroves. The main driver of mangrove deforestation is timber extraction (32 %). Mangrove wood is mainly used to fuel woodburning ovens for cooking. In fact, it is the most widely used fuel (Folega et al., 2017Folega, F., Rakotondrasoa, M.A., Wala, K., Woegan, Y.A., Kanda, M., Pereki, H., Polo-Akpisso, A., & Batawila, K.A.K., 2017. Écologie et dynamique spatio-temporelle des mangroves au Togo. VertigO - la revue électronique en sciences de l'environnement. 17(3): 1-24. 10.4000/vertigo.18791; Schure et al., 2012Schure, J., Marien, J.N., De Wasseige, C., Drigo, R., Salbitano, F., Dirou, S., & Nkoua, M., 2012. Contribution du bois énergie à la satisfaction des besoins énergétiques des populations d’Afrique centrale : perspectives pour une gestion durable des ressources, Les forêts du bassin du Congo : État des forêts 2010. Luxembourg, pp. 109- 122; Trefon et al., 2010Trefon, T., Hendricks, T., Kabuyaya, N., & Ngoy, B., 2010. L’économie politique de la flière du charbon de bois à Kinshasa et à Lubumbashi : appui stratégique à la reconstruction post conflit en RDC. Institute of Development Policy and Management, Anvers, Belgique, 110 p.; CIFOR et al., 2007CIFOR, Banque mondiale & CIRAD, 2007. La forêt en République Démocratique du Congo post-confli: Analyse d’un agenda prioritaire. Jakarta, Indonesia: Center for International Forestry Research. https://www.cifor.org/publications/pdf_files/Books/BCIFOR0701F.pdf (Page consulted on October 01, 2022); MECNT, 2012MECNT (Ministère de l’Environnement Conservation de la Nature et Tourisme), 2012. Etude qualitative sur les causes de la déforestation et de la dégradation des forêts en République Démocratique du Congo, 165 p. Rajoharison, 1990Rajoharison, H.M., 1990. Impacts de l’érosion continentale sur zone de Mangroves, 8 p.; MAFET, 2002MAFET (Ministère des Affaires Foncières, Environnement et Tourisme), 2002. Septième réunion de la conférence des parties à la convention sur les espèces migratrices appartenant à la faune sauvage, Bonn / Allemagne, 18 – 24 septembre 2002, 22 p.). However, a tone of wood contains 500 kg of carbon, so 365 kg of carbon are released into the atmosphere after each carbonization of a tone of wood, with a low-performance technique. Conversely, with an improved carbonization technique, these emissions can be reduced to 275 kg (Trefon et al., 2010Trefon, T., Hendricks, T., Kabuyaya, N., & Ngoy, B., 2010. L’économie politique de la flière du charbon de bois à Kinshasa et à Lubumbashi : appui stratégique à la reconstruction post conflit en RDC. Institute of Development Policy and Management, Anvers, Belgique, 110 p.; Mangion, 2010Mangion, I. 2010, Le futur mécanisme REDD face aux moteurs de la déforestation et de la dégradation des forêts au Brésil, en Indonésie et en RDC. Mémoire de Fin de Master en Sciences et Gestion de l’Environnement, ULB. 152 p.).
The rate of mangrove area loss in Africa is estimated at around 13.8 % (Polidoro et al., 2010Polidoro, B.A., Carpenter, K.E., Collins, L., Duke, N.C., Ellison, A.M., Joanna, C.E., Elizabeth, J.F., Edwino, S.F., Kandasamy, K., Nico, E.K., Suzanne, R.L., Toyohiko, M., Gregg, E.M., Vien Ngoc, N., Jin Eong, O., Jurgenne, H.P., & Severino, G.S., 2010. The Loss of Species: Mangrove Extinction Risk and Geographic Areas of Global Concern. PLoS ONE5(4): e1009510.1371/journal.pone.0010095). The same authors (Polidoro et al., 2010Polidoro, B.A., Carpenter, K.E., Collins, L., Duke, N.C., Ellison, A.M., Joanna, C.E., Elizabeth, J.F., Edwino, S.F., Kandasamy, K., Nico, E.K., Suzanne, R.L., Toyohiko, M., Gregg, E.M., Vien Ngoc, N., Jin Eong, O., Jurgenne, H.P., & Severino, G.S., 2010. The Loss of Species: Mangrove Extinction Risk and Geographic Areas of Global Concern. PLoS ONE5(4): e1009510.1371/journal.pone.0010095) report that 16% of species exclusive to mangroves are at high risk of extinction. The destruction of these forests not only leads to changes in their structure, but also to the loss of ecological, biological and economic functions (Folega et al., 2017Folega, F., Rakotondrasoa, M.A., Wala, K., Woegan, Y.A., Kanda, M., Pereki, H., Polo-Akpisso, A., & Batawila, K.A.K., 2017. Écologie et dynamique spatio-temporelle des mangroves au Togo. VertigO - la revue électronique en sciences de l'environnement. 17(3): 1-24. 10.4000/vertigo.18791). Population growth in rural areas plays an important role in the loss of forest cover (Tungi-Tungi et al., 2022Schure, J., Marien, J.N., De Wasseige, C., Drigo, R., Salbitano, F., Dirou, S., & Nkoua, M., 2012. Contribution du bois énergie à la satisfaction des besoins énergétiques des populations d’Afrique centrale : perspectives pour une gestion durable des ressources, Les forêts du bassin du Congo : État des forêts 2010. Luxembourg, pp. 109- 122; Tungi-Tungi et al., 2021Sharma, S., MacKenzie, R.A., Tieng, T., Soben, K., Tulyasuwan, N., & Resanond, A., Blate, G., & Litton, C.M., 2020. The impacts of degradation, deforestation, and restoration on mangrove ecosystem carbon stocks across Cambodia. Science of the Total Environment, 706(1): 135-416. 10.1016/j.scitotenv.2019.135416; Defourny and Kibambe, 2012Defourny, P. & Kibambe, L., 2012. Croissance des populations, sécurité alimentaire et couverture forestière en RDC : causes directes et moteurs de la déforestation. Séminaire, UC. Louvain (Belgique), 40 p.; Defourny et al., 2011Defourny, P., Delhage, C., & Kibambe, J.P., 2011. Analyse quantitative des causes de la déforestation et de la dégradation des forêts en République Démocratique du Congo. Rapport final, UC. Louvain (Belgique), 105 p.).
The immediate consequence of mangrove overexploitation is highlighted in several studies addressing the loss of the original mangrove area (Marjolaine et al., 2022Marjolaine, O.G., Rogombe, L.G., Ondo, AE., Lembe, B.A., Akendengue, A.I., & Mombo, J.B., 2022. Les moteurs de la déforestation des mangroves urbaines du Grand Libreville (Gabon). VertigO - la revue électronique en sciences de l’environnement. 22(1): 1-35. https://journals.openedition.org/vertigo/35668 (Page consulted on July 07, 2022); Sharma et al., 2020Sharma, S., MacKenzie, R.A., Tieng, T., Soben, K., Tulyasuwan, N., & Resanond, A., Blate, G., & Litton, C.M., 2020. The impacts of degradation, deforestation, and restoration on mangrove ecosystem carbon stocks across Cambodia. Science of the Total Environment, 706(1): 135-416. 10.1016/j.scitotenv.2019.135416; Bahamondez and Thompson, 2016Bahamondez, C., & Thompson, I.D., 2016. Determining Forest degradation, ecosystem state and resilience using a standard stand stocking measurement diagram: theory into practice, Forestry, 89, (3): 290–300. 10.1093/forestry/cpv052; Roche and Van Cu, 2015Roche, Y., & Van Cu, P., 2015. Les mangroves face aux changements climatiques : Le cas à la fois typique et particulier du Vietnam. Vertigo, Hors-série23. 10.4000/vertigo.16600). Rajoharison (1990Rajoharison, H.M., 1990. Impacts de l’érosion continentale sur zone de Mangroves, 8 p.) points out that the disappearance of mangrove plant cover exposes the area to various natural disasters such as coastal erosion, flooding and the multiplication of flash and flash floods; this is the case of the river disasters that caused the gradual accumulation of alluvial deposits in lagoons, mangroves and coral reefs as a whole in the 1990s in Madagascar (Guillet et al., 2008Guillet, M., Renoux, E., Robin, M., Debaine, F., Rakotonavalona, H.D., & Ratsivalaka, S., 2008. Suivi et analyse de l’évolution de la mangrove de Mahajamba (Nord-Ouest de Madagascar). Actes ducolloque international pluridisciplinaire “Le littoral : subir, dire, agir” - Lille, France, 16-18 janvier 2008. 8 p.).
In addition to human activities, natural factors also contribute to mangrove disturbance, such as successive deposits of sand on ocean shores, which cover the lenticels of Avicennia pneumatophores, leading to the physiological death of mangroves; violent waves, which cause mangroves to fall, etc. However, they are less disruptive to those coastal ecosystems than manmade cuttings. And they are less disruptive to coastal ecosystems than man-made logging (Nfotabong, 2011Nfotabong, A., 2011. Impacts des activités anthropiques sur la structure de la végétation des mangroves de Kribi, de l’embouchure du fleuve Nyong et de l’estuaire du Cameroun. Phd ThesisUniversité de Douala, Cameroum.; Rajoharison, 1990Rajoharison, H.M., 1990. Impacts de l’érosion continentale sur zone de Mangroves, 8 p.).
The Moanda Mangrove Marine Park in the Democratic Republic of the Congo (DRC) is no exception to the degradation and deforestation that mangroves are experiencing worldwide. Despite its status as a protected area, farmers are clearing fields and cutting mangroves for carbonization. Indeed, the absence of a permanent culinary energy source other than wood is one of the causes of the use of mangrove wood as an energy source by coastal households (MECNEF, 2007MECNEF (Ministère de l’Environnement, Conservation de la Nature, Eaux et Forêts), 2007. Profil de la zone côtière de la RDC. Kinshasa, 65 p.). OSFAC (2014OSFAC (observatoire satellital des forêts d’Afrique centrale), 2014. Forêts d’Afrique centrale Evaluées par Télédétection (FACET). Document statistique, 68 p. https://www.osfac.net/images/data_and_products/facet/docs/FACET_RDC_Statistic.pdf (Page consulted on August 20, 2022)) estimated the annual deforestation rate of this Mangrove Marine Park at 0.04% for the period 2000-2010. This situation is becoming problematic due to the lack of incentives to restore the natural landscape of this Mangrove Marine Park.
The aim of this study is to map and quantify the dynamics of land cover in the Moanda Mangrove Marine Park between 2002 and 2020; in particular: (i) to assess the annual rate of change in the mangrove land cover; (ii) to estimate the influence of wood energy on the decline in mangrove forest area; and (iii) to propose a number of measures for the sustainable management of the park's wood resources.
2. Material and method
⌅2.1. Study area
⌅This research is being carried out in the Moanda Mangrove Marine Park in DRC. The park is located in the extreme south-west of the DRC, just at the Congo River mouth, in the Mer sector, Moanda territory in the province of Central Kongo.
The Moanda Mangrove Marine Park is covered by a vast vegetation characterized by savannah, degraded forest land, high or old mangrove dominated by the species Rhizophora racemos and Rhizophora mangle, low or young mangrove dominated by the species Avicenia germinans, and herbaceous mangrove. Mangrove vegetation covers the banks of the Congo River (Figure 2). The mangroves of Moanda are declared a protected area under ministerial decree n°044/CM/ECN/92 of 02.V.1992, known as Nature Reserve or Mangrove Marine Park (MECN, 1992MECN (Ministère de l’Environnement et Conservation de la Nature), 1992. Arrêté ministériel n°044/CM/ECN/92 du 02 Mai 1992, portant création et délimitation d’une réserve dénommée Reserve Naturelle des mangroves ou Parc marin. République du Zaïre, 5 p.). This Marine Park covers an administrative area of 76,000 ha, of which the Atlantic Ocean occupies around 20% (MECNT and WRI, 2009MECNT (Ministère de l’Environnement Conservation de la Nature et Tourisme) et WRI (World Resources Institute), 2009. Atlas forestier interactif de la république démocratique du Congo. 68 p.).
Moanda’s soil is of the Guinean-Congolese type, specifically sandy-clay, which is favorable for growing cassava, groundnuts, and perennial crops. Its northwestern part is dominated by a vast swampy area, suitable for lowland crops (Moanda Territory Office, 2015Moanda Territory Office, 2015. Annual report 2015, 60 p.).
Nearly 45 villages are settled in or close to Mangrove Marine Park, depending largely on the resources of the said park (Moanda Territory Office, 2015Moanda Territory Office, 2015. Annual report 2015, 60 p.). Most of the population of these villages lives mainly from slash-and-burn agriculture, artisanal fishing, small livestock and poultry rearing, as well as partially from small game hunting. Moanda is home to two oil companies. These are the French company PERENCO-REP for oil exploration and exploitation both offshore and onshore, and the Congolese company for refining industries (COCIR) (ULB-coopération, 2021ULB-coopération, 2021. Caractérisation des contextes socio-économiques et environnementaux de cinq villages du Parc Marin des Mangroves et de sa périphérie en République Démocratique du Congo. Rapport de formation/action au diagnostic agraire. https://www.ulb-cooperation.org/wp-content/uploads/2021/09/202107-rapport-de-formation-action-au-diagnostic-agraire-parc-marin-des-mangroves.pdf (Page consulted on June 10, 2022); Vambi et al., 2018Vambi, N.B., Subi, M.O., & Tasi, M.J.P., 2018. Ruée vers les ressources halieutiques dans le Parc Marin des Mangroves à Moanda en République Démocratique du Congo. RAFEA-Revue africaine environnement et agriculture, l.1 (2) : 21-28. http://www.rafea-congo.com/admin/pdfFile/Rafea-Article-Vambi-et-al-2018.pdf (Page consulted on August 19, 2022); Moanda Territory Office, 2015Moanda Territory Office, 2015. Annual report 2015, 60 p.).
2.2. Methods
⌅2.2.1. Data
⌅Field data were collected in two ways: (1) in the form of geospatial data, using a Global Positioning System (GPS) receiver, (2) in the form of socio-economic survey data, using a pre-established questionnaire validated by the research team. Through this questionnaire, the rural population was interviewed about their various activities having a positive or negative impact on the forest cover of the Mangrove Marine Park. The survey was carried out in 22 villages in and around the Moanda Mangrove Marine Park. Nearly 250 rural households were surveyed, with ± 11 households per locality.
To complete the data, we used raster data. This included Landsat 7 satellite imagery acquired on April 08, 2002, and Landsat 8 on May 3, 2020, and forest cover evolution data from intervening years acquired on the Global Forest Watch (GFW) platform. The GFW dataset has a spatial resolution of 30 m (Hansen et al., 2013Hansen, M.C., Potapov, P.V., Moore, R., Hancher, M., Turubanova, S.A., Tyukavina, A., Thau, D., Stehman, S.V., Goetz, S.J., Loveland, T.R., Kommareddy, A., Egorov, A., Chini, L., Justice, C.O., & Townshend, J.R.G., 2013. High-Resolution Global Maps of 21st-Century Forest Cover Change. In Science, Vol 342, Issue 6160, pp. 850-853. 10.1126/science.1244693).
Satellite data underwent radiometric correction to eliminate contamination introduced by the atmosphere. To improve localization accuracy, geometric correction involving esampling and ortho-rectification was applied. These images were then normalized to take account of the effect of anisotropy, to reduce highlighting in the east of the image compared with the west.
Data relating to the road network, hydrographic network and administrative boundaries were downloaded from various databases (RGC, WRI and OSM RDC). Given the size of the study area and the inaccessibility of certain zones, the geolocalized data collected in the field were completed using Google Earth, to validate the results of the land cover classification carried out using satellite images.
2.2.2. Data Geospatial data processing and analysis
⌅Land cover classes in the Moanda Mangrove Marine Park were discriminated using supervised classification of Landsat satellite images (Kharki et al., 2021Kharki, O., Mechbouh, J., Ducrot, D., & Mvogo, J.N., 2021. Panorama sur les méthodes de classification des images satellites et techniques d’amélioration de la précision de la classification. In Revue Française de Photogrammétrie et de Télédétection210(210): 23-38; Pony et al., 2000Pony, O., Descombes, X., et Zerubia, J., 2000. Classification d’images satellitaires hyperspectrales en zone rurale et périurbaine. [Rapport de recherche] RR-4008, INRIA. 2000, 64 p. inria-00072636). Supervised classification requires a priori knowledge of the area under study, to create training area. This classification was carried out using the Maximum Likelihood algorithm in ENVI 5.0 software. This algorithm classifies pixels using a probabilistic approach. For each pixel in the image, it calculates the probability of being assigned to each class. These calculations are based on the mean of the training area, the spectral signature of the pixel and the standard error margin of the covariance matrix of the pixels in the training area. The pixel is then assigned to the class with the highest probability (Kharki et al., 2021Kharki, O., Mechbouh, J., Ducrot, D., & Mvogo, J.N., 2021. Panorama sur les méthodes de classification des images satellites et techniques d’amélioration de la précision de la classification. In Revue Française de Photogrammétrie et de Télédétection210(210): 23-38; Pony et al., 2000Pony, O., Descombes, X., et Zerubia, J., 2000. Classification d’images satellitaires hyperspectrales en zone rurale et périurbaine. [Rapport de recherche] RR-4008, INRIA. 2000, 64 p. inria-00072636).
Land cover classes were separated and then cross-referenced to discriminate forest evolution in the Moanda Mangrove Marine Park. Deforestation in the Moanda Mangrove Marine Park was monitored for the intervening years (between 2002 and 2020) using Global Forest Change data.
2.2.3. Evaluation of classification results
⌅The concordance between ground reality and the land cover classes produced from satellite image classification was assessed using the confusion matrix (Tungi tungi et al, 2021Tungi Tungi, LJ., Madibi, M.P., Nsimba, N.E., Lutete, L.E., Lendo, M.C., Baraka, L.P., Ndiyo, E.S., Lumbuenamo, S.R., Tshimanga, M.R., & Mwamba, T.C., 2021. Impact de la croissance démographique et de l’expansion urbaine sur la dynamique forestière des zones environnantes de la ville de Kikwit en République Démocratique du Congo. Revue Africaine d’Environnement et d’Agriculture, 4 (4): 23-30). The confusion matrix is a contingency table that compares the results of interpretation with field data recognized as “the truth”, after which accuracy indicators are produced.
2.2.3. Statistical analysis
⌅Statistical analyses of land cover data and survey data were carried out based on the following formulas:
Where σ2: variance; Mi: middle of class; x̅ : mean of observations; fi: frequency of observations; Σ: sum; σ: standard deviation; n: sample size
Bernier’s formula (1992) was used to highlight the rate of annual change in land cover classes:
T: Annual rate of spatial expansion in %;
: Neperian logarithm;
e: The base of the neperian logarithm (e = 2.71828);
S1: Area of first-year land cover;
S2: Area of recent year’s land cover;
t: Number of years in the period concerned.
Survey data were analyzed using Epidata and SPSS 28.0.1 statistical software and Microsoft Excel.
3. Results
⌅3.1. Riparian forest characteristics: tree species, sizes, and ages
⌅In the initial year (2002), the landscape of the Moanda Mangrove Marine Park was mainly savannah (25%), Rhizophora sp. forest (17.5%), young secondary forest (15.6%), Avicenia sp. forest (11.8%) and water (11%). Anthropogenic area (8.7%), herbaceous mangroves (7.4%) and mature secondary forest (2.6%) occupied very small areas. In 2002, this landscape was less disturbed, but over the years it has undergone significant changes, generally of anthropogenic origin (Table 1 and Figure 4).
The result of the land cover classification shows a very good match with the data collected in the field. This agreement is justified by the accuracy indicators, overall accuracy, and Kappa index of 82.4% and 0.81 respectively.
3.2. Dynamics of land cover in the Mangrove Marine Park between 2002 - 2020
⌅The Rhizophora sp. forest (high mangroves) has decreased from around 12633 ha in 2002 to around 12459 ha in 2020, representing a loss of 1.38% of its initial area (174.5 ha). The Avicenia sp. forest (low mangroves) covered around 8559 ha in the initial year (2002), and by the year 2022 it covers 8478 ha, representing a 0.95% reduction in its initial area. The greatest losses are observed in the mature secondary forest class, with 388.6 ha representing 20.5% of its initial area, and the young secondary forest class, with 1343 ha, or 11.8% of its initial area. Unlike the forest classes, which saw a reduction in their initial areas, the savannah, herbaceous mangrove and anthropogenic area classes expanded by 265 ha (4.96%), 396.63 ha (2.19%) and 1325.25 ha (21%) respectively (Figure 5).
Spatial analysis of satellite images providing information on GFW forest cover for the study area from 2002 to 2020 revealed average annual deforestation of around 79.62 ± 86.19 ha, a rate of around 0.07%. Annual trends in the loss of forest cover show a progression in terms of total number of occurrences, although there were years of inflexion in 2014 (369.5 ha), 2011 (141.9 ha) and 2010 (167 ha) in terms of the severity of this phenomenon. On the other hand, deforestation was low in 2003 (7.9 ha) and 2004 (21.5 ha) (Figure 6).
Analysis of land cover dynamics revealed an exceptional rate of change in land cover classes in the Moanda Mangrove Marine Park. The analysis revealed a more accelerated annual rate of regressive change for the mature secondary forest (-1.27%) and young secondary forest (-0.7%) classes. The annual rate of deforestation of Rhizophora sp. forest and Avicenia sp. forest is very low compared with that of young secondary forest and adult secondary forest. The most pronounced annual rate of expansion is observed in anthropogenic area (1.06%), herbaceous mangroves (0.27%) and savannah (0.12%) (Table 2).
3.3. Survey results
⌅Fishing is the main income-generating activity for most of the households surveyed (31%); government employment and sales account for 3% and 17% respectively. 26% of households depend mainly on agriculture, and 22% of households surveyed identified charcoal production as their main income-generating activity (Figure 7). According to field data, mangrove charcoal is in greater demand on the Moanda market, due to its high calorific value. Charcoal is generally produced by groups of charcoal-makers, although some charcoal-makers remain solitary. There are also a few reluctant charcoal burners, even in the fully protected area. The exploitation of wood energy, mainly charcoal, is most intense in and around the Mangrove Marine Park, with monthly production reaching around 252 ± 160 bags.
Slash-and-burn agriculture is one of the activities adversely affecting forest cover in and around the Moanda Mangrove Marine Park. Farmers grow more cassava, maize, groundnuts, beans, eggplants, and tomatoes. A coastal household clears an average of 2 ± 0.872 fields per year, each covering an area valued at around 0.375 ± 0.218 ha.
4. Discussion
⌅The results of the satellite data analysis revealed strong landscape dynamics in the Moanda Mangrove Marine Park between the years 2002 and 2020. Major changes were observed in the adult secondary forest (-20%), young secondary forest (-11.8%), anthropogenic area (21%) and herbaceous mangroves (4%) classes. As for mangrove forest, the loss amounts to 1.38%, i.e. 174.5 ha for Rhizophora sp. forest, and 0.95%, i.e. 81.09 ha for Avicenia sp. forest. The negative change is recorded only in the forest classes, while non-forest areas have all seen an extension of their initial areas.
Overall, deforestation dynamics in the Moanda Mangrove Marine Park are estimated at an annual average of around 79.62 ± 86.19 ha, or a rate of 0.07%. The park lost large tracts of forest in 2014 (369.5 ha), 2011 (141.9 ha) and 2010 (167 ha). This high level of deforestation is thought to be due to the intensification of anthropogenic activities within the Mangrove Marine Park, such as unsustainable agriculture and the cutting of wood for various uses (construction, firewood, charcoal, etc.).
The analysis revealed a more accelerated annual rate of regressive change for the adult secondary forest (-1.27%) and young secondary forest (-0.7%) classes. The annual rate of deforestation of Rhizophora sp. forest and Avicenia sp. forest is very low compared with that of young secondary forest and adult secondary forest. The most pronounced annual rate of expansion is observed in anthropogenic area (1.06%), herbaceous mangroves (0.27%) and savannah (0.12%).
The annual deforestation rate found by our analyses (0.07%) is higher than that (0.04%) found by OSFAC (2014OSFAC (observatoire satellital des forêts d’Afrique centrale), 2014. Forêts d’Afrique centrale Evaluées par Télédétection (FACET). Document statistique, 68 p. https://www.osfac.net/images/data_and_products/facet/docs/FACET_RDC_Statistic.pdf (Page consulted on August 20, 2022)), as the latter’s analyses were based on the period 2000-2010, while our analyses considered the broader period 2002-2020. In addition, our results on the evolution of land-use classes confirm the high production of fuelwood and the activity of shifting cultivation in the park, as reported during field surveys. In addition, our analyses revealed very high deforestation values in years not considered by the study conducted by OSFAC (2014OSFAC (observatoire satellital des forêts d’Afrique centrale), 2014. Forêts d’Afrique centrale Evaluées par Télédétection (FACET). Document statistique, 68 p. https://www.osfac.net/images/data_and_products/facet/docs/FACET_RDC_Statistic.pdf (Page consulted on August 20, 2022)), namely 2014 and 2011.
Analysis of the survey data revealed that Moanda's coastal population earns its living from fishing (31%), state civil service (3%), sales (17%), agriculture (26%) and charcoal production (22%). Fishing is the most important activity for the coastal population. It is most intense around the villages of Nsiamfumu/Vista, Banana, Ile Mateba, Tompo, Kimuabi, Tshonda and Inga. This fishing is generally traditional and artisanal in Moanda, using large-mesh and sometimes small mesh nets. Nevertheless, the local population has reported the presence of foreign fishing boats on the Congolese coast. The use of small mesh nets does not guarantee the sustainability of Moanda's coastal fishery; the presence of foreign boats on Congolese territory is due to the absence of a good number of Eco-guards serving this Marine Park.
According to the local population, mangroves are more in demand for charcoal, due to their high calorific value. The analysis shows a monthly production of around 252 ± 160 bags of charcoal per village. Charcoal production in the Moanda Mangrove Marine Park corroborates the findings of MECNEF (2007MECNEF (Ministère de l’Environnement, Conservation de la Nature, Eaux et Forêts), 2007. Profil de la zone côtière de la RDC. Kinshasa, 65 p.). The high consumption of wood energy by Congolese urban centers has already been reported by Schure et al, (2012Schure, J., Marien, J.N., De Wasseige, C., Drigo, R., Salbitano, F., Dirou, S., & Nkoua, M., 2012. Contribution du bois énergie à la satisfaction des besoins énergétiques des populations d’Afrique centrale : perspectives pour une gestion durable des ressources, Les forêts du bassin du Congo : État des forêts 2010. Luxembourg, pp. 109- 122); Trefon et al, (2010Trefon, T., Hendricks, T., Kabuyaya, N., & Ngoy, B., 2010. L’économie politique de la flière du charbon de bois à Kinshasa et à Lubumbashi : appui stratégique à la reconstruction post conflit en RDC. Institute of Development Policy and Management, Anvers, Belgique, 110 p.); CIFOR et al, (2007CIFOR, Banque mondiale & CIRAD, 2007. La forêt en République Démocratique du Congo post-confli: Analyse d’un agenda prioritaire. Jakarta, Indonesia: Center for International Forestry Research. https://www.cifor.org/publications/pdf_files/Books/BCIFOR0701F.pdf (Page consulted on October 01, 2022)).
Slash-and-burn agriculture is gaining ground in the landscape of the Moanda mangrove marine park. It is one of the driving forces behind the anthropization of the park's landscape. A coastal household's field covers an average of 0.375 ± 0.218 ha; a household can clear 2 ± 0.872 fields per year. This result confirms those of Marjolaine et al. (2022Marjolaine, O.G., Rogombe, L.G., Ondo, AE., Lembe, B.A., Akendengue, A.I., & Mombo, J.B., 2022. Les moteurs de la déforestation des mangroves urbaines du Grand Libreville (Gabon). VertigO - la revue électronique en sciences de l’environnement. 22(1): 1-35. https://journals.openedition.org/vertigo/35668 (Page consulted on July 07, 2022)), Bhomia et al. (2016Bhomia, R.K., MacKenzie, R.A., Murdiyarso, D., Sasmito, S.D., & Purbopuspito, J., 2016. Impacts of Land Use on Indian Mangrove Forest Carbon Stocks: Implications for Conservation and Management. Ecological Applications, 26(5): 1396–1408. 10.1890/15-2143), Webber et al. (2016Webber, M.H., Calumpong, B., Ferreira, E., Granek, S., Green, R., Ruwa, & Soares, M., 2016. Mangroves. New York, USA: World Ocean Assessment, United Nations. https://www.un.org/depts/los/global_reporting/WOA_RPROC/Chapter_48.pdf) and Mpoyi et al. (2013Mpoyi, A.M., Nyamwoga, F.B., Kabamba, F.M., & Assembe-Mvondo, S., 2013. Le contexte de la REDD+ en République Démocratique du Congo : Causes, agents et institutions. Document Occasionnel, CIFOR, Bogor, Indonésie, 84 p.), who refer to the regression of mangrove woody spaces due to the expansion of agricultural land.
Wood contains 50% carbon, and a low-performance carbonization technique yields around 63.5% charcoal and 36.5% carbon dioxide emissions (Mangion, 2010Mangion, I. 2010, Le futur mécanisme REDD face aux moteurs de la déforestation et de la dégradation des forêts au Brésil, en Indonésie et en RDC. Mémoire de Fin de Master en Sciences et Gestion de l’Environnement, ULB. 152 p.). Considering that a bag of charcoal weighs on average 65 kg (Trefon et al., 2010Trefon, T., Hendricks, T., Kabuyaya, N., & Ngoy, B., 2010. L’économie politique de la flière du charbon de bois à Kinshasa et à Lubumbashi : appui stratégique à la reconstruction post conflit en RDC. Institute of Development Policy and Management, Anvers, Belgique, 110 p.), the carbon dioxide emissions of a coastal village in Moanda amount to almost 9415 ± 5978 kg for its monthly charcoal production. With an improved carbonization technique, carbon dioxide emissions are reduced by up to 27.5% (Mangion, 2010Mangion, I. 2010, Le futur mécanisme REDD face aux moteurs de la déforestation et de la dégradation des forêts au Brésil, en Indonésie et en RDC. Mémoire de Fin de Master en Sciences et Gestion de l’Environnement, ULB. 152 p.); with this technique, carbonization emissions are estimated at 6213 ± 3945 per village.
This result illustrates the role of anthropization of the Mangrove Marine Park land cover on climate degradation. Activities such as slash-and-burn agriculture and carbonization make a significant contribution to greenhouse gas emissions.
5. Conclusions
⌅The main aim of this study was to map and quantify the dynamics of land cover in the Moanda Mangrove Marine Park between 2002 and 2020. It reveals that the landscape of the Moanda Mangrove Marine Park is undergoing increasing changes over the years, because of anthropization, despite its protected status. Forests are mainly disappearing to make way for crops, savannah, and other anthropogenic occupations. Through diachronic analysis of Landsat images, and analysis of field survey data. Accuracy analysis showed very good agreement between land cover classes and the corresponding field data, with a Kappa value of 0.81 and overall accuracy of 82.4%.
The study showed the high dependence of the coastal population of Moanda on the natural resources of the Mangrove Marine Park via fishing (31%), agriculture (26%) and charcoal production (22%). Average village production is around 252 ± 160 bags of charcoal per month. As for slash-and-burn agriculture, an average field covers an area of 0.375 ± 0.218 ha. A coastal household can clear an average of 2 ± 0.872 fields per year.
Average deforestation in the Moanda Mangrove Marine Park is estimated at around 79.62 ± 86.19 ha per year, or a rate of 0.07%. The years 2014, 2011 and 2010 are characterized by high deforestation in the Moanda Mangrove Marine Park, with loss values of 369.5 ha, 141.9 ha and 167 ha respectively. Low levels of deforestation were recorded in 2003 (7.9 ha) and 2004 (21.5 ha) (Figure 7). The study highlighted the influence of population growth on the loss of forest cover in the Marine Park.
Although mangrove deforestation is a consequence of satisfying certain human needs, it also has far-reaching and sometimes devastating consequences, such as social conflict, extinction of plants and animals, and climate disruption.
Restoring mangrove stands (Rhizophora sp and Avicenia sp) by reforesting deforested areas would improve the ecological conditions of species at risk of extinction and provide a response to natural disasters. In addition, redevelopment of the Moanda Mangrove Marine Park using an integrated approach would reduce pressure on the park. The promotion of renewable energy, as an alternative to wood energy; the promotion of improved stoves; and the promotion of sustainable agriculture are effective ways of countering deforestation in the Moanda Mangrove Marine Park.