The Impact of Land Conversion on Plant Biodiversity in the Forest Zone of Cameroon
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Biodiversity and Conservation 11: 2047–2061, 2002. 2002 Kluwer Academic Publishers. Printed in the Netherlands. The impact of land conversion on plant biodiversity in the forest zone of Cameroon LOUIS ZAPFACK12,3 , STEFAN ENGWALD* , BONAVENTURE SONKE , GASTON ACHOUNDONG45 and BIRANG A MADONG 1Department of Plant Biology, Faculty of Science, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon; 2Botanical Institute of the University of Bonn, Nibelungenallee 19a, D-60318 Frankfurt am Main, Germany; 3Department of Biology, Higher Teachers’ Training College, University of Yaounde I, P.O. Box 047, Yaounde, Cameroon; 4National Herbarium of Cameroon, P.O. Box 1601, Yaounde, Cameroon; 5Institut de la Recherche Agricole pour le Developpement and Humid Forest Ecoregional Center, International Institute for Tropical Agriculture, P.O. Box 2008, Messa, Yaounde, Cameroon; *Author for correspondence (e-mail: [email protected]; fax: 149-69-90502864) Received 13 April 2001; accepted in revised form 10 December 2001 Key words: Biodiversity, Cameroon, Carbon sequestration, Land conversion, Primary forest, Secondary vegetation Abstract. Floristic surveys were carried out in different land use systems (primary and secondary forest, fallows of different ages, cocoa plantations, crop fields) within the forest zone of Cameroon, to assess the impact of land conversion on above-ground plant biodiversity. Beside various diversity studies, plant density was measured and diameter at breast height was estimated. The results showed that the forest areas, which represent the historic biodiversity of the region, preserve the greatest number of species (160 species in primary forest and 171 in secondary forest). Our results indicate the relatively great importance of secondary forests as refuge areas for primary forest plant species that may function as a starting point for possible regeneration of original biodiversity. Species richness is reduced progressively from the original forest (160 spp.) and secondary forests (171 spp.), to Chromolaena odorata (Asteraceae) fallow fields (149 spp.), to an old fallow field (139 spp.), to a cocoa plantation (116 spp.) and to the farmland (64 spp.), where only weeds and crops contribute essentially to plant biodiversity. Also the number of species that are used for non-timber products (construction, food and medicines) decreased with increased land conversion. Introduction Tropical deforestation proceeds at a rate of 154000 km2 year21 (Aldhous 1993) with approximately 0.32 Gt C year21 being lost to the atmosphere due to land conversion from forest to other uses in Africa (Brown et al. 1993). The classification of Letouzey (1985) showed that the Cameroonian rain forest is subdivided into three important types: the evergreen rain forest, the mesophyllous rain forest and the semi-deciduous forest. These types of forests are subjected to shifting cultivation, the creation of industrial tree plantations and timber operations. Thenkabail (1999) identified seven classes of land use systems that derived from these transformations in the semi-deciduous rain forest areas. In 1993, Duguma signalled the existence of the following types of land use system in the semi-deciduous rain forest zone: 2048 • Groundnut-based mixed food crop fields • Plantain-banana crop fields • Cocoa-fruit trees and plantain fields • Fallow field (more than 10 years old) • Forest land controlled by households • Cucumeropsis pepo based mixed food fields • Home gardens • Other monocultural fields • Horticultural fields We conducted a floristic survey in the southern part of Cameroon in primary and secondary forests, a cocoa plantation, an old fallow field, a young Chromolaena odorata (L.) R.M. King & H. Rob (Asteraceae) fallow field, and in a groundnut- based mixed food crop field. The other crop stands listed above were not included in this study. Chromolaena odorata (L.) R.M. King & H. Rob, also cited sometimes as Eupatorium odoratum L. (Balick et al. 2000), is a widely distributed neotropical shrub introduced into many parts of the tropics. It forms pure stands in disturbed areas, grasslands, fallows and forestry plantations, spreading rapidly due to its efficient short- and long-distance dispersal abilities. The objective of this study was to elucidate the impact of land conversion on plant biodiversity. This impact can be noticed not only by analysing species richness. Different land use characteristics also strongly affect vegetation structure and carbon sequestration. Methodology Site selection This study was implemented within a semi-deciduous rain forest region where seven villages were selected (Figure 1). These villages belong to three general areas, which form the ASB Benchmark: (1) Yaounde, at the northern extreme, with no original forests remaining. (2) Mbalmayo, at the centre of the benchmark, where most of the forest has been logged and where slash-and-burn cultivation is a common feature. (3) Ebolowa, to the south, which is partially logged due to a few populations that practise slash-and-burn agriculture, but still with some primary forest areas remaining. The study area is situated between the geographical co-ordinates 28359 N and 48159 N and 118489 E and 118159 E. The altitude varies between 450 and 715 m above sea level. The mean annual precipitation reaches 1820 mm. The soil is a typical Ferralsol haplique (FAO classification) and the pH varies from 4.29 to 5.43 (Kotto-Same et al. 1997). The texture is made of clayey and sand, while the change horizon is silty. The original tree vegetation of the study area is not uniform: the evergreen species only occur in the southern part, whereas the north part is characterized by semi-deciduous species (for a more detailed description see also Zapfack et al. 1996). 2049 Figure 1. Map of Cameroon and the three study areas with the forest margins benchmark. The benchmark is subdivided into three blocks: Yaounde, Mbalmayo, and Ebolowa. Plant diversity, ecological data and tree biomass determination Three quadrats of 625 m2 (25 3 25) were established at each of the main land use systems found in the zone. They comprise: primary forest (Forest I), secondary forest (Forest II), fallow (old fallow and C. odorata fallow which are younger than the first), crop and cocoa fields. Altitude and the geographical co-ordinates of each quadrat were identified. The 625 m2 was further subdivided into five blocks of 125 m2 each (5 3 25). Inventories were made from the first block to the last. All woody species with a diameter above 2.5 cm were measured and other vascular species, including herbs and epiphytes, were only listed for species richness, not counted. Their diameter at breast height (DBH) as well as their full height were measured or estimated and basal area was calculated. Life forms of woody species have been reported sensu Raunkiaer (1934). Their leaf size was classified after Webb et al. (1976). Due to the negligible number of other leaf forms, only picophyllous, leptophyllous, nanophyllous, microphyllous, platiphyllous, and macrophyllous leaves have been classified in Figure 5. Scientific and vernacular names (the latter given by a traditional local prac- titioner) were annotated. Uses of each species (medicinal plants, nutritive, fertilizers, timber etc.) were also determined. Species that could not be identified in the field were collected, pressed in between newspaper and conserved in alcohol for later identification and storage in the National Herbarium of Cameroon (YA), and Royal Botanic Garden Kew (UK). Tree biomass estimates were assigned using the allometric approach of Brown et al. (1989) applying the equation for moist life zones based on diameter (adj R2 5 0.78). 2050 Data compilation and analysis Data on floristics were subjected to a factor analysis in order to determine the affinity between land uses. Only species composition has been taken into account in order to identify the floristic affinity between the land use systems. In this study, three of the most common indices have been calculated to describe and compare the floristic diversity of the various land use forms: the Shannon–Weaver Index (H9), the diversity index of Simpson (D) and the equitability of Pielou´´ (Pielou 1966; Daget 1976; Dajoz 1982; Frontier and Pichod-Viale 1993). H9 tends to be weighted slightly towards less abundant or rare species, while D takes mostly into account the more abundant or dominant species. These two indices are complementary and considered together they give a good description of the a-diversity of communities. We also performed the Sørensen Index (CCs , Equation 1) of floristic similarity in combination with the number of shared species (Brower and Zar 1977). 2c CC 5]] (1) s a1b with a 5 species number of releve´´ 1; b 5 species number of releve 2; c 5 number of shared species in both relevees.´ The Sørensen Index gives a strong importance to the shared species and puts them into relation with the mean species number of both. The results multiplied with 100 correspond to the percentage of floristic coincidence. Results Taxonomic diversity Four hundred and forty-six species were recorded in the different land use types surveyed during this study. The number of species recorded in each type of land use is given in Table 1. The secondary forest comprised the highest number of species. Nevertheless, the most diverse land use system considering diversity indices was the C. odorata fallow with a Shannon–Weaver Index of 7.04 and a Simpson Evenness Index of 0.008. The Pielou´ equitability is 0.97 (Table 1). The most abundant plants at the farmland were herbaceous weeds; hence in the fallow dominated by C. odorata, other herbs occurred in its understory. A few pioneer species of semi-woody character were also found casually in this type of land use system. The old fallow carried mostly pioneer species, among them shrubs, lianas and many species in the Marantaceae. In the primary and secondary forests, large trees formed the canopy with shrubs and herbs in the understory. Floristic comparison and b-diversity Considering the Sørensen Index (CCs ), the floristic similarity of the studied areas decreased with the increase of human impact on these study sites (Table 2).