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 ZAPFACK 1 , STEFAN ENGWALD 2, *, BONAVENTURE SONKE 3 ,
GASTON ACHOUNDONG 4 and BIRANG A MADONG 5
1
Department of Plant Biology, Faculty of Science, University of Yaounde I, P.O. Box 812, Yaounde,
Cameroon; 2 Botanical Institute of the University of Bonn, Nibelungenallee 19 a, D-60318 Frankfurt am
Main, Germany; 3 Department of Biology, Higher Teachers’ Training College, University of Yaounde I,
P.O. Box 047, Yaounde, Cameroon; 4 National Herbarium of Cameroon, P.O. Box 1601, Yaounde,
Cameroon; 5 Institut 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: stefan.engwald@ t-online.de; 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 154 000 km 2 year 21 (Aldhous 1993)
with approximately 0.32 Gt C year 21 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 m 2 (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 m 2 was further subdivided into five blocks of 125
m 2 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 R 2 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 (CC s , Equation 1) of floristic similarity in
combination with the number of shared species (Brower and Zar 1977).
2c
CC s 5]] (1)
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 (CC s ), the floristic similarity of the studied areas
decreased with the increase of human impact on these study sites (Table 2).2051
Table 1. Diversity parameters for the different land use systems. Total species number 5 446.
Primary Secondary Chromolaena Old Cocoa Farmland
forest forest plantation fallow
Species number 160 171 149 139 116 64
H9 6.64 6.68 7.04 6.47 4.39 5.99
D 0.02 0.02 0.009 0.02 0.24 0.016
12D 0.98 0.98 0.992 0.98 0.76 0.984
EQ 0.91 0.9 0.97 0.91 0.64 0.99
H9 – Shannon–Weaver Index; D – Simpson Index; EQ – Pielou ´ equitability; 12D – Simpson’s
dominance. 12D has been calculated to confirm the value of EQ. Values lower than 0.8 mean high
abundance of one or two species and the value of 12D must be close to EQ.
Table 2. Sørensen Index (CC s ) and number of shared species between land use systems.
CC s For I For II Jac Co
For II 41.8
Jac 31.5 43.2
Co 18.1 29.6 46.0
Cul 12.5 27.8 40.3 47.6
Number of shared species
For II 67
Jac 44 65
Co 25 51 69
Cul 27 49 60 76
For I – primary forest; For II – secondary forest; Jac – old fallow; Co – C. odorata field; Cul – farmland.
Comparing the primary forest (For I) with all other analysed land use systems, the
floristic affinities reduce from nearly 42% (within the secondary forest For II) to
12.5% (farmland). The highest similarity in floristic composition occurs between
the C. odorata fallow and the farmland.
The factor analysis obtained after the input of data on floristic composition of
each vegetation type reveals – following the X-axis – an opposition of the Forests I
and II to the fallow fields and the farmland (Figure 2A, B). The three analysed
factors can explain a total of 83.9% of the variances (the different percentages for
each factor are given in Figure 2A and B). X-axis was positively influenced mainly
by Forest I, which harboured almost 160 species. It was negatively influenced by 64
species of the farmland. We thus notice a decreasing gradient of species richness
from primary forest to the farms. The cocoa fields comprised numerous species
more related to this kind of vegetation use: Pteris spp., Laportea ovalifolia,
Ampelocissus bombycina, Pethersianthus macrocarpus. This explains its isolation
in relation to the other vegetation types. The same observation can be made on the
figure obtained from axes 1 and 3 (Figure 2B). Some species are more related to
particular land uses than others (Tables 3 and 4). They are characteristic for their
habitats and may function as indicators of the type of land conversion.2052 Figure 2. Factor analysis of land use systems of the Cameroonian semi-deciduous rain forest based on floristic data. (A) Performance of axes 1 and 2; (B) Performance of axes 1 and 3. For I – primary forest; For II – secondary forest; Jac – old fallow; Cac – cocoa fields; Cul – farmland. Vegetation structure and basal area The distribution of trees within diameter classes and the total number of stems are given in Table 5 for each vegetation type. The primary and secondary forests show the highest stem densities of all study sites. Their regeneration capacity seems to be high: they consisted mainly of juvenile trees and treelets within the 5 cm diameter class. The lowest density was recorded in the farmland. Among the different types of vegetation, only secondary and primary forests exhibit a classical tree distribution into diameter classes; all other vegetation types had an irregular distribution of diameters. The value of the basal area was calculated by means of diameter measurements of all stems. Basal area of the secondary forest was 44.9 m 2 ha 21 and 39.2 m 2 ha 21 for the primary forest. The lowest value was obtained in the farmland with 0.07 m 2 ha 21 (Figure 3). Life form The proportion of life forms (sensu Raunkiaer 1934) in different land uses indicates
2053 Table 3. Non-timber forest products identified in the five land use systems in the Cameroonian semi-deciduous rain forest area. Species For I For II Jac Co Cul Vernacular name Uses Cola acuminata 1 Coula edulis 1 1 1 Edible fruits Vitex grandifolia 1 1 Edible fruits Albizia adiantifolia 1 1 1 Sayem Medicine, feet oedema Artropteris sp. 111 Zing Green vegetable Costus afer 1 1 1 1 Mien Female fertility Posinistalia 1 1 1 Akeng Stomach ache Piper guineense 1 1 1 Ondods Spices Paullinia pinata 1 Aloum Medicine Vernonia stelulifera 1 Hypertension Pycnanthus angolense 1 1 1 1 Eteng Stomach ache Irvingia gabonense 11 1 Aandoofan Female fertility – fruits sold Enantia chlorantha 11 1 Hepatitis Picralima nitida 1 Ebam Malaria Tabernanthe iboga 1 Malaria Diospiros conocarpa 1 Ovinefan Sculpture Polyalthia suaveolens 1 Insecticides Megaphynium macrostachyum 1 1 1 1 See Leaves used as packing paper Sarcophynium priogonium 1 1 1 1 See – Maranthochloa purpurea 1 1 1 1 See – Calamus deeratus 1 Nlon Providing Eremospatha wenlandiana 1 Nlon – Ricinodendron heudelotii 1 1 1 1 Spices (seeds) For I – primary forest; For II – secondary forest; Jac – old fallow; Co – C. odorata field; Cul – farmland; 1 denotes present; 11 abundant; 111 most abundant. the predominance of the chamaephytes in all sampled habitats (Figure 4). The phanerophytes were best represented in the pristine forest compared to other land use systems. Therophytes were absent in the primary forest and showed relative abundance in the crop fields. Hence, only a few individuals in all of the six study sites represented cryptophytes. Leaf size Microphylls and mesophylls are the most abundant leaf sizes in the six land use systems (Figure 5). The picophylls and megaphylls are poorly represented. Carbon sequestration Figure 6 points out a strong decrease in tree biomass (expressed as Carbon t / ha) from primary forest to farmland, along a gradient of disturbance, respectively of conversion. Ethnobotany In the fallow fields, the cocoa fields and the farmland, the species preserved by
2054 Table 4. Tree species peeled or collected for medical purposes by the local peoples. Genus Species For I For II Cac Jac Co Cul Aframomum sp. 2 Aidia micrantha 3 Albizia adianthifolia 2 Albizia sp. 2 Alchornea cordifolia 6 Allanblackia floribunda 5 Blighia welwitschii 5 4 Chytranthus sp. 6 Cleistopholis patens 2 Cola lepidota 10 Combretum sp. 3 Dacryodes edulis 2 Dialium guineense 7 3 Dichostemma glaucescens 7 Dioscorea sp. 3 2 Drypetes afranensis 4 Elaeis guineensis 2 Ficus exasperata 3 3 2 Ficus mucuso 3 2 Garcinia mannii 3 Heisteria zimmereri 3 Ipomoea sp. 2 Irvingia gabonensis 3 Landolphia sp. 7 Macaranga assas 3 Macaranga grandis 5 Margaritaria discoidea 2 2 Markhamia lutea 3 Microdesmis puberula 4 Milicia excelsa 2 Millettia sp. 7 9 2 Penianthus longifolius 8 Pentaclethra macrophylla 3 6 Persea americana 3 Petersianthus macrocarpus 3 Phyllanthus sp. 3 Plagiostyles africana 5 Polyalthia sauveolens 4 3 Pycnanthus angolensis 4 Rinorea sp. 6 Rothmannia hispida 5 Rothmannia sp. 8 Salacia sp. 6 Sida sp. 2 Solanum sp. 2 2 Sorindeia sp. 5 4 Tabernaemontana crassa 14 Terminalia superba 2
2055 Table 4. (continued) Genus Species For I For II Cac Jac Co Cul Tetracera alnifolia 3 Tetrorchidium didymostemon 10 Theobroma cacao 77 Trichilia monadelpha 4 Triplochiton scleroxylon 3 Uapaca paludosa 3 Uncaria africana 2 Vernonia conferta 2 For I – primary forest; For II – secondary forest; Cac – cocoa fields; Jac – old fallow; Co – C. odorata field; Cul – farmland (numbers represent the quantity of nominations of plants by locals). farmers are normally either medicinal species, fertilizers, nutritive or timber. Many non-timber forest products were found in those three land use systems (Table 3). Marantaceae (Marantochloa purpurea, Sarcophrynium prionogonium and Megap- hrynium macrostachyum) and Sterculiaceae are used for packing paper and ropes (string) as well as ‘Kola’ and aphrodisiacs. Many of the forest species are the source of fruits that are sold at local markets (Coula edulis, Vitex grandifolia). Gnetum aficanum (Okok), little known by the local population but highly requested on the international market, was seen in the Mengomo, in Forest II. Its popularisation might contribute to increase the standards of living of the peasants. About 60–70% of the trees is peeled off for medical purposes in the cocoa farms, fallow fields and the farmland (Table 4). Discussion In terms of species richness the farmland ranges at the lowest end of all study sites, but by far the lowest values were observed in the cocoa field due to the dominance of one species, Theobroma cacao. Most of the values obtained here are higher than those reported by White (1992), Sokpon (1995), Sonke´ (1998) and Sonke´ and Lejoly (1998). Rather unexpectedly, the species richness of the secondary forest (Forest II) exceeds all others of our study areas. Particularly, pioneer species and those characteristically belonging to secondary forest are responsible for the high species number: Macaranga spp., Musanga cecropioides, Terminalia superba, Ceiba pentandra, Triplochiton scleroxylon. This might be evidence for the importance of secondary forests as a starting point for a possible regeneration of original biodiversity. The proportion of shared species of each surveyed vegetation area with the total flora (446 spp.) ranges between 14% in the farmland and 38% in the secondary forest. This and the relatively low Sørensen indices seem to be evidence for a high species turnover and therefore a high b -diversity. However, the primary forest (Forest I) hosts the highest number of ‘original’
2056
Table 5. Distribution of tree diameter classes within the land use systems.
Diameter class center (cm) Primary forest Secondary forest Old fallow Chromolaena odorata field Cocoa Farmland
No % No % No % No % No % No %
5 2008 72.7 1920 73.7 1144 70.6 8 12.5 1208 77.5 8 100
15 456 16.5 360 13.8 376 23.2 16 25 232 14.9 0 0
25 112 4.1 160 6.1 0 0 24 37.5 32 2.1 0 0
35 88 3.1 72 2.8 16 0.9 0 0 24 1.5 0 0
45 24 0.9 40 1.5 40 2.6 8 12.5 24 1.5 0 0
55 32 1.2 8 0.3 0 0 0 0 16 1 0 0
65 0 0 0 0 16 0.9 0 0 8 0.5 0 0
75 8 0.3 16 0.6 0 0 0 0 16 1 0 0
85 8 0.3 16 0.6 0 0 0 0 0 0 0 0
95 24 0.9 0 0 16 0.9 0 0 0 0 0 0
105 0 0 16 0.6 16 0.9 8 12.5 0 0 0 0
Total 2760 100 2608 100 1624 100 64 100 1560 100 8 100
Absolute number of stems and percentage per 625 m 2 .2057 Figure 3. Basal area (in m 2 / ha) of six land use systems in the semi-deciduous rain forest of Cameroon. Figure 4. Distribution of life forms among the following land use systems: For I – primary forest; For II – secondary forest; Cac – cocoa field; Jac – old fallow; Co – C. odorata field; Ba – banana field; Cul – farmland. ph – phaneropyte; ch – chamaephyte; hc – hemicryptophyte; c – cryptophyte; th – therophyte.
2058 Figure 5. Proportion of leaf size in the six land use systems of the Cameroonian semi-deciduous rain forest. pi – picophyll; le – leptophyll; na – nanophyll; mi – microphyll; pl – platiphyll; ma – macrophyll (due to the neglectable number of species other leaf forms have not been classified). For I – primary forest; For II – secondary forest; Cac – cocoa field; Jac – old fallow; Co – C. odorata field; Ba – banana field; Cul – farmland. species that belong exclusively to undisturbed forest vegetation. At species level, we notice that about 12% of the woody species found in the primary forest can also occur in the secondary forest, but most of them as seedlings. It is almost as diverse as Forest II. This pristine forest, which was used as the point of reference, includes species that probably have reached their growth climax. The understory is covered with many lianas, and very few seedlings were observed. The high humidity and the Figure 6. Biomass (Carbon in t / ha) per five land use systems in the Cameroonian semi-deciduous rain forest area. For I – primary forest; For II – secondary forest; Co – C. odorata field; Jac – old fallow; Cul – farmland.
2059 rapid decrease of light transmission below the level of canopy closure permit the regeneration of species restricted to primary forest (dispersed by wind and animals) and the progressive elimination of pioneer species. Succession in tropical rain forests generally involves the following different steps: forest gaps are first colon- ized by herbaceous and woody pioneer species. Continuously, these species are replaced by secondary forest tree species, followed later on by typical elements of the primary forest. In our case, secondary forest also holds seedlings of primary forest species and sometimes those of pioneer plants. In contrast, the primary forest does not bear many seedlings but many shrubs. Reduction of species number from forest to farmland comprises a total of 37.4% as far as species richness is concerned (not species identity). This is considerably more than the species loss reported by Turner et al. (1994) in a study of plant species extinction in Singapore. They recorded a decrease of species number for inland forests of 29% due to logging activities and land conversions. But their extinction rates differed enormously between different habitats (coastal region 39%; open sites 5%) and for different life forms (trees 19%, shrub species 34%, herb species 23%, climbers 23%, epiphytes 62%). In the young fallow fields as well as in the farmland the herbaceous plants dominate. Seedlings of species that are purely from the primary forest are almost absent. This can be explained by the fact that micro-organisms of the soil – responsible for the growth of the dominant diaspores from the forest – were eliminated in the course of burning. This can be noticed in particular between the clumps of C. odorata. With regard to species richness, the cocoa fields occupy an intermediate position between the forest areas and the farms. Here, many of the ‘primary forest species’ were left standing in the course of burning, fruit trees were planted and other species (seedlings) were protected for further multiple uses. Species richness of herbaceous plants in the cocoa farm highly depends on the period of the year. They are frequently cut in order to care for a good yield. The values of the basal area are similar to those of Malaisse (1984) in the Democratic Republic of Congo and those of Devineau (1984) in Ivory Coast. Those different values confirm the idea that the basal area is a good tool to classify the vegetation structure (Malaisse 1984). Among all the types of land use, and despite their low diversity indices, the cocoa fields are the most protected land use systems. They are in fact the gardens where the medicinal plants are collected in case of urgency, while Forests I and II are being more oriented towards hunting and gathering. The species discovered in all study sites as medicinal plants (46) are less important compared to the 120 species that Fujisaka et al. (1995) assessed in the Amazonian forest. Conclusions Using the primary forest as the point of reference, our present study of the diversity of different types of land use has shown that:
2060
• The secondary forest yielded most of the encountered plant species, despite
effects of slight human impact.
• The primary forest, which has not yet been affected neither by slash-and-burn
agriculture nor by other forms of anthropogenic disturbance, is the second most
diverse area, but the richest in non-timber products (e.g. medicinal plants,
subsistence).
• The floristic similarity between the studied areas declines along a gradient of
anthropogenic disturbance. Similarity of species composition between primary
and secondary forest is almost 42%. The primary forest held only 36% of the
total flora. These two observations indicate a relatively high b -diversity.
• The farmland, which suffered recently from continuous burnings, is markedly
poorer in species richness. Large trees are absent and they do not dispose of
many useful species (non-woody secondary forest products).
• The fallow fields, still more diverse than the farms, show a progressive succes-
sion towards the forest.
• The cocoa fields have a potential richness in terms of medicinal and edible plants
that could increase the peasant’s revenues. A good management in this area may
assure the amelioration of the standards of living of the peasants and stabilize
them in order to protect the forest. Management strategies should include the
selection and reintroduction of original forest species into plantations of cocoa
trees.
Acknowledgements
This study was partly supported by grants from IRAD/ IITA / WAU and the IRAD/
ASB Projects. We are very grateful to Prof. Brussaard, co-ordinator of ‘Soil macro-
fauna diversity, density and function along a land-use gradient in the humid forest
zone of southern Cameroon’, for helpful suggestions and constructive criticism that
improved the manuscript. We thank our colleagues of the Cameroon National
Herbarium (YA) for their help on plant identification and conservation. We owe
special thanks to two anonymous reviewers.
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