Rarity and Abundance in a Diverse African Forest
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Biodivers Conserv DOI 10.1007/s10531-006-9065-2 ORIGINAL PAPER Rarity and abundance in a diverse African forest David Kenfack Æ Duncan W. Thomas Æ George Chuyong Æ Richard Condit Received: 10 April 2006 / Accepted: 15 May 2006 Ó Springer Science+Business Media B.V. 2006 Abstract We censused all trees ‡1 cm dbh in 50 ha of forest in Korup National Park, southwest Cameroon, in the central African coastal forest known for high diversity and endemism. The plot included 329,519 individuals and 493 species, but 128 of those taxa remain partially identified. Abundance varied over four orders of magnitude, from 1 individual per 50 ha (34 species) to Phyllobotryon spathulatum, with 26,741 trees; basal area varied over six orders of magnitude. Abundance patterns, both the percentage of rare species and the dominance of abundant species were similar to those from 50-ha plots censused the same way in Asia and Latin America. Rare species in the Korup plot were much less likely to be identified than common species: 42% of taxa with <10 individuals in the plot were identified to species, compared to 95% of the abundant taxa. Geographic ranges for all identified species were gleaned from the literature and online flora. Thirteen of the plot species are known only from Korup National Park (all discovered during the plot census), and 39 are restricted to the Nigeria–Cameroon coastal zone. Contrary to expectation, species with narrow geographic ranges were more abundant in the plot than average. The small number of narrow endemics (11% of the species), many locally abundant, mitigates short-term extinction risk, either from demographic stochasticity or habitat loss. D. Kenfack (&) Missouri Botanical Garden, 4500 Shaw Blvd., St, Louis, MO 63110, USA e-mail: [email protected] D. W. Thomas Department of Forest Science, Oregon State University, Corvallis, OR 97331-2902, USA G. Chuyong Department of Life Sciences, University of Buea, P.O. Box 63, Buea, Cameroon R. Condit National Center for Ecological Analysis and Synthesis, 735 State St. Suite 300, Santa Barbara, CA 93101, USA R. Condit Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948, USA 123 Biodivers Conserv Keywords Korup Æ Cameroon Æ Tree abundance Æ Dominance Æ Rarity Æ Geographic range Introduction Rarity is central to tropical forest conservation. Diverse communities inevitably include large numbers of species which are seldom recorded: the singletons in many inventories. In typical forest plots of a single hectare or less, 30% of the tree species may be singletons (Pitman et al. 1999). To exacerbate the difficulties with our understanding of tropical communities, species identification is often problematic. Even experts leave many specimens as ‘morphospecies’—recognizable within a site, but not matched to named collections in herbaria. For these reasons—rarity and difficult taxonomy—quantitative information on the abundances and distributions of tropical organisms, not just trees, remains problematic, and conservation planning is hindered by lack of basic knowledge about which species are most endangered. This is probably true in Africa more than any other region. In addition, African forests can have unusual abundance patterns. One or a few species sometimes approach abundances observed in temperate forests (e.g. San- kovski and Pridnia 1995; Shaw et al. 2004), where a single dominant tree comprises more than half the forest (Makana 1998; Hart unpublished, 1990). This is known as monodominance, and although it is known in tropical forests elsewhere, it is most important in Africa (Marimon et al. 2001; Nascimento and Proctor 1997). Mono- dominance should go hand-in-hand with low diversity (Connell and Lowman 1989) and rarity, because the dominant trees force the scarce species to even lower abundance. Enhanced rarity exacerbates the problems of studying the forests. More exhaustive inventories are needed to document tree species abundances, and rarity might enhance extinction risk for many species. The coastal forests of Western Africa, especially Cameroon, are increasingly recognized as important for the conservation of forest diversity. They are the richest in plant species across Africa and repeatedly appear as a center of endemism for plants and for animals (Lovett et al. 2000; Linder 2001; Ku¨ per et al. 2004; Rodrigues et al. 2004; Burgess et al. 2005). Because conservation of species and communities depends on more than just species counts, we sought more detailed information about the forests of coastal Cameroon. As part of a global network of large census plots within the tropics, coordinated by the Center for Tropical Forest Sciences (CTFS) of the Smithsonian Institution, we initiated a large-scale and precise forest inventory, aimed at documenting the abundances of all tree species and testing hypotheses about factors that regulate diversity and species composition. The plot covers 50 ha in the Korup National Park, and all individual trees have been mapped and identified (Thomas et al. 2003; Chuyong et al. 2004). Along with a companion plot in the Democratic Republic of the Congo (Makana et al. 2004), these are the largest forest plot inventories in Africa, and provide quantitative assessments of abundance and rarity. Here we describe the floristic composition, structure, and physiognomy of the Korup plot. We answer several basic questions about tree abundance: is the forest monodominant, and how abundant are the dominant canopy tree species? Is the understory also dominated by one or a few species? At the opposite extreme, how 123 Biodivers Conserv many species are rare in 50 ha? We then consider whether the rare species at Korup have narrow geographic ranges, as predicted by macroecological theory (Brown 1995; Gaston 2003). Conservation planning often relies on the local abundance of species as well as their geographic ranges, and here we provide information for judging the conservation importance of southwest Cameroon (Rodrigues and Gas- ton 2000). Because a dozen other sites in the world now have comparable inventories (Condit et al. 2005), abundance patterns at Korup can be compared against the rest of the world, so that we can determine whether African forests differ fundamentally, as Richards (1973) once suggested. Materials and methods The park and forest The Korup plot (NW corner 5°03.86¢ N, 8°51.17¢ E) is located in southern Korup National Park (4°54¢ to 5°28¢ N latitude and 8°42¢ to 9°16¢ E longitude), near the coast of Cameroon and the Nigerian border. It is within a belt of evergreen forest extending from southeast Nigeria to the mouth of the Congo River (Fig. 1) that is called the Lower Guinea forest, containing one or more Pleistocene refugia (White 1979; Maley 1987). The belt is characterized by wet forested lowlands, often backed by mountain ranges, and is generally reported to be rich in species (Linder 2001). Letouzey (1968, 1985) described the southern part of the Korup National Park as Biafran coastal forest, rich in gregarious Fabaceae-Caesalpinioideae. Fig. 1 Map of tropical Africa showing the three major forest blocks, from west to east: Upper Guinea, Lower Guinea, Congo (or Congolian). Inset in lower left is the detailed map of southeast Nigeria and southwest Cameroon, with Korup National Park indicated 123 Biodivers Conserv The plot measures 1,000·5,00 m2 and is 150–240 m above sea level. The long axis of the plot crosses a permanent creek which is fed by a number of small, seasonal drainages (Fig. 2). One-third of the plot is a steep, rocky ridge, while the remaining two-thirds are flat terrain (Fig. 2). Soils of southern Korup are mostly non-hydric, skeletal, with a high sand content and low pH (Gartlan et al. 1986). Detailed soil mapping within the plot has been initiated but will be reported elsewhere. Census We first surveyed the 50 ha, placing permanent stakes precisely every 20 m with minimal damage to the vegetation (Condit 1998). Subsequently, all individuals ‡1 cm diameter-breast-height (dbh) were tagged with numbered aluminum tags, mapped, measured, and identified to morphospecies. Stem diameter was measured 1.3 m above the ground, with swollen or buttressed trees measured at a spot where the trunk was more regular; in these cases, the measurement spot was painted so future measurements could match. Any stem fork or branch <1.3 m above the ground was treated as a secondary stem and also measured. To map stems, ropes were strung between adjacent stakes (including diagonals) and temporary stakes were placed at 5-m intervals, then the position of each stem was marked by eye on maps covering 10·10 m2. Additional details are given in Condit (1998). Taxonomy and collection Most of the trees enumerated lacked flowers and fruits, so we had to rely on veg- etative characters to segregate morphospecies: color, odor, and texture of bark slash; color of exudates from bark or leaves; leaf arrangement; petiole length; leaf shape; the number and form of secondary veins; and indumenta. Vouchers were collected from each morphospecies in the plot when first encountered. The first specimen was considered the ‘‘type’’ and was carefully described. Each morphospecies was sub- sequently collected at least five times (excepting those with <5 individuals) and compared side-by-side with the ‘‘type’’ until we were confident we could recognize Fig. 2 Topographic map of the Korup 50-ha plot, 1000 m · 500 m with 2 m contours. North and the highest point in the plot is to the left 123 Biodivers Conserv it. Flowers or fruits were eventually collected from most taxa. Matching and rec- ognizing morphospecies is a matter of judgment, and our opinions changed through time, but two of us (DK, DT) spent 700 days over 3 years in the plot, viewed nearly every one of 330,000 individuals, and checked all morphospecies against reference collections at herbaria in Cameroon, the U.K., and the U.S.