1720 TROPICAL ECOSYSTEMS / Swietenia (American Mahogany) Swietenia (American Taxonomy and Genetics Mahogany) American or true mahoganies (Swietenia spp.) are J P Cornelius, World Agroforestry Centre, Lima, Peru the most valuable species of the Meliaceae, a K E Wightman, Purdue University, West Lafayette, IN, pantropical family which includes other high-quality USA timber trees such as Spanish cedar (Cedrela odorata), J E Grogan, Yale University, New Haven, CT, USA andiroba (Carapa guinanensis), the African maho- S E Ward, Mahogany for the Future, Puerto Rico ganies (Lovoa, Entandrophragma, Khaya), and the & 2004, Elsevier Ltd. All Rights Reserved. Asia-Pacific red cedars (Toona spp.) (all of the Swietenioideae subfamily), as well as less closely related species of value, e.g., neem (Azadirachta indica) and pride of India (Melia azedarach). The genus, named after the Austrian botanist Gerard von Introduction Swieten, consists of three largely allopatric species: The Swietenia genus (‘true’ or American mahoganies) S. humilis (dry-zone, Pacific, Pacific Coast, or consists of three closely related species (S. humilis, Mexican mahogany), S. macrophylla (big-leaf, Bra- S. mahagoni, and S. macrophylla) whose largely zilian or Honduras mahogany), and S. mahagoni allopatric natural distributions are concentrated in (Cuban, Dominican, West Indian, or small-leaf the seasonally dry, lowland neotropics. The latter two mahogany) (Figure 1). The latter, restricted naturally species have also been widely planted elsewhere, to the Bahamas, Cuba, Jamaica, Hispaniola, and particularly in tropical Asia and Oceania. The long- southern Florida, is the type species. The first to enter established, sustained demand for mahogany timber into international trade, its rapid depletion led is largely due to its high stability, durability, easy quickly to commercial displacement by S. macro- workability, and beauty. It is used principally in the phylla. This has the largest natural distribution of manufacture of high-quality furniture, flooring, the three, ranging from southern Mexico (231 N) to doors, window frames, and decorative veneers. the southern Amazon of Bolivia and Brazil (181 S). Swietenia macrophylla, the only species still with Swietenia humilis, the least-known species, is re- commercially exploitable natural populations, is a stricted principally to the dry Pacific watersheds of massively buttressed, light-demanding canopy-emer- Mesoamerica from the Mexican states of Durango gent, reaching heights of up to 70 m and diameter at and Sinaloa to northern Costa Rica; although some- breast height (dbh) of 3.5 m. It occurs principally at times referred to as Pacific Coast mahogany, it also À low densities of 0.1–3.0 merchantable trees ha 1, occurs well inland. Within and outside their natural although higher densities may occur as a result of ranges, the species are generally known as caoba (in catastrophic disturbance, e.g., hurricanes, fires, and Spanish-speaking states except Bolivia, where the floods. The natural ranges of S. humilis and S. name mara is used), mahogany (English-speaking mahagoni are largely deforested; the ecology of these states), and mogno (Brazil), i.e., without the epithets species in natural forest is little known, but is found in the technical literature. Many other names probably similar to that of S. macrophylla. All three are used locally by indigenous peoples. species are subject to restrictions on international Biologically, the three species are rather poorly trade under Appendix II of the Convention on defined; the current taxonomy is maintained essen- International Trade in Endangered Species (CITES), tially on the basis of the largely allopatric distri- a consequence both of general deforestation and the bution described above, coupled with general mor- unsustainable exploitation to which they have been phological differences. The species are not easily subjected. Sustained production of S. macrophylla in distinguished on the basis of leaf and flower natural forests is thought to be feasible. However, due morphology, and appear to hybridize readily when to the species’ requirement for high light and in proximity (e.g., S. humilis and S. macrophylla relatively low competition in its early years – where their ranges overlap in northeastern Costa conditions unlikely to be met under selective logging Rica; S. macrophylla and S. mahagoni as exotics in – it will require intensive methods. The establishment Puerto Rico). Although both common-garden experi- of Swietenia plantations has been inhibited by the ments and preliminary studies with DNA markers attacks of mahogany shoot borers (Hypsipyla spp.). confirm that the species represent distinct genetic However, accumulated experience and growing entities, it may be that S. humilis and S. macrophylla, experimental knowledge indicate that, with appro- at least, are no more genetically distinct than priate pest management methods, mahogany can be regional variants of other species that range over successfully grown in plantations. similarly diverse conditions, e.g., Pacific and Atlantic TROPICAL ECOSYSTEMS / Swietenia (American Mahogany) 1721 Figure 1 (a) Mature Swietenia humilis tree, Comayagua, Honduras (courtesy of JP Cornelius); (b) S. macrophylla saplings in 3-year- old plantations, Quintana Roo, Mexico (courtesy of KE Wightman); (c) foliage of S. mahagoni, Puerto Rico. Courtesy of JP Cornelius. Central American ecotypes of Cedrela odorata and pollination has been demonstrated in S. humilis, Cordia alliodora (Boraginaceae). The taxonomic and probably occurs in all three species, contributing significance of observed interspecific variation in to the maintenance of genetic variation even in chromosome number is questionable because wide apparently isolated populations. Sporadic tree im- intraspecific variation in chromosome number has provement activities have been implemented, e.g., also been reported. clonal seed orchards in Mexico and Fiji (S. macro- The little provenance work that has been carried phylla), and seedling seed orchards and progeny tests out (S. macrophylla and S. humilis only) suggests in Costa Rica, Mexico (S. macrophylla), and Hon- substantial genetic variation between widely sepa- duras (S. humilis). The absence of large, sustained rated provenances, but little differentiation between planting programs explains the lack of more intensive populations at a more local scale (e.g., within the breeding activities. Yucatan peninsula of Mexico). Joint consideration of Swietenia macrophylla and, to a lesser extent, randomly amplified polymorphic DNA (RAPD) S. mahagoni, have been widely planted outside their studies in Mesoamerica and allozyme work in Boli- native ranges, particularly in Southeast Asia and via suggests that neutral genetic variation shows Oceania; plantations established in Fiji, Indonesia, a similar trend. Quantitative and molecular (allo- Philippines, Solomon Islands, and Sri Lanka total zymes, RAPDs, microsatellites) data indicate abun- more than 190 000 ha. There are also increasing dant genetic variation at the within-population level, areas of S. macrophylla plantations within the as is common in tropical trees. Long-distance natural range, particularly in southern Mexico and 1722 TROPICAL ECOSYSTEMS / Swietenia (American Mahogany) Guatemala. Worldwide, however, mahogany remains a minor plantation species, with total area less than one-twentieth that of teak. In large measure, this can reasonably be attributed to the problem – real or perceived – of mahogany shoot-borer attack. Ecology Swietenia macrophylla Typically, Swietenia are species of relatively low- altitude (r1400 m), seasonally dry forests, and are largely absent from or rare in perhumid regions within their natural ranges. These factors are Figure 2 Opened capsule and seeds of Swietenia macrophylla. responsible for an interruption in the natural range Courtesy of KE Wightman. of S. macrophylla, formed by the coast-to-coast humid and/or high-altitude belt that runs diagonally Two particular patterns of frequency and abun- from northeastern Costa Rica to north-Pacific dance of big-leaf mahogany have been widely noted. Panama. Similarly, S. macrophylla’s arc-shaped range First, and more rarely and locally, mahogany may in South America traces the seasonal tropical forests form relatively dense aggregations of up to 40–50 À along the northern, western, and southern rims of the mature trees ha 1. These formations, reported parti- Amazon basin. cularly from the Yucatan peninsula, seem to derive Big-leaf mahogany is one of the giants of the from the resistance of mature mahogany trees to tropical forest, reaching heights of up to 70 m and hurricanes and fires, both relatively common in the diameter (above often massive buttresses) of 3.5 m. Caribbean region. Such catastrophic disturbances Trees of such dimensions are probably more than 400 favor the species by opening the canopy and destroy- years old and are now rarely found; across its range, ing seed trees and advance regeneration of other the average merchantable big-leaf mahogany tree is species. The copious seed rain of surviving mature likely to be 20–30 m tall with dbh of 60–120 cm. mahogany trees can then lead to seedling densities of À Young trees have narrow, shallow crowns and may up to 1000 ha 1. The annual height growth of young remain unbranched for 6–8 m. The mature crown, seedlings, which may exceed 2 m during the first 5 composed of a few, large primary branches, tends to years, is sufficiently rapid to permit high adult be