| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Wild lives Baobabs Certain tribes in the Transvaal wash baby boys in water soaked in the bark of a baobab. Then, like the tree, they will grow up mighty and strong. The baobab is entrenched in the folklore of much of Africa. G. E. Wickens, in his comprehensive article "The Baobab: Africa's Upside-Down Tree" in the Kew Bulletin describes a number of superstitions centered on this strange tree. The Creator is said to have initially planted the baobab in the rainforests of the Congo Basin, but the tree complained that the dampness made its trunk swell. So the Creator moved it to the high slopes of the Ruwenzori range, East Africa's Mountains of the Moon. But the baobab continued to grumble about the humidity. Angered by the incessant wailing, the Creator took the swollen trunk and tossed it into a dry part of Africa. The tree landed upside down with its roots in the air. In some parts of Africa it is thought that baobab flowers are inhabited by spirits, and that anyone who picks a flower will be devoured by a lion. In Madagascar, the Grandidier's baobab (Adansonia grandidieri), the most statuesque of the Malagasy species, is believed to be the dwelling place of spirits. Offerings are placed at its base to ensure fertility, a fine harvest, and good fortune. For centuries, much of what was known about baobabs was based exclusively on the African baobab (A. digitata). The first recorded reference was by the fourteenth-century Arab traveler Ibn Batuta who mentions the water-storage capacity of its massive trunk. Much of the early familiarity of Europeans with the baobab came from its fruits, which were commonly sold in the herb and spice markets of Egypt during the sixteenth century, probably for their medicinal value in reducing or removing fever. The origin of the name baobab is uncertain. Some have suggested that it comes from "bu hobab," a name used for the plant in the markets of Cairo. Or perhaps it was derived from "bu hibab," an Arabic designation for "the fruit with many seeds." It was Linnaeus who, in 1759, gave the baobab its scientific name, Adansonia. In so doing, he honored the celebrated French botanist Michel Adanson (1727-1806) who had lived in Senegal and who provided the first technical description and illustration of the tree. Baobabs are members of the Bombacaceae, a pantropical family containing a number of better-known economically important plants like kapok, balsa wood, and durian. Six of the eight species of baobabs are restricted to western and southern Madagascar, a seventh is endemic to northwestern Australia, and the eighth is widespread in sub-Saharan Africa but now introduced by humans throughout the warm tropics. All baobabs are deciduous trees ranging in height from five to 30 meters. They have leaves with segments that radiate somewhat like the fingers of a hand, showy flowers, and large, many-seeded gourdlike fruits that are covered with a velvety thatch of hairs. In height, trunk shape, and girth, there is no such thing as a typical baobab. Even within a single species, individuals can vary greatly in size and shape. Many of the Malagasy trees have single trunks that can be cylindrical, bottle-shaped, or tapered from bottom to top. The Australian baobab often has multiple trunks that are barrel- or bottle-shaped at the base, becoming quite constricted just beneath the branches. But the grotesque trunk of the African baobab is surely the standout. This species can attain a height of 25 meters and a diameter of ten meters with a trunk that is single or multiple, cylindrical or fluted, and often buttressed with branches irregularly distributed along the trunk or confined to the top. The enormous swollen trunk, actually a reservoir of water, together with its stunted branches and wide root system, has prompted the natives of Africa to refer to the baobab as the upside-down tree. It is hardly surprising that the African baobab has been described as a "grizzled distorted old goblin with the girth of a giant, the hide of a rhinoceros, and twiggy fingers clutching at empty air." The tremendous girth of some African baobabs has led observers to suggest that individual trees can reach an age of several thousand years. Like many large tropical trees, the baobab has no reliable tree rings, and the largest trees aren't necessarily the oldest because variation in water content of the trunk can cause large fluctuations. Adanson, who first attempted age calculations, estimated that two trees on an island off Cape Verde were 5,150 years old. Since those trees have since disappeared it is difficult to know if he was right. According to Wickens, foresters studying trees with diameters exceeding ten meters estimated that they had lived for over 2,000 years. This seems reasonable in view of a confirmed radiocarbon date of just over 1,000 years for a tree 4.5 meters in diameter from a site near the Zambezi River. There are few tropical trees with as many uses as baobabs. The diary of David Livingstone, the English missionary, mentions an old baobab near the Mozambique coast with a room-sized central cavity that would afford shelter for an entire local family. The hollowed-out trunk of a living tree can be used to store water. Fiber from the bark is used to make rope, baskets, cloth, musical instrument strings, and waterproof hats. While stripping the bark from the lower trunk of most trees usually leads to their death, baobabs not only survive this common practice, but they regenerate new bark. Fresh baobab leaves provide an edible vegetable similar to spinach which is also used medicinally to treat kidney and bladder disease, asthma, insect bites, and several other maladies. The tasty and nutritious fruits and seeds of several species are sought after, while pollen from the African and Australian baobabs is mixed with water to make glue. Baobabs are also hospitable hosts to animal visitors seeking food, shelter, or nesting sites. Recent research by David A. Baum of Harvard University has shed new light on the pollination systems of baobabs. All baobabs have spectacular, odoriferous flowers that open at night and produce generous amounts of nectar. While the African species has long been known to be pollinated by bats, Baum's work has shown that the two rarest Malagasy species, A. suarezensis and A. grandidieri, are pollinated by fruit bats and nocturnal lemurs, respectively. In contrast, the Australian species and the four other Malagasy species are pollinated by long-tongued hawkmoths. Baum's studies have brought the story of baobab pollination into sharper focus, but like the incremental advances in scientific research generally, new answers provoke new questions. How are the fruits dispersed and what are the conditions necessary for seedling establishment? How large an area does a species need to maintain a healthy reproducing population? How much habitat destruction can a species stand before its pollinator spectrum begins to unravel? Answers to these kinds of questions are essential in any effort to make informed decisions about conservation and resource management. At present, the future of the Australian baobab seems secure. Survival of the African baobab and at least two Malagasy species (A. rubrostipa and A. za) also seems likely because of their comparatively widespread ranges and broad ecological tolerances. The long-term prognosis for the four Malagasy baobabs with more restricted distributions is a matter of concern because of ongoing habitat destruction that is accelerating along with population growth. I recently visited the degraded agricultural land around Morondava, Madagascar, where mature stands of Grandidier's baobab silhouetted against a cloudless sky provided a magical scene that I'll long remember. Like sentinels of a past era, baobabs have been able to survive in denuded landscapes because of the phenomenal water-storage capacity of their trunks and the extensive horizontal root systems, which will glean any available surface moisture. However, I found the lack of seedlings and young trees at this site distressing. It served as a potent reminder that recurrent fires, seed predation, and perhaps competition from aggressive weeds are restricting regeneration of these gnarled giants. Frank Almeda is Curator of Botany at the California Academy of Sciences. |
Spring 1997
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