Adansonia digitata
Adansonia digitata, the African baobab, is the most widespread tree species of the genus Adansonia, the baobabs, and is native to the African continent and the southern Arabian Peninsula. These are long-lived pachycauls; radiocarbon dating has shown at least one individual to be 1,275 years old. They are typically found in dry, hot savannas of sub-Saharan Africa, where they dominate the landscape and reveal the presence of a watercourse from afar. They have traditionally been valued as sources of food, water, health remedies or places of shelter and are a key food source for many animals. They are steeped in legend and superstition. In recent years, many of the largest, oldest trees have died, for unknown reasons. Other common names for the baobab include monkey-bread tree, upside-down tree, and cream of tartar tree.
Description
African baobabs are trees that often grow as solitary individuals, and are large and distinctive elements of savanna or scrubland vegetation. They grow to a height of. The trunk is typically very broad and fluted or cylindrical, often with a buttressed, spreading base. Trunks may reach a diameter of, and may be made up of multiple stems fused around a hollow core. The hollow core found in many tree species is the result of wood removal, such as decay of the oldest, internal part of the trunk. In baobabs, however, many of the largest and oldest of the trees have a hollow core that is the result of a fused circle of three to eight stems sprouting from roots. The bark is grey and usually smooth. The main branches can be massive. All baobabs are deciduous, losing their leaves in the dry season, and remaining leafless for about eight months of the year. Flowers are large, white and hanging. The fruit are rounded with a thick shell.The leaves are palmately compound with five to seven leaflets in mature trees, but seedlings and regenerating shoots may have simple leaves. The transition to compound leaves comes with age and may be gradual. African baobabs produce simple leaves much longer than most other Adansonia species. Leaflets are stalkless to short-stalked and size is variable.
Flowering occurs in both the dry and the wet season. Buds are rounded with a cone-shaped tip. Flowers are showy and sometimes paired, but usually produced singly at the end of a hanging stalk about in length. The calyx is typically made up of five green triangular bent-back lobes with a cream-coloured, hairy interior. The petals are white, roughly the same width and length – up to, and are crumpled in bud. Flowers open during the late afternoon, staying open and fertile for only one night. The fresh flowers have a sweet scent, but after about 24 hours, they start to turn brown and emit a carrion smell. The androecium is white and made up of a long tube of fused stamens surrounded by unfused filaments 3–5 cm long. There are a large number of stamens, 720–1,600 per flower, with reports of up to 2,000. Styles are white, growing through the staminal tube and projecting beyond it. They are usually bent at right-angles and topped with an irregular stigma. Pollen grains are spherical with spikes over the surface, typical of the Malvaceae family. Pollen grain diameter is around 50 microns.
All Adansonia develop large rounded indehiscent fruit which can be up to long with a woody outer shell. African baobab fruit are quite variable in shape, from nearly round to cylindrical. The shell is thick. Inside is a fleshy, light beige coloured pulp. As it dries, the pulp hardens into a crumbly powder. The seeds are hard and kidney-shaped with a.06-mm-thick coat. They show long-term dormancy, only germinating after fire or passing through an animal's digestive tract. It is thought that this is because the seed coat needs to be cracked or thinned to allow to water to penetrate before the seed can germinate.
Water storage
Baobab trees store water in their trunks and branches on a seasonal basis as they live in areas of sustained drought and water inaccessibility. The U-shaped branches allow for water to trickle down, allowing for maximum absorption over an extended period of time even after the rain stops. The water is absorbed into the vascular tissue of the tree, where it can be moved into the tree's parenchyma cells for long-term storage, or used. A large Baobab can store as much as 136,400 liters of water.During the dry season, the trees expel all of their leaves. During this period, the circumference of the trunk will shrink about 2–3 cm and the water content of the stem will drop by about 10%. Dropping leaves during the dry season is done to prevent water loss through transpiration out of the stomata, which would cause the water potentials in the vascular tissue to drop too low and pull water out of the vacuoles in the parenchyma cells. This would lead to the parenchyma cells, which make up the majority of the trunk and branches, to plasmolyse destroying the tree.
Baobab trees have much higher water and parenchyma content than most trees, which allows them to grow very large with less energy expenditure. Parenchyma are soft plant tissue cells that are commonly used for water storage in other drought tolerant species like cactus and succulents. The water fluxes from the vascular tissue into the parenchyma cells at the center of the tree with the help of actively transported ions. The ion flux into the cell will shift the concentration gradients, causing water to rush into the cells for long-term storage.
Another reason why the water in the trunk can only be used as a buffer for long-term deficits is the distance between the vascular tissue and the parenchyma. The transportation of water from the vascular tissue into storage cells is a very slow process as it is a high-resistance path. The water in the cells at the core of the trunk and the branches would take too much energy from the tree to move back into the vascular tissue for daily use.
Bark
Baobab trees have a thick fibrous bark that possesses an exceptional capacity to heal from even severe damage. Unlike most trees, the depth of the bark is due to thickening of the secondary phloem rather than the periderm, with the former accounting for 75% of the bark by volume. In addition to water storage, the abundance of parenchyma enhances the structural integrity of the tree, as they complement the phloem fibers to provide compressive and tensile strength, respectively. Moreover, a high density of parenchyma also contributes to the healing capacity of the bark, as these cells may undergo further division, and have some capacity to redifferentiate. The spongy material of the bark allows water to be absorbed deeper into the tissue, as there is rarely enough rain during the wet season to penetrate the litter layer of soil.Longevity
The growth rate of baobab trees is determined by ground water or rainfall. The trees produce faint growth rings, but counting growth rings is not a reliable way to age baobabs because some years a tree will form multiple rings and some years none.Radiocarbon dating has provided data on a few individual A. digitata specimens. The Panke baobab in Zimbabwe was around 2,450 years old when it died in 2011, making it the oldest angiosperm ever documented, and two other trees—Dorslandboom in Namibia and Glencoe in South Africa—were estimated to be approximately 2,000 years old. Another specimen known as Grootboom was dated after it died and found to be at least 1,275 years old. Baobabs may be so long-lived in part due to their ability to periodically sprout new stems.
Taxonomy
The scientific name Adansonia refers to the French explorer and botanist, Michel Adanson, who wrote the first botanical description for the full species. The species name digitata refers to the digits of the hand, as the baobab has compound leaves with normally five leaflets, akin to a hand. A. digitata is the type species of the genus Adansonia and is the only species in the section Adansonia. All species of Adansonia except A. digitata are diploid; A. digitata is tetraploid. Some populations of African baobab have significant genetic differences and it has been suggested that the taxon contains more than one species. For example, the shape of the fruit varies considerably from region to region. In Angola, the fruit are elongated, rather than round.A proposed new species, was described in 2012, found in high-elevation sites in eastern and southern Africa. This is now however no longer considered a distinct species, but a synonym of A. digitata. Some high-elevation trees in Tanzania show different genetics and morphology but further study is needed to determine if they should be considered a separate species.
History
The earliest written reports of African baobab are from a 14th-century travelogue by the Arab traveler Ibn Batuta. The first botanical description was by Alpino looking at fruit that he observed in Egypt from an unknown source. They were called Bahobab, possibly from the Arabic "bu hibab", meaning "many-seeded fruit". The French explorer and botanist, Michel Adanson observed a baobab tree in 1749 on the island of Sor, Senegal and wrote the first detailed botanical description of the full tree, accompanied with illustrations. Recognising the connection to the fruit described by Alpino he called the genus Baobab. Linnaeus later formally named the genus Adansonia, to honour Adason, but use of baobab as the commonest vernacular name has persisted. Additional common names include monkey-bread tree, upside-down tree, and cream of tartar tree because of the powdery fruit pulp.Distribution and habitat
The African baobab is associated with tropical savannas. It is found in drier climates, is sensitive to water logging and frost and is not found in areas where sand is deep. It is native to mainland Africa, between the latitudes 16° N and 26° S. Some references consider it as introduced to Yemen and Oman while others consider it native there. The tree has also been introduced to many other regions including Australia and Asia.The northern limit of its distribution in Africa is associated with rainfall patterns; only on the Atlantic coast and in the Sudanian savanna does its occurrence venture naturally into the Sahel. On the Atlantic coast, this may be due to spreading after cultivation. Its occurrence is very limited in Central Africa, and it is found only in the very north of South Africa. In East Africa, the trees grow also in shrublands and on the coast. In Angola and Namibia, the baobabs grow in woodlands, and in coastal regions, in addition to savannas. The African Baobab is native to Mauritania, Senegal, Guinea, Sierra Leone, Mali, Burkina Faso, Ghana, Togo, Benin, Niger, Nigeria, northern Cameroon, Chad, Sudan, Congo Republic, DR Congo, Eritrea, Ethiopia, southern Somalia, Kenya, Tanzania, Zambia, Zimbabwe, Malawi, Mozambique, Angola, São Tomé, Príncipe, Annobon, South Africa, Namibia, Botswana. It is an introduced species in Java, Nepal, Sri Lanka, Philippines, Jamaica, Puerto Rico, Haiti, Dominican Republic, Venezuela, Seychelles, Comoros, India, Guangdong, Fujian, Yunnan and has been planted in Penang, Malaysia, along certain streets. Arab traders introduced it to northwestern Madagascar where baobab trees were often planted at the center of villages.