Tropical savannas are areas that border tropical rainforests but do not receive enough water to support very many trees. Typically a savanna is covered with grass with scattered and isolated trees. Some have dry woodlands where the tree canopy is not closed. Savannas have a wet season and a dry season. Dry seasons often have no rainfall for six months or more. Savannas occur in Africa, Australia, South America and southern Asia.
Boabab trees (Adansonia digitata) dominate the tropical African savanna. They are one of the largest tree species in the world but not the tallest, only reaching up to 80 feet tall. The size of the boabab comes from its massive trunk which can reach up to 50 feet in diameter. The fleshy wood swells with water during the rainy season, then the tree uses that water during the long dry season. The trunks have the habit of becoming hollow as they age and have been used as houses and storage. One tree in Limpopo Province, South Africa has even been converted into a bar for local gold miners.
Baobab trees have green leaves arranged in groups of three to nine that fall from the tree in the dry season to conserve water. They have large white flowers in the summer that are pollinated by fruit bats. The fruit is hard and lumpy and covered with small yellow hairs. It hangs down from the branches and from a distance looks like dead rats, which is why this tree is commonly called the dead rat tree. The white pulp inside the fruit is the source of cream of tartar.
Senegal Gum Acacia
The Senegal gum acacia (Acacia senegal) is a widespread tree on the tropical African savanna and has been cultivated as an agricultural crop around the world in warm climates like Pakistan and India. The sap from this tree is commercially called gum Arabic and is used in almost all processed food, especially candy, and in medicine.
Senegal gum acacias grow as small bushes or as trees up to 60 feet tall. The trees form a dense canopy with a rounded flat top. The branches produce thorns to protect the foliage from grazing predators. The seed pods resemble dried pea pods and are only produced on stressed trees. The powdery blue-green leaves are made up of many small leaflets arranged on a central stem.
Fire fern (Cycas armstrongii) is a deciduous cycad native to the Australian tropical savanna in the Northern Territory near Darwin. Cycads are closely related to palm trees with similar structures. They grow up to 20 feet tall, sometimes with branching trunks, and have 3-foot long fronds with numerous leaflets along a central midrib. The plants are either male or female. Male plants produce a large oval cone in the central apex that releases pollen. Female plants produce fruit that look like yellow-orange eggs that hang from the apex of the trunk. The tree is called a fire fern because of the almost annual fires that sweep across the savanna at the end of the dry season. The trunk of the fire fern survives, but the foliage is burned. It re-sprouts as the wet season starts.