Academic Reading — Test 52

Reading passage

Across the dry savannahs of sub-Saharan Africa stands one of the continent's most distinctive plants: the African baobab, Adansonia digitata. Recognisable by its swollen trunk and a crown of bare, root-like branches, the tree has long been associated with endurance. In regions where the rainy season may last only a few months and is followed by a long, harsh dry period, the baobab continues to flourish where many other large trees would perish. Its survival is not the result of a single remarkable feature but of a combination of structural, behavioural and physiological adaptations that together allow it to manage scarce water with extraordinary efficiency. The most obvious of these adaptations is the trunk itself. Unlike the dense, woody stems of many hardwood trees, the baobab's trunk is composed of soft, fibrous tissue that can hold large quantities of water. During the brief wet season, the tree absorbs moisture rapidly through its extensive root system and stores it within this spongy wood. As the dry months advance, the stored water is gradually withdrawn, and the trunk may visibly shrink in circumference before swelling again when the rains return. Some of the largest specimens are thought to hold many thousands of litres of water, acting as living reservoirs that buffer the plant against months without rainfall. It is worth noting, however, that this water is not stored in a hollow cavity but is held within the living tissue of the wood. The tree's behaviour through the year reinforces this water-saving strategy. For much of the dry season the baobab is deciduous, shedding its leaves entirely and standing bare. Leaves are the main site at which plants lose water, because the tiny pores on their surface, known as stomata, must open to admit the carbon dioxide needed for photosynthesis. By dropping its foliage when water is scarce, the baobab dramatically reduces the surface area through which moisture can escape. The leafless period coincides with the hottest, driest part of the year, so the tree effectively pauses much of its growth until conditions improve. When the rains return, new leaves emerge quickly, allowing the tree to resume photosynthesis without delay. Below ground, the baobab's roots contribute to its resilience in two complementary ways. A network of shallow roots spreads widely just beneath the surface, enabling the tree to capture light or irregular rainfall before the water evaporates or drains away. At the same time, deeper roots can reach moisture held lower in the soil. This dual arrangement means the tree is not wholly dependent on any single source of water and can exploit whatever becomes available. The bark, too, plays a protective role: it is thick and fibrous, and unlike that of many trees it can regenerate after damage. This same bark helps shield the trunk from the intense heat of bush fires, which are common in the savannah during the dry season. The baobab's slow and steady approach to life further supports its longevity. The tree grows gradually and can live for many centuries; some individuals are estimated to be well over a thousand years old, though determining a precise age is difficult because the soft wood does not form the clear annual rings found in temperate trees. Slow growth places relatively modest demands on water and nutrients, which suits an environment where both are unreliable. The tree also flowers and produces large fruits, and its pale flowers open at night and are pollinated chiefly by bats and the insects active after dark. The fruit that follows is rich in nutrients and provides food for numerous animals as well as for people, which has made the baobab an important resource in many rural communities. Taken together, these features explain why the baobab is so often described as a survivor. No single adaptation would be sufficient on its own; rather, it is the way in which water storage, leaf loss, an efficient root system, protective bark and unhurried growth operate in concert that allows the tree to endure. As climate patterns across Africa become less predictable, scientists have taken a renewed interest in the baobab, both as a subject of study and as a species whose own future may be tested by the very droughts it has historically been so well equipped to withstand. Understanding precisely how it copes with water stress may yet offer wider lessons for agriculture in a warming world.