Baobab trees are fat because their massive trunks are essentially living water tanks. A mature baobab can store up to 120,000 liters of water inside its swollen trunk, enough to survive years of drought in the African savanna. Everything about the tree’s anatomy, from its unusually light wood to its sponge-like cellular structure, is built around this single survival strategy.
A Trunk Built Like a Sponge
Most trees have dense, rigid wood packed with structural fibers. Baobabs took a completely different evolutionary path. Their wood is made up of 69 to 88 percent parenchyma, a type of soft, flexible tissue that acts like a biological sponge. These cells absorb and hold enormous quantities of water during the rainy season, then slowly release it as conditions dry out. The water content of baobab wood can reach 79 percent, meaning the trunk is more water than wood by weight.
This design makes baobab wood astonishingly light. At an average density of 0.13 grams per cubic centimeter, it is actually lighter than balsa wood, which averages around 0.15. For comparison, a typical eucalyptus hardwood has a density of 0.40 to 0.46. If you could somehow cut into a baobab trunk, you wouldn’t find the hard, ringed wood you’d expect from a tree that can live for millennia. You’d find something closer to a waterlogged foam.
Why Water Storage Matters in the Savanna
Baobabs grow in semi-arid environments across Africa, Madagascar, and parts of Australia, places where rain falls intensely for a few months and then stops entirely. The tree drops all its leaves during the dry season to reduce water loss, standing bare for months. During this leafless period, the water locked inside the trunk keeps the tree alive. It’s a strategy that trades structural strength for survival: instead of investing energy in dense, costly wood, the baobab builds cheap, flexible tissue that can swell and shrink with the seasons.
Research published in the American Journal of Botany found that the cost of building baobab wood, in terms of the energy the tree spends per unit of volume, is several times lower than in typical trees. That efficiency is the whole point. The tree can grow an enormous trunk without the metabolic expense that a comparable volume of hardwood would require. The tradeoff is that the wood becomes less stiff as it fills with water, meaning the tree’s mechanical stability is closely tied to its hydration. If a baobab loses too much water, its own trunk can become structurally compromised.
How Big Baobab Trunks Actually Get
The numbers are hard to believe. A baobab trunk can reach a circumference of more than 25 meters (82 feet) and a diameter of 9 meters (29 feet). That’s wide enough to park several cars inside. Some of the largest specimens have been used as bars, chapels, storage rooms, and even a prison. The oldest baobab with accurate radiocarbon dating, a tree in Angola known locally as “The biggest baobab of Africa,” is approximately 2,100 years old.
That kind of age raises a question: how does a tree made of spongy, lightweight tissue survive for two millennia? Part of the answer lies in how baobabs grow. Many large baobabs aren’t single-trunked trees at all. They’re made of multiple stems that sprouted from roots or fallen trunks and gradually fused together over centuries into what looks like one enormous trunk. Radiocarbon dating of cavity walls has revealed that the stems in a ring-shaped baobab can be different ages, sometimes separated by hundreds of years.
The Mystery of Hollow Centers
Many old baobabs are hollow inside, and for a long time people assumed this was simply rot. The real story is more interesting. Research using radiocarbon dating has identified two distinct types of internal cavities. Normal cavities form the way you’d expect: fungi, fire, animal damage, or human activity eats away at the oldest wood in the center, creating an irregular hollow typically one to three meters tall.
But many of the most impressive baobab cavities, the ones large enough to walk around in, aren’t caused by decay at all. These “false cavities” are natural empty spaces between multiple stems that grew in a ring and fused together on the outside, leaving an open chamber in the middle. These false cavities are larger (3 to 8 meters tall), more regular in shape, and always extend down to ground level. And unlike true rot cavities that grow bigger over time, false cavities actually shrink as the surrounding stems continue to grow inward.
This multi-stem architecture helps explain why baobabs can get so wide. The trunk isn’t expanding outward from a single center the way an oak does. In many cases, it’s a coalition of separate stems that merged into one colossal structure, each one packed with water-storing tissue, collectively creating the fat, bottle-shaped silhouette that makes baobabs unmistakable on the savanna horizon.
Fat Trunk, Thin Branches
The contrast between a baobab’s enormous trunk and its relatively spindly, root-like branches is so striking that many cultures call it “the upside-down tree,” as if it were planted with its roots in the air. This shape isn’t accidental. The branches stay small because the tree’s growth strategy prioritizes trunk volume over canopy spread. A wider trunk means more water storage, which directly determines whether the tree survives the next drought. Broad, leafy branches would lose water through evaporation, working against the whole point of the design.
During the wet season, the trunk swells visibly as it fills with water, and it contracts during the dry months as reserves are drawn down. The flexible parenchyma tissue allows this expansion and contraction without cracking or splitting. It’s a living accordion, breathing with the rhythm of the rains. The moisture leaking from a baobab’s trunk and the nutrients from its fallen leaves also sustain surrounding plants and animals, making these trees ecological anchors in dry landscapes where water is the limiting resource for everything alive.