The thick trunks of baobab trees give them a distinct appearance. One species, Adansonia digitata, grows up to 25m tall and can reach a diameter of 10m. The Baobab’s large trunk size has long been thought of as a way to increase water storage, since the climate where they grow can have extended periods without rainfall. Studies examining trunk water usage during the dry season show conservative use of its stored water, as it has also been shown that drawing from the trunk water negatively affects the tree’s structural integrity. Moderate water use, however, is compensated for by its trunk geometry and outer bark.
Baobab wood is characteristically soft and less dense than other types of wood, and the water content in the trunk is so high that the amount of solid wood in a given volume is as low as 5% in some species. The large amount of water within the inner wood directly affects the wood’s stiffness by affecting the cell turgor pressure, which is the pressure exerted by water inside a cell against the cell wall. The more water pushing against the cell wall, the greater the turgor pressure and the more rigid the cell becomes; with less water, the cell becomes flaccid. This in turn affects the overall stability of the tree, particularly the possibility of it buckling under the weight of its own mass during water shortages. Reported instances show that a large withdrawal of water has actually caused trees to collapse. In contrast, the water content of the thick outer bark remains constant throughout the year, which may help compensate for moderate use of water from the rest of the trunk.
The height of the Baobab tree and lack of material stiffness in its wood could cause the tree to collapse under its own weight, if it had a smaller trunk diameter similar to that of other trees. Instead, its large trunk compensates and allows the tree to grow to the same height and with the same resistance to buckling as other studied trees. Increasing trunk diameter directly increases its strength against buckling, which would cost more energy to construct for trees with denser wood. Testing of the baobab’s low-density wood has shown that the energy to construct such a large trunk is not more than other trees of the same height. Additionally, the trunk has a thick outer bark which helps increase its overall stiffness.Edit Summary
“Recently completed studies of baobab water use contradict the often‐stated assumption, both in the popular and the scientific literature, that baobab trees rely heavily on the water stored in their stems to survive in arid climates. In this study, we have demonstrated that baobab trees are not more overbuilt than other trees, nor does the construction of their stem require a larger energy investment. We propose, therefore, that the large stem diameter of the baobab tree is necessary to prevent the stem from collapsing under its own mass and the mass of the extensive crown.” (Chapotin et al. 2006; 1262)
A biomechanical perspective on the role of large stem volume and high water content in baobab trees (Adansonia spp.; Bombacaceae)American Journal of BotanySeptember 1, 2006
Rethinking the value of high wood densityBritish Ecological SocietyJuly 13, 2010
Integrative biomechanics for tree ecology: beyond wood density and strengthJournal of Experimental BotanyNovember 1, 2013
A systematic revision of Adansonia (Bombacaceae)Missouri Botanical Garden PressJanuary 1, 1995
“With regard to the second question, as to why more extensive use of stored water does not occur, results from two related studies indicate that stored water may be unavailable because of tissue water relations, transport limitations and biomechanical considerations. Baobab stem wood is highly vulnerable to cavitation and has a turgor‐loss point near actual field water potentials (Chapotin et al., 2006). In addition, the conductive portion of the sapwood is restricted to the 1–2 cm just beneath the bark, so that radial transport of water in the stem to the transpiration stream is through a relatively high resistance pathway, and is likely to occur very slowly. This rate of water movement may be sufficient for the needs of growing tissues, but is unlikely to meet the demand of actively transpiring leaves. Furthermore, the wood is soft and weak, and comprised primarily of living tissues which may exhibit decreased mechanical stability under excessive water withdrawal.” (Chapotin et al. 2006; 557-558)