| Baobab trees (Adansonia L., Bombacaceae) are widely thought to store water for use during drought, yet few studies have addressed this topic. My thesis questions the assumption that the swollen stems serve a major water storage role through an investigation of water relations and biomechanics in the baobab species from Madagascar. I quantified the relative contributions of stem and soil water on a seasonal and daily scale in three species of baobabs, and assessed the structural and anatomical properties of the wood in six species. The results suggest a limited role for stored water and I propose that the unusual morphology of the baobab tree is instead due to biomechanical constraints associated with the low density, parenchyma-filled wood. Little to no use of stored water occurred during the dry season, when baobab trees are leafless, but some use occurred during the leaf flushing period preceding the rainy season, as evidenced by decreasing stem water content. Sap flow measurements indicate no water uptake from the soil during this time, and moderate flow from the stem into the branches. Water use during leaf flushing was low and the use of stem water was limited to supporting new leaf growth and cuticular transpiration. During the rainy season, I found no role for stored water in buffering daily water deficits. Stem capacitance was high, yet the amount of water that could be withdrawn prior to turgor loss was small. Xylem vulnerability was high and the pattern of stomatal conductance suggests a strategy of embolism avoidance. Stem anatomy restricts water movement between storage tissues and the conductive pathway, making stored water usage more appropriate to longer-term water deficits than as a buffer against daily water deficits. The volumetric wood construction cost is several times lower than in typical trees and the elastic modulus approaches that of parenchyma tissue. Stem safety factors were low, indicating that baobab trees are not more overbuilt than other trees; however energy investment into stem material is comparable. Furthermore, the elastic modulus of the wood decreases with water content, such that excessive water loss could affect mechanical stability. |
