A biomechanical perspective on the role of large stem volume and high water content in baobab trees (Adansonia spp.; Bombacaceae)

Abstract

The stems of large trees serve in transport, storage, and support; however, the degree to which these roles are reflected in their morphology is not always apparent. The large, water-filled stems of baobab trees (Adansonia spp.) are generally assumed to serve a water storage function, yet recent studies indicate limited use of stored water. Through an analysis of wood structure and composition, we examined whether baobab morphology reflects biomechanical constraints rather than water storage capacity in the six Madagascar baobab species. Baobab wood has a high water content (up to 79%), low wood density (0.09-0.17 g · cm(-3)), high parenchyma content (69-88%), and living cells beyond 35 cm into the xylem from the cambium. Volumetric construction cost of the wood is several times lower than in more typical trees, and the elastic modulus approaches that of parenchyma tissue. Safety factors calculated from estimated elastic buckling heights were low, indicating that baobabs are not more overbuilt than other temperate and tropical trees, yet the energy investment in stem material is comparable to that in temperate deciduous trees. Furthermore, the elastic modulus of the wood decreases with water content, such that excessive water withdrawal from the stem could affect mechanical stability.

The Parenchyma of Secondary Xylem and Its Critical Role in Tree Defense against Fungal Decay in Relation to the CODIT Model

“…Ultimately, they are all interlinked to some degree through a continuum via plasmodesmata connections between cells within the xylem matrix and their collective link to the phloem. For instance, the mechanical role of living cells is commonly accepted to be more closely related to the RP cells, where, owing to their orientation, they increase radial strength (Beery et al, 1983; Burgert et al, 1999; Burgert and Eckstein, 2001; Reiterer et al, 2002); however, AP cells also play a mechanical role through turgor pressure (Niklas, 1992; Chapotin et al, 2006). For most woody species the fibers or ‘living’ fibers assume the mechanical role.…”

The Role of Xylem Parenchyma in the Storage and Utilization of Nonstructural Carbohydrates

“…It is not known if excessive water loss from parenchyma cells during drought can compromise their physiological functions, although desiccation-induced damage to the protoplasm has been documented in wood parenchyma cells during cold stress (Ristic and Ashworth 1994 ). If the maintenance of turgor pressure is important, for instance for biomechanical reasons (Chapotin et al 2006 ), increased concentration of soluble sugars could provide means for reducing the capacitive discharge from wood parenchyma cells.…”

“…8.1b ). Last but not least, wood of some trees such as Adansonia (Chapotin et al 2006 ) or Ceiba ( Fig. 8.1f ) is extremely parenchymatous, with axial parenchyma comprising most of the matrix between vessels and rays.…”

Coordination of leaf and stem water transport properties in tropical forest trees

“…Surveys of diffuseporous angiosperms have shown that living axial and ray parenchyma tissue often constitute up to 40% or more of the total sapwood volume (Panshin and de Zeeuw 1980). In some tropical trees with low wood density and high water 4y Springer storage capacity, xylem parenchyma can occupy >75% of the total sapwood volume (Chapotin et al 2006). Thus, sapwood pressure-volume curves should reflect the volumeweighted average of the water retention characteristics of both living and dead cells.…”