Biomechanical Characteristics of the Ontogeny and Growth Habit of the Tropical Liana Condylocarpon guianense (Apocynaceae)

Abstract

Mechanical analyses were carried out on stem segments representing young to old ontogenetic stages of a mature specimen of the tropical liana Condylocarpon guianense (Apocynaceae). Two-, three-, and four-point bending tests demonstrated that during ontogeny, the Young's modulus (E) of the stem decreased from a mean of 2722 MN m-2 in early ontogenetic stages characterized by the twigs of the crown to a mean of 306 MN m-2 in older stages, including the main vertically orientated axis extending to the ground. This trend is consistent with that observed among other tested lianas. The reduction in the Young's modulus of the stem in C. guianense takes place during ontogeny by the rapid transition from early dense wood lacking large vessels to one consisting of a higher-volume percentage of large-diameter vessels. Later shifts in development include separation of the wood cylinder into lobes. Measurements from specific interconnecting branch hierarchies indicate that trends in the reduction of Young's modulus vary according to opportunistic shifts toward lianoid mechanical characteristics (lowering of Young's modulus) apparently because of the availability and acquirement of supports. Canopy branches of the same hierarchy may show different Young's moduli, flexural stiffness, and either self-supporting or non-self-supporting branch postures. The developmental transition between individual axes from self-supporting to lianoid is highly opportunistic and may be recognized from the mechanical attributes of individual branches.