The growth form of Croton pullei (Euphorbiaceae) - functional morphology and biomechanics of a neotropical liana
Croton pullei is a woody climber of the lowland rain forest in French Guiana and Suriname. During ontogeny, a shift from a juvenile free-standing growth phase to an older supported growth phase is observed. The following biomechanical parameters were studied in French Guiana during 1997-99: structural Young's modulus [modulus of elasticity], structural torsional modulus, flexural stiffness and bend to twist ratios. Changes in anatomical development were also analysed for different stages of development of C. pullei which differed significantly in their mechanical properties. Free-standing plants showed a nearly constant structural Young's modulus and structural torsional modulus during ontogeny, with flexural stiffness increasing proportionally with the axial second moment of area. These patterns were typical for "semi-self-supporting plants". In contrast, supported plants showed a significant decrease in structural Young's modulus in older stem parts, as well as a decrease in structural torsional modulus. Due to the decrease in structural Young's modulus, flexural stiffness did not increase proportionally with the axial second moment of area. These patterns were typical for non-self-supporting lianas. In all supported plants, a sudden transition occurs from dense earlywood to a wood type with a much higher proportion of large diameter vessels. In contrast, only the dense wood type was present in all tested free-standing plants. The data were compared with results from other climbing species of the same study area and discussed with reference to the observed features characterizing the growth form and life history of C. pullei.