A biomechanical method to distinguish between self-supporting and non self-supporting fossil plants

Abstract

A method is described that allows a (semi-)quantitative distinction between extinct self-supporting and non self-supporting plants (i.e. lianas and semi-self-supporting plants). This method is derived from experimental findings in extant plants. In the extant lianas: Aristolochia macrophylla, Clematis vitalba and Fallopia aubertii experimentally derived data of flexural stiffness for older ontogenetic stages are clearly below a neutral line, that is calculated by using as a slope the mean Young's modulus of the youngest ontogenetic stage. Plants of this growth habit show a decrease in Young's modulus during ontogeny. Conversely, the stems of self-supporting woody plants, Syringa vulgaris, Alnus viridis, Alnus glutinosa, show an increase in Young's modulus during ontogeny and for older ontogenetic stages the values of flexural stiffness are clearly above the neutral line. These changes in stem mechanics are caused by changes in anatomy. In lianas the contribution of the peripheral strengthening tissues towards axial second moment of area drastically decreases during ontogeny, whereas in self-supporting taxa, the contribution of the secondary wood considerably increases. These findings make it possible to recalculate the mechanical properties for fossil taxa from which permineralized stems of different ontogenetic stages are preserved. By calculating the axial second moment of area of the different stem tissues and by estimating the Young's modulus of each tissue type with data from similar living tissues, the flexural stiffness and Young's modulus for the fossil stems can be (semi-)quantitatively estimated. For the arborescent lycopsid Diaphorodendron vasculare, the results are very similar to the experimental findings of extant self-supporting trees. In the seed fern Lyginopteris oldhamia, on the contrary, the results exclude a self-supporting growth habit. This taxon was probably a semi-self-supporting plant having either a more lianescent or scrambling growth mode or a habit characterized by individuals providing mutual support in dense monotypic stands. By using a formula that calculates the critical global buckling length for different tapering modes and taking into consideration the stem's weight and any additional load acting at the stem's tip, the theoretical maximum height of trees for different safety factors against global buckling can be estimated. For D. vasculare the calculation points to a maximum height of about 15 m for plants with a basal stem radius of 0.15 m. This corroborates very well, the maximum height inferred by DiMichele (1981).