Journal of Experimental Botany, Vol. 51, No. 344, pp. 617-633,
March 2000
© 2000 Oxford University Press
Brittleness of twig bases in the genus Salix: fracture mechanics and ecological relevance
1 University of Freiburg, Institute of Biology II/Geobotanik, Schänzlestraße 1, D-79104 Freiburg, Germany
2 Harvard University, Herbaria, 22 Divinity Avenue, Cambridge, MA 02138, USA
3 CIRAD-Forêt, 73 rue J.-F. Breton, Bâtiment 16, BP 5035, F-34032 Montpellier, Cedex 1, France
4 University of Freiburg, Institute of Biology III, Schänzlestraße 1, D-79104 Freiburg, Germany
5 University of Freiburg, Botanic Garden, Schänzlestraße 1, D-79104 Freiburg, Germany
The twig bases within the genus Salix were investigated. Brittleness of twig bases as defined in the literature neither correlates with Young's modulus nor with growth strains, which were measured for S. alba, S. fragilis and S. xrubens. For the species S. alba, S. appendiculata, S. eleagnos, S. fragilis, S. purpurea, S. triandra, S. viminalis, and S.xrubens, fracture surfaces of broken twigs were investigated and semi-quantitatively described in terms of ‘relative roughness’ (ratio of rough area of fracture surface over whole area of fracture surface). The relative roughness clearly corresponds with the classification into brittle and non-brittle species given in the literature. An attempt was made to quantify brittleness with mechanical tests. The absolute values of stress and strain do not correlate with the brittleness of the twig bases as defined by the relative roughness. However, the ‘index stress’ (ratio of stress at yield over stress at fracture) or the ‘index strain’ (ratio of strain at yield over strain at fracture), correlate well with the relative roughness. The graphic analysis of index stress against index strain reveals a straight line on which the eight species are ordered according to their brittleness. Depending on growth form and habitat, brittle twig bases of willows may function ecologically as mechanical safety mechanisms and, additionally, as a propagation mechanism.
Key words: Growth strains, scanning electron microscopy, elasticity, fracture, plant ecology.
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