Brittleness of twig bases in the genus Salix: fracture mechanics and ecological relevance

The twig bases within the genus Salix were investigated. Brittleness of twi g bases as defined in the literature neither correlates with Young's modulu s nor with growth strains, which were measured for S. alba, S. fragilis and S. x rubens, For the species S. alba, S. appendiculata, S. eleagnos, S. fr agilis, S. purpurea, S. triandra, S. viminalis, and S. x rubens, fracture s urfaces of broken twigs were investigated and semiquantitatively described in terms of 'relative roughness' (ratio of rough area of fracture surface o ver whole area of fracture surface). The relative roughness clearly corresp onds with the classification into brittle and nonbrittle species given in t he literature. An attempt was made to quantify brittleness with mechanical tests. The absolute values of stress and strain do not correlate with the b rittleness of the twig bases as defined by the relative roughness. However, the 'index stress' (ratio of stress at yield over stress at fracture) or t he 'index strain' (ratio of strain at yield over strain at fracture), corre late well with the relative roughness. The graphic analysis of index stress against index strain reveals a straight line on which the eight species ar e ordered according to their brittleness. Depending on growth form and habi tat, brittle twig bases of willows may function ecologically as mechanical safety mechanisms and, additionally, as a propagation mechanism.