Amplified fragment length polymorphism (AFLP) analysis of 26 trees of three Salix taxa: Salix alba L. (White Willow), S. fragilis L. (Crack Willow) and their hybrid S. x rubens Schrank, across an example of their habitat range in south‐west Germany, supported the distribution previously determined using morphological characterization. UPGMA and principal coordinates analysis of the AFLP data revealed three distinct clusters corresponding to the three taxa. In addition, AFLP analysis on individuals which were difficult to identify morphologically revealed that they were either the hybrid S. x rubens or S. fragilis. Four specimens of S. fragilis were indistinguishable with three primer combinations suggesting they are members of one clone.

The quantitative description of local buckling of hollow plant stems requires the knowledge of Young's modulus in the longitudinal and tangential directions for the different tissues of which the stem is composed. For thick-walled stems the shear modulus for the radial-tangential plane is needed for an advanced treatment of the process of ovalization. The primary causes of failure can be predicted if critical compressive strains in the longitudinal direction and critical tensile strains in the tangential direction are known. All of these mechanical properties and their variation along the length of the stem can be measured in Arundo donax.

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. x rubens. For the species S. alba, S. appendiculata, S. eleagnos, S. fragilis, S. purpurea, S. triandra, S. viminalis, and S. x rubens, fracture surfaces of broken twigs were investigated and semiquantitatively 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 nonbrittle 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.

Biomechanical responses of stems of 6-to 7-year-old spruce [Picea abies (L.) Karst.] and beech (Fagus sylvatica L) trees were studied after 4 years of growth in elevated atmospheric CO 2 in combination with a nitrogen treatment and on two different soil types. At the end of the treatment, stems were harvested and tested in fresh and air-dried status. Bending characteristics of juvenile wood (modulus of elasticity, termed rigidity) were determined by bending tests. Fracture characteristics (termed toughness) were determined by stroke-pendulum tests. From the base disk of each stem densitometric data were obtained. In spruce, wood produced under elevated CO 2 was tougher on both soil types; enhanced N deposition made wood less rigid only on acidic soils. In contrast, beech wood samples showed no significant reaction to CO 2 but were significantly tougher under high nitrogen depositions on acidic soil. Effects on wood density of both CO 2 and N treatments were not significant, but wood density was higher on acidic soil and so were rigidity and toughness (soil effect). Different genotypes of spruce and beech reacted significantly differently to the treatments. Some genotypes reacted strongly to CO 2 or N, whereas others did not react or showed interactions between CO 2 and N. This underlines the importance of genetic diversity in tree communities.

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