Tensile strength of the bark of Dirca L. and other genera of the Thymelaeaceae1

The bark from many species in the Thymelaeaceae (Daphne, Mezereum, fiber-bark, or rope-bark family) can be peeled from stems in long strips and used for cordage, clothing, decorations, and paper. Although observations in the field indicate that the bark from members of this family is unusually resistant to breakage, the physical strength of bark from members of the Thymelaeaceae has not been measured and compared with that of bark of species from other families. We therefore characterized the ultimate tensile strength of bark of 19 species: 13 within the Thymelaeaceae and one from each of six other families. The four species of the genus Dirca L. were of particular interest to us because, unlike most other members of the Thymelaeaceae, they are indigenous to North America, where practical uses of bark from two of the species have been documented. The bark of the four species of Dirca did not differ in ultimate tensile strength and averaged 82 MPa. Ultimate tensile strength of the bark of Dirca mexicana Nesom and Mayfield, which was selected to represent the genus in subsequent analyses, was not different from the ultimate tensile strength of bark of 9 of 15 other species; eight of those nine species were members of other genera in the family Thymelaeaceae. Of the six species with lower ultimate tensile strength than that of D. mexicana, only one, Peddiea africana Hook, was in the Thymelaeaceae. This pilot study of bark tensile strength suggests that unusually break-resistant bark may be a common but not universal trait within the Thymelaeaceae. The comparatively high ultimate tensile strengths of bark of Dirca spp. are consistent with reports of practical uses of bark from these species.