Relationship between bark thickness and diameter at breast height for six tree species used medicinally in South Africa

Bark is the most popular product harvested for traditional medicine in South Africa. Harvesting is sometimes selective for particular stem size-classes and the effect of bark removal and the sustainability of harvesting practices are species-specific. However, baseline autecological data that would assist conservation and trade monitoring efforts are not easily measured and rarely available. In an effort to link bark thicknesses recorded during three ethnobotanical surveys in the traditional medicine markets of Johannesburg, the relationship between bark thickness and stem diameter at breast height (dbh) was investigated for six species used medicinally in South Africa. Samples of bark were removed from 207 stems and subsequently weighed and measured. Thereafter, the samples were placed in a phytotron chamber to dry out over a period of 12 weeks. The change in bark thickness over time was regressed with stem diameter in order to predict stem diameter from bark thickness records. The strength of the relationship between bark thickness and diameter was strongly influenced by the macroscopic bark morphology of the species. Species where the rhytidome tended to stay on the stem exhibited similar stronger r2 values (r2 = 0.80–0.88) compared to the lower values for species that shed their bark (r2 = 0.005). Using Warburgia salutaris as an example, the prevalence of bark of certain thicknesses in the medicinal markets was used to evaluate the change in tree size-classes over a 6-year period. Results showed that whereas trees larger than 40 cm dbh were available in 1995, in 2001 bark from trees less than 25 cm dbh were more prevalent.