Trade-off relationships between tree growth and defense: a comparison of Ocotea bullata and Curtisia dentata following bark harvesting in an evergreen moist South African Forest

Abstract

Two artificially debarked Afrotemperate tree species showed different trade-offs between wound closure and compartmentalisation of decay in the stem. One species had a relaxed trade-off but inefficient defence, and the other showed more efficient defence but a substantial trade-off. Bark stripping for medicinal use is a common cause of damage in several indigenous tree species in natural forests of South Africa. Ocotea bullata and Curtisia dentata are in high demand for their bark for medicinal purposes. The study aimed at revealing intra-specific and inter-specific differences of tree growth rates wound closure and decay containment responses ten years after experimental bark harvesting. The results obtained on computer tomography scans of twenty trees showed that C. dentata had significantly higher decay percentages compared to O. bullata, indicating that O. bullata was able to more efficiently contain decay than C. dentata. While decay in O. bullata was confined at the wounded tissues, decay in the C. dentata extended below and above the wounded area. Intra-specifically, O. bullata showed strong positive correlations between tree growth rates and wound closure rates, and wound closure rates with relative volume of decayed wood, indicating that individuals with higher rates of growth and wound closure suffered higher percentages of decay. Results confirmed an intra-specific trade-off between growth rate and defense investment, with fast growing trees showing high percentage decay (poor compartmentalization of decay). Inter-specifically, however, growth versus defense trade-off did not present itself. For C. dentata, no correlations were found between rates of wound closure and percentage decay. The findings and conclusions derived from this study reveal complex, species-specific responses to damage. This study highlights the need to gain an in-depth understanding of underlying morphological, phylogenic, physiological characteristics of species to further explain the observed species differences.