Resin-producing plants form a large group, of which the majority is woody species. Most resins are complex mixtures of terpenes. They are of significant commercial interest due to their flavour and fragrance properties, such as damar from Dipterocarpaceae, rosin and turpentine from Pinus spp, and frankincense from Boswellia spp. (Coppen 1995, Boer and Ella 2000; Langenheim, 2003). The genus Boswellia of Burseraceae family comprises 20 species of trees and shrubs, with the centre of geographic distribution in the Horn of Africa (Lovett and Friis 1996). Today, at least five Boswellia species are used for their resin (called frankincense or olibanum ), mainly in local communities during religious and coffee ceremonies, but also internationally.
When a tree is damaged, viscous liquid substances (wood exudates) may emerge from the wound. The injury may be natural (by insect, animal or wind damage), or human induced (by tapping). Wood exudates mainly contain carbon-based compounds, considered as secondary plant compounds as they are not required for growth, in contrast to primary compounds such as proteins, sugars and fats. The chemical composition of frankincense is about 60-85% resins (i.e. mixtures of terpenes), 6-30% gums (i.e. mixtures of polysaccharides), and 5-9% essential or volatile oil (Baser et al. 2003). Wood exudates are generally assumed to provide plant defence, because they seal-off injured tissue, prevent desiccation from injured tissue, protect against attack by insects and fungi (e.g. Phillips and Croteau 1999). There are important concept developed in predicting patterns of carbon allocation to growth and secondary metabolites (here, wood exudates).
Plants allocate carbohydrates for (1) resource capture and growth, (2) reproduction, (3) storage purposes, and (4) for the protection of already captured resources. Tapping for resin likely affects the pattern of carbohydrate allocation, as it enhances the competition for assimilates between growth processes and resin production (e.g. Lorio and Sommers 1986). Production of resin increases substantially after deliberately wounding the stem and trees with larger crowns produce more (Ruel et al. 1998, Lombardero et al. 2000). This probably is a result of a greater overall carbon budget of large trees (Sterck and Bongers 2001). Resin production in juvenile trees may originate from current photosynthesis, as stored carbohydrates are few (Lieutier et al. 1993). Tapping of wood exudates not necessarily reduces tree vitality (i.e. the ability to grow and reproduce under the present environmental conditions). However, increasing demands for gums and resins often result in harvesting techniques that maximise short-term economic gains only. Indeed, uncontrolled commercial tapping may result in retarded growth and reproduction (Lorio and Sommers 1986, Berman and DeJong 2003).
This project will investigate the physiology of resin removal in the incense tree Boswellia papyrifera and evaluate the consequences of tapping for the life history of the species.The project aims to determine to what extent removal of resin affects carbohydrate allocation in terms of tree growth, reproduction, storage and resin production. Particularly, we seek to develop practical criteria that ensure sustainable exploitation of frankincense in trees growing in contrasting environments (i.e. wet-dry areas and low-high altitude).