C.T. de Wit Graduate School for Production Ecology and Resource Conservation, Wageningen University, 6708 PB Wageningen, THE NETHERLANDS.
The degradation of frankincense tree dominated woodlands has been attributed to climatic conditions and human activities. We lack however information on how such factors influence the resource balance and productivity of trees. The aim of this study was to evaluate the impact of resin tapping on the whole tree carbon gain, storage and allocation pattern of frankincense trees (Boswellia papyrifera (Del.) Hochst) in the dry woodlands of northern Ethiopia. I hypothesized that the intensive resin tapping of frankincense trees reduces tree vitality, particularly under relatively dry conditions. I established experimental plots in the highland woodlands of Abergelle and the lowland woodlands of Metema, and applied tapping treatments to similar sized adult trees (DBH 20 +/- 3cm). For these trees I also collected data on leaf gas exchange, crown traits, carbon storage, carbon allocation, growth and frankincense production during a period of two years (2008-2009).
Trees follow similar leaf gas exchange patterns in contrasting environments, but differ in annual crown carbon gain between highland and lowland sites. Highland trees of Boswellia had a higher photosynthetic capacity, were exposed to higher light conditions, but had a shorter leaf lifespan than lowland trees. Integrating these effects, I showed that the annual crown carbon gain is higher in the highland trees than in lowland trees. Lowland trees are mainly constrained by clouded conditions and resultant low light levels during the wet season, limiting their carbon gain. Moreover, carbon gain was also restricted by atmospheric drought, and much less by soil water deficit during the growing season. The production of frankincense was not affected by the annual tree carbon gain implying that trees with smaller total leaf area may suffer sooner from carbon starvation by tapping.
Tapping reduced storage carbohydrate concentrations in wood, bark and root tissues indicating that continuous tapping depletes the carbon reserves. A large part of the carbohydrate concentration in the plant tissues was starch. Boswellia trees have more total nonstructural carbohydrates (TNC) concentrations and pool sizes in wood than in root and bark tissues. Because tapped trees face depleting carbon storage pools during the dry tapping season and cannot fully replenish these pools during the wet season, tapped trees may face higher risks of carbon starvation compared to untapped trees in the long term.
Estimated total annual carbon sinks to the different plant components were 38-68% of the annual carbon gain in both study sites. However, Boswellia trees also establish mycorrhizal associations which may consume an additional 20% of gross primary production. On a whole-tree basis, the percentage of autotrophic respiration may exceed all other costs. The foliage construction costs and incense production are the second and third largest carbon sinks, respectively. Contrary to our expectation, the sum of all dry season carbon costs was higher than the total amount of consumed TNC during the dry season. The high carbon costs during the dry season imply that trees do not fully depend on TNC to pay for the carbon costs during the dry season. With the exception of carbon allocation to foliage production and maintenance, a higher gross primary production does not enhance an overall increase in carbohydrate investments in the other sinks. Therefore, the carbon allocation pattern is constrained not exclusively by the absolute amount of carbon gained but also by other factors.
The results clearly indicate that continuous tapping depletes the amount of stored carbon, the leaf area production and the reproductive effort. These negative effects were however site specific and could possibly be apparent sooner for smaller trees than for larger ones. Thus, guidelines for resin tapping of Boswellia trees should consider tapping intensity, tapping frequency, environmental conditions and tree size and should focus on maintaining vital trees and populations for the future.