The effect of tapping for frankincense on sexual reproduction in Boswellia papyrifera.
In the Horn of Africa, frankincense (an aromatic hardened wood resin) is obtained by tapping Boswellia papyrifera. World-wide, frankincense is of great economic and social importance as an important element of incense and perfumes. The production is declining as a result of poor natural regeneration of the Boswellia woodlands, possibly as a result of the low production of viable seeds. We hypothesize that this is because of the current intensive tapping regime, which might favour allocation of carbohydrates for synthesis of resin at the expense of allocation for generative growth. Investigations were carried out at sites in different agro-ecological zones in the southwestern part of Eritrea with annually tapped trees and with trees that had not been tapped for several years. Seed viability and germination success were determined for 200 randomly collected seeds in each site. For three stands, the sexual reproduction (number of flowers, fruits and seeds) was determined for different sized trees subjected to three experimental tapping intensities (no, normal and heavy tapping). At the stand level, non-tapped trees produced three times as many healthy and filled seeds as tapped trees. Germination success was highest in stands with non-tapped trees (>80%) and lowest for those with tapped trees (<16%). At the tree level, sexual reproduction decreased with increasing tapping regime irrespective of tree size. Overall, large trees tended to produce slightly heavier seeds than small trees, and seeds from non-tapped trees were heavier than those from tapped trees. In the stands where tapping was prohibited changes in tapping regimes had the greatest effect on sexual reproduction. Trees subjected to annual tapping always showed the lowest sexual reproduction. Tapping for frankincense results in limited flower and fruit production, and low production of mainly non-viable seeds in B. papyrifera. We argue that tapping causes competition for carbohydrates between frankincense production, and fruit and seed setting. Consequently, the current tapping regimes will cause tree exhaustion and eventually a decline in vitality. Tapping may potentially reduce natural regeneration of the species. New tapping regimes are suggested that include periods of time in which tapping is prohibited in order for trees to recover and replenish their stored carbon pool, and a reduction in the number of tapping points per tree. This is important in view of the long-term sustainability of frankincense production, an internationally highly valued resource.