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Conservation biology

Unkind cuts for incense

Gold, frankincense and myrrh — the three royal gifts of the Christmas story — remain valuable commodities. But as Toon Rijkers et al. report in the Journal of Applied Ecology (43, 1188–1195; 2006), the latter-day story of frankincense is also a tale for our times.

Frankincense is a resin produced by several small trees of the genus Boswellia, which grow in sub-Saharan Africa, from Nigeria to the Horn of Africa, and Arabia and the Himalayan foothills. The resin is exuded naturally from leaves and shoots, but especially from wounds. Its natural function is probably to ward off grazers and fungal attack. But the resin's fragrance has also made it prized as a perfume, fumigant, flavouring and medicinal compound.

Credit: C. OSBORNE/CORBIS

Rijkers et al. looked at frankincense production in Eritrea, where it is used locally for cultural and medicinal purposes, and is also exported. Wild Boswellia papyrifera trees are tapped by making incisions around the trunk of the tree, starting in mid-September at the end of the summer monsoon. The resin is harvested every three weeks by reopening the incisions, and harvesting continues throughout the dry season. Boswellia trees also produce their flower buds at the end of the wet season, however. Could the constant harvest of resin, commencing at the time of floral induction, be a serious drain on the trees' carbon resources? Not least because leaves are lost at the beginning of the dry season, so carbon for flowering and resin production must be obtained from storage products.

To investigate the effects of resin-tapping on the tree life-cycle, and especially seed quality, Rijkers et al. checked the seed-germination potential of tapped and untapped trees. They found that seeds from annually tapped trees had a germination success of only about 20% compared with a figure of 80–90% for untapped trees. Tapped trees also produced fewer inflorescences, fruits and seeds. So tapping is evidently costly to the individual tree.

There has been a severe reduction in the natural regeneration of Boswellia in Eritrea, to which more intense tapping may be contributing. Actions that would help to reverse this trend include reducing the amount of resin taken per tree, and resting selected trees for a few seasons; the Boswellia populations also need protection from intense grazing.

Here, however, the spectre of the 'Tragedy of the Commons' arises: the resources for both frankincense harvesting and grazing are freely available, and so prone to overexploitation unless the common good is taken into account. Self-regulation in the cause of long-term sustainability is a difficult lesson to learn, as a wider world has found out from the over-exploitation of fisheries.

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Moore, P. Unkind cuts for incense. Nature 444, 829 (2006). https://doi.org/10.1038/444829a

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