"The tree is overused," says Frans Bongers, a tropical forest ecologist at Wageningen University in the Netherlands. He adds that the resource is "exploited very intensively in some areas."
Frankincense comes from these small, deciduous trees that grow in the hills of Ethiopia and Eritrea as well as other East African nations. The trees are tapped for resin during the region's dry period from mid-September to June. A small incision is made in the tree stem and the sap drips and solidifies into a "tear" that is collected three weeks later. After the tears are harvested, the incision is reopened and the process repeated. In a typical season each tree can be tapped as many as 15 times to extract between two and three kilograms of frankincense.
Previous studies suggested that agriculture, drought and low seed production all threaten the species as well as the supply of frankincense, which is used around the world for religious ceremonies, traditional medicine and perfume. In the current study conducted in Eritrea, researchers looked at how tapping for the economically and socially important extract affects seed production in B. papyrifera.
In one experiment Bongers and his colleagues collected seeds from trees that had not been harvested in at least four years and from trees tapped each year. Seeds from the untapped trees germinated more successfully than those from tapped trees, according to the report in the December issue of the Journal of Applied Ecology. The researchers also compared the flowers, fruits and seeds in trees that were not milked with those that formed in trees that were tapped normally (at six points along the tree trunk) or heavily (at twelve points). Overall, the number of such reproductive structures, along with the seed mass and viability, decreased with increased tapping.
The researchers suggest that extracting frankincense weakens the trees, which then use their energy to replace the resin rather than for growth and reproduction.
"In the long term, there is no reproduction because the energy is drawn out of the trees," Bongers says. "In 20 or 30 years time, that means no new individuals."
Already B. papyrifera stands are dominated by mature trees and have few seedlings. Goats and other livestock grazing freely on the land eat the young trees. The species is considered threatened in the region and the Dutch researchers warn that current frankincense tapping practices "may contribute to the lack of natural regeneration of the species in Eritrea" even further.
Bongers offers several possible solutions: Areas of untapped trees could be set aside and healthy seeds collected and replanted. Trees could also be less intensively tapped or given a few years of rest between tapping to recuperate.
"If more is invested in managing the forest, better and more products can get out," Bongers says. It's "good for the forest and good for the people."