Encouraging medicinal tree diversity within agricultural landscapes

Chris Mesiku

A recent study from the World Agroforestry Centre and published in the Forests, Trees and Livelihoods journal reveals that middle aged Prunus africana trees that are middle sized, give the best yields of an extract capable of reducing prostate gland disorders. The study also found that the trees from Timboroa had the highest extract yield while those from Kakamega and Kinale had the highest number of the tree’s chemical compounds. The scientists envisage that the information on the different populations of Prunus africana trees will enable tree breeders to breed higher yielding trees with quality extracts. Lead Scientist Peter Gachie says “A higher yielding tree will add value to a rural farmer’s income generation potential.”

These days, diseases largely associated with lifestyle choices are on the rise including cancer and prostate disorders. So natural extracts that can help alleviate some of these are very much in demand. The Prunus africana tree is a source of such natural extracts and currently very high in demand. Without the conservation of high yielding populations, these trees could be harvested out of extinction leading to reduced potential sources of income for smallholder farmers.

The study by Peter’s team focused mainly on species that are found in natural forests around Kenya because they were assumed to be the true origin of the said trees.  Peter says “if for example, trees from farms where chosen, we would have relied on word of mouth to know the origin of the seeds and this is not scientifically robust.” They investigated whether there was a correlation between yield and age of trees. Importantly they wanted to discover whether there was origin based variation in the yield and chemical composition of the bark extract.  They compared bark extracts from different Kenyan forest zones. These zones were Timboroa, Eburu, Kinale, Kakamega and Kobujoi. They discovered that youthful trees in the 40-55 age range had the highest extract yields, with the yield dropping as the tree’s aged. These results are in line with other Centre studies that have shown that it usually takes 15 years for the active prostate cancer curing ingredient to form within the Prunus africana bark. These trees were, on average measuring between 30cm and 50cm dbh. A tree’s dbh is its “diameter at breast height” defined as a height of 1.37 meters above the forest floor.

 Trees from Timboroa had the highest extract yields, followed by Eburu with Kobujoi registering the least. This implied that the higher the tree was in terms of altitude, the better its yield tended to be but they found temperature had an opposite effect on yield. The important question was whether different Kenyan zones gave vastly different yields. The results showed a significant difference between the yields of bark samples from Timboroa and Kobujoi. Peter said “We found that two trees which are the same species but grow in a different natural forests yielded different amounts of the medicinal extract.” “This led as to conclude that place of origin plays a significant role in determining crude bark yields,” he continued.   

A technique called gas chromatography was used to work out the number of compounds in the bark extracts. Kakamega samples were found to be superior with 8 compounds but interestingly, when the compounds were analysed by a technic called Thin Layer Chromatography, Kinale had the highest number of compounds, closely followed by Kakamega and Kobujoi. In total, 13 different compounds were depicted in the 5 Kenyan zones when analysed under gas chromatography.

Mr Gachie says “the Prunus africana is commonly used in households as a treatment for a variety of ailments but its market potential is yet to be realized.  In the future, when more research is available, the benefits of this tree will be better appreciated by all people involved in the Prunus africana value chain.”

This study could be a step to creating a database of high yielding Prunus africana trees. The findings should be integrated into breeding programs. Tree breeders are urged by the scientists to integrate as many qualities from different Prunus africana trees as possible to improve the quality of the medicinal extract. They also recommend that the superior tree traits should be correlated with easily identifiable physical characteristics such as bark colour. “The physical characteristics will make it easier to identify superior species in the future” Says Peter.

Access the press release here.

Download the research paper.