Genetic diversity and differentiation of the frankincense tree (Boswellia papyrifera (Del.) Hochst) across Ethiopia and implications for its conservation

Abstract

Boswellia papyrifera is used to produce frankincense, a bark resin that has been a commodity of domestic and international trade since ancient times. It is harvested from natural forests. The tropical dry forest (Terminalia–Combretum) woodland ecosystems in which B. papyrifera is one of the dominant species, are facing anthropogenic threats. In Ethiopia B. papyrifera populations have decreased tremendously to smaller and isolated remnant patches, and many forests in the North-western and North-eastern parts of Ethiopia completely lack recruitment of saplings. This regeneration bottleneck, in combination with adult mortality, threatens the persistence of the species. Devising an effective strategy to conserve wild genetic resources needs information on the genetic diversity and the pattern of genetic differentiation across the species area. In the present study we analysed adult trees sampled in twelve populations across the growing area of the species in Ethiopia for genetic diversity and spatial genetic differentiation using 10 polymorphic microsatellite loci. The mean level of observed and expected heterozygosity were 0.669 and 0.681 respectively, and these levels were similar for trees from larger populations and those from degraded populations. A moderate level of among populations genetic differentiation (FST=0.084) was detected. Genetic distance between populations was correlated with geographic distance (r=0.663, p<0.05). STRUCTURE analysis distinguished four distinct genetic clusters corresponding to regions with different environmental conditions. In the Western populations we detected recruitment of many seedlings and saplings, which is a significant novel finding as most of the other populations are completely devoid of saplings. We conclude that currently a high level of genetic variation is still maintained in B. papyrifera adult trees across the species’ range in Ethiopia including the highly degraded remnant B. papyrifera population patches scattered on farm and pasture lands. An effective conservation strategy for the species has to take into account the geographic distribution of source populations.