Population structure and morphology of the frankincense tree Boswellia papyrifera along an altitude gradient in Eritrea

In Eritrea, the frankincense tree Boswellia papyrifera is a multipurpose plant. Human induced factors such as land clearing for agriculture, overgrazing by livestock and overtapping of resin are threatening its distribution. Against this background, a study was carried out to investigate the species current population structure and tree morphology in five Boswellia areas along an altitude gradient (range 800 - 2000 m a.s.l.). In each area sample plots of 20 by 20 m were inventoried; a total of 144 plots were studied. The population structure analysis showed that there was an overall absence of juvenile trees between 1 and 8 cm DBH. Natural regeneration was found only in two areas in which trees were not tapped for resin and inaccessible to livestock. Tree height, DBH, crown depth and crown diameter decreased with increasing altitude. In the lowland areas trees were about two times taller (10 to 12 m) with deeper crowns than those growing in the highland areas. This indicates that the species grows better in the warm moist lowlands than in the moist and dry highlands. To promote natural regeneration and seedling establishment in existing Boswellia woodlands control measures are needed including proper tapping procedures and controlled grazing.