The distribution of the frankincense tree Boswellia papyrifera in Eritrea: the role of environment and land use

Aim We determined the present and past distribution, and the abundance, of Boswellia papyrifera in Eritrea, and the environmental and land-use factors determining its distribution limits. Location Eritrea, in the Horn of Africa. Methods In 1997 a Boswellia field survey was conducted in 113 village areas covering four administrative regions. Species occurrence was related to rainfall, air temperature and length of growing period. Additionally, the relationship between the abundance of Boswellia trees and selected physical and chemical soil factors, topography and land-use types was determined for five study areas (with a total of 144 plots) situated along an altitude gradient of 800¿2000 m a.s.l. Results The geographical distribution of B. papyrifera was limited to the southwestern and southern parts of the country between 800 and 1850 m altitude receiving a mean annual rainfall of 375¿700 mm, with a growing period of 45¿100 days. Species abundance was affected by, in order of importance: altitude, land-use intensity and soil organic matter. Most trees were found in hilly areas; tree density increased from the foot slope to the hill summit; no trees occurred in valleys. Land-use intensity, especially agriculture, fallow and grazed areas, had a profound negative effect on tree abundance. Natural regeneration of the species was promoted in areas where grazing by livestock was not allowed or regulated. Main conclusions The distribution of B. papyrifera in Eritrea has decreased during past decades, mainly due to an increasing human population, resulting in the conversion of woodlands into agricultural fields and increasing livestock pressure hindering natural regeneration. Consequently, Boswellia trees are found mainly in hilly areas on steep slopes with shallow soils of low fertility. The species appears to be able to adapt to these harsh growing conditions: in adjacent countries it was also found in comparable growth habitats.