Seedling mortality causes recruitment limitation of Boswellia papyrifera in northern Ethiopia

The Frankincense tree Boswellia papyrifera is an important resource in the semiarid lower highlands of eastern Africa but its populations are declining due to human pressure and environmental degradation. To assess the perspectives of (assisted) natural regeneration for Boswellia woodland restoration, we examined Boswellia stand structure and seedling densities in a grazed woodland and in a livestock grazing exclosure. We also tested topsoil scarification as a means to increase seedling survival. Boswellia populations lacked small diameters, indicating recruitment limitation. During the rainy season, seedling densities were 8331 ha(-1) in the exclosure and 3325 ha(-1) in the grazed woodland. Respectively, 19% and 11% of these seedlings survived the first dry season. However, the lack of saplings suggests a lack of surviving seedlings over successive dry seasons and thus a cumulative seedling mortality approximating 100% within each generation. Topsoil scarification had an adverse effect on seedling survival. Dry season seedling mortality seriously limits the potential of natural Boswellia woodland recovery. To restore a healthy population structure in exclosures, additional management interventions such as shading to support early seedling survival or planting of large rooted cuttings need to be tested. (c) 2007 Elsevier Ltd. All rights reserved.