Dispersal and recruitment of Olea europaea ssp. cuspidata in degraded Afromontane savanna : implications for forest restoration in the highlands of northern Ethiopia


Land cover changes and erosion triggered by overgrazing, unsustainable use of resources and population growth are some of the most important causes for severe dryland degradation and desertification in the highlands of northern Ethiopia. The consequences are poverty and food insecurity as well as large-scale loss of biodiversity. To control further ecosystem degradation, the government has adopted a forest rehabilitation strategy, using formally declared, community-managed protected areas where removal of remnant vegetation and free grazing are currently suspended. Land rehabilitation efforts in these exclosures aim at restoring the natural Afromontane forest vegetation, but in contrast to the relative rapid recovery of grass and shrub cover, regeneration of trees is slow and often unnoticed. Assisting the regeneration of trees may expedite the productivity of the exclosures, hereby serving both conservation and poverty-alleviation and thus encouraging the adoption of environmental rehabilitation efforts by highland smallholders. In limiting environments, tree seedlings may benefit from growing near pioneer shrubs that buffer against extreme site conditions, improve resource availability or protect against herbivory. The application of this effect, known as facilitation, may improve the establishment of tree seedlings, mimicking natural succession. Therefore, this research focuses on the natural regeneration of Olea europaea ssp. cuspidata (African wild olive) in exclosures. O. europaea is a fleshy-fruited, secondary climax species of dry Afromontane forest and at present a major constituent of the rather species-poor forest relics in the region. Once established, the tree is drought resistant, but because of its multiple uses (e.g. durable timber, traditional ox-ploughs, firewood and charcoal) both young and mature trees have been over-harvested dramatically in Ethiopia and as a result this valuable tree is now under threat of local extinction. In this study a simple model of the plant life cycle, composed of life stages connected by transition probabilities that are determined by different processes, was proposed. The influence of biotic and abiotic factors at different recruitment stages was analyzed, and based on an elimination process the critical steps in the regeneration cycle were identified. Comparison of transition probabilities between microhabitats – under the canopy of pioneer shrubs and in patches of bare soil between shrubs – showed that recruitment depends on shelter by shrubs. Not the dominant thorn shrub Acacia etbaica but dense multi-stemmed Euclea racemosa ssp. schimperi shrubs with a wide base and crown on a mulch-rich mound are the key recruitment foci. The observed facilitative effect of shrubs on germination and seedling survival may be attributed to reduction of solar radiation by shrub canopies and moisture retention effects of the ectorganic layer, promoting imbibition of seeds and preventing desiccation of seedlings. Establishment limitation (i.e. germination limitation and survival limitation) clearly affects Olea recruitment, but based on the estimated Olea seed density in exclosures, particularly dissemination limitation may be accountable for the slowness of forest succession. Limited seed disperser activity and seed production at population level due to reduced forest vitality, lack of fit parent trees and fragmentation of the landscape are most likely the major problems for natural regeneration of Olea in exclosures. The most important notion is that basically all life stage transitions pose problems for Olea regeneration, and that management guidelines for assisted forest restoration must therefore take as much of these limitation problems into account. Increasing the size of small forest patches and placing exclosures in the vicinity of these patches, creation of suitable patches under pioneer shrubs, especially under Euclea, direct sowing of Olea seeds, pre-treated by birds, under shrubs and planting seedlings under pre-established pioneer shrubs, particularly in years with above-average summer rainfall are some techniques that may help to overcome the recruitment problems of Olea in exclosures. For an increased transferability of the proposed management guidelines for dealing with recruitment limitation, further research should evaluate whether similar facilitative effects between climax tree species and pioneer shrubs exist in other semiarid regions.


facilitation, rehabilitation, forest, succession, semiarid, seed ecology, seedling survival, stable states, grazing, exclosures, development cooperation


Landscape Ecology {Research discipline}
Conservation and Biodiversity {Research discipline}
Ecology and Evolution not elsewhere classified {Research discipline}
Management and Environment {Research discipline}
Agroforestry {Research discipline}
Forestry Sciences not elsewhere classified {Research discipline}
Environmental Management and Rehabilitation {Research discipline}
Natural Resource Management {Research discipline}
Ecosystem {Integration level}
International (non-EU) {Cooperation status}
With High Priority Countries {Cooperation status}
Effect of Biodiversity on Ecosystem Functioning {Research orientation}
Systematics, Inventorying and Classification {Research orientation}
Conservation, Restoration and Sustainable use of Biodiversity {Research orientation}
Northeast Tropical Africa {Geographical scope}
East Tropical Africa {Geographical scope}
Ethiopia {Geographical scope}
Dry and Sub-humid Lands {Habitat type}
Forest {Habitat type}
Mountain {Habitat type}
General Measures for Conservation and Sustainable Use {Research purpose}
Technical and Scientific Cooperation {Research purpose}
In-situ Conservation {Research purpose}
Sustainable Use of Components of Biological Diversity {Research purpose}
Research and Training {Research purpose}
Public Education and Awareness {Research purpose}
Aves {Taxonomical scope}
Pteridophyta {Taxonomical scope}


Name Role Amount
Post-doctoral Fellowship unknown


Name Role Start End
Aerts, Raf member 2003-10-01 2006-03-01
Muys, Bart promotor
Hermy, Martin member


Name Role Start End
Division Forest, Nature and Landscape Research unknown


Name Role Start End
Forest rehabilitation through natural regeneration in Tigray, Ethiopia. partner


Reference Role
Aerts R. 2007. Church forests in Ethiopia. Frontiers in Ecology and the Environment 5(2):66-66 author
Aerts R., Maes W., November E., Behailu M., Poesen J., Deckers J., Hermy M., Muys B. 2006. Surface runoff and seed trapping efficiency of shrubs in a regenerating semiarid woodland in northern Ethiopia. Catena 65(1):61-70 author
Aerts R., Maes W., November E., Negussie A., Hermy M., Muys, B. 2006. Restoring dry Afromontane forest using bird and nurse plant effects: Direct sowing of Olea europaea ssp cuspidata seeds. Forest Ecology and Management 230(1-3):23-31 author
Aerts R., Negussie A., Maes W., November E., Hermy M., Muys B. 2007. Restoration of dry afromontane forest using pioneer shrubs as nurse-plants for Olea europaea ssp cuspidata. Restoration Ecology 15(1):129-138 author
Aerts R., November E., Vander Borght I., Behailu M., Hermy M., Muys B. 2006. Effects of pioneer shrubs on the recruitment of the fleshy-fruited tree Olea europaea ssp cuspidata in Afromontane savanna. Applied Vegetation Science 9(1):117-126 author
Aerts R., Van Overtveld K., Haile M., Hermy M., Deckers J., Muys B. 2006. Species composition and diversity of small Afromontane forest fragments in northern Ethiopia. Plant Ecology 187(1):127-142 author
Aerts R., Wagendorp T., November E., Behailu M., Deckers J., Muys B. 2004. Ecosystem thermal buffer capacity as an indicator of the restoration status of protected areas in the northern Ethiopian highlands. Restoration Ecology 12(4):586-596 author

created:2011-12-14 14:18:59 UTC, source:iweto

© 2012 by the Belgian Biodiversity Platform