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The flora of woody plants and vegetation on the Horn of Africa

Other Research Project | 03/01/17

There are about one thousand species of woody plants that occur naturally on the Horn of Africa, including trees and large shrubs, and they have many functions in the highly varied ecosystem on the Horn, including soil conservation and the prevention of flooding during tropical rainstorms. For historical reasons, the woody flora and the vegetation types they form have been less studied than in most other parts of Africa, and new species of even rather large shrubs or trees are still discovered, named and described. This project, which is part of an involvement with the flora and vegetation on the Horn since 1970, aims at describing and mapping the woody plants, the vegetation types they form, and the ways in which the local population interacts with them. The nearer aims are to produce manuals and scientific publications on the flora, including an improved map of the vegetation. Due to long-term collaboration with Ethiopian scientists and the involvement of many Ethiopian Ph.D. students, some of which will eventually become staff members at the new universities of the country, we have as an aim for the future to spread the information not only in international academic circles, but also to relevant places in the vast countries of the Horn.

Background – long experience with the Horn of Africa

The project has its roots in grants from the Carlsberg Foundation in 1970 and 1972. These two grants were intended to cover studies of flora and vegetation in the humid south-western part of Ethiopia, but it was soon realised that the complex topography and climate of the Horn made it necessary to broaden the work. The Ethiopian revolution from 1973 and onwards made it difficult to continue fieldwork until the beginning in 1980 of Swedish sponsored projects to write floras of Ethiopia and Eritrea (and soon after also Somalia). The Danish involvement in particularly the Ethiopian and Eritrean project, which was concluded in 2007 with the publication of the 9th and 10th volumes of the flora manual, lead to fieldwork over as many parts of the countries that could be visited for reasons of politics and security. This fieldwork was almost entirely sponsored by the Carlsberg Foundation, and can be seen on the attached map, where the older study areas are marked with blue dots and the ones sponsored by the present grants from 2013 and 2014 are marked with green dots. The work in Eritrea took place in 1986 and the work in Somalia in 1987.

[1] Map based on Ib Friis' data.

Since our work with the flora of Ethiopia and Eritrea there has been close collaboration between Ethiopian and Danish botanists, mainly Ib Friis at the Natural History Museum of Denmark and Prof. Sebsebe Demissew at the National herbarium of Ethiopia, Addis Ababa University.

[2] Left image, from left to right: Three technical staff members from Addis Ababa University, professor Sebsebe Demissew and one of his Ph.D.-students on a trip with Ib Friis to forests in SW Ethiopia (phot. Ib Friis). Right image: Ib Friis in Acacia-Commiphora bushland in SE Ethiopia (phot. Sebsebe Demissew).

Based on fieldwork up to 2010, we have published an atlas of the potential (natural) vegetation of Ethiopia:
Ib Friis, Sebsebe Demissew, & Paulo van Breugel. 2010. Atlas of the potential vegetation of Ethiopia. Biologiske Skrifter 58: 1-307.
The full text and all the maps of the atlas was also publised in Ethiopia and an individual book:
Ib Friis, Sebsebe Demissew, & Paulo van Breugel. 2011. Atlas of the potential vegetation of Ethiopia. 2. udg. Addis Ababa University Press, 2011. 306 pp.
The purpose was to secure a cheaper edition available in Ethiopia and within reach of all who might need it. The maps in both editions of the atlas was in 1:2,000,000. The illustration below [3] is a reproduction of the entire map on a small scale.

Topography, climate and population on the Horn of Africa

In tropical environments with moderate to high population density the indigenous woody plants can be useful indicators of the health of the environment, but also pointers to the structure and composition of the natural vegetation in the past. In the Horn of Africa (1.9 million km2) the average population density is close to 50 inhabitants per km2 (i/ km2), but the figures vary dramatically from one region to another, from close to 500 i/ km2 in the most densely populated and most intensely cultivated parts of the Ethiopian highlands to less than 50 i/ km2 or virtually no resident population in the lowlands with nomadic population. There is also very large variation in altitude and climate within the Horn.

Field studies supported by the Carlsberg Foundation in 2013-2015

As appears from the map of field studies 1970-2013 and 2013-2015, the recent studies took place on the Western and Eastern slopes of the Ethiopian highlands (see map of sudy sites above). The main purpose was to test various theories formed during the work with the Atlas of the potential vegetation of Ethiopia and gather new information to our publications on the trees of the Horn of Africa. In addition to these planned studies, we made several new discoveries of taxonomic, biogeographical and ecological importance.

The less studied areas of Ethiopia and the future of environmental studies in that country

[4] The attached map shows some of the less studied areas: A: South-Eastern slope of the highlands. We have established a new vegetation type in this area (the transitional semi-evergreen bushland) and made interesting discoveries of hitherto poorly and completely unknown species, but the highly varied topography and geology makes it very likely that much is still undiscovered. B: Western slope of the central Ethiopian highlands. This area is less varied than A with regard to geology, but it is very little studied and of interest with regard to the North-South gradient. The areas C and D are currently rather less accessible for security reasons.

Diversity in topography and climate on the Horn of Africa

[5] The maps are constructed from the data set termed WorldClim (, which we also use in the modelling of species distribution and distribution of vegetation types.

Ethiopia's largest area, where 80% of the population lives, is the central highland, which is divided into a Western and an Eastern part of the Rift Valley. Most of the highland is located at altitudes between 1500 and 2400 m above sea level, but many massifs and peaks reach above 4000 m. To the north-east the Rift Valley expands into the afar, and here is the Dallol Depression, which is more than 100 m below sea level. To the south and east the highlands slope towards Somalia Towards Sudan. West of the highlands is a narrow strip of lowland. The highlands have temperate climate with large temperature fluctuations between day and night. 

The rainfall is unevenly distributed both geographically and during the year, but generally the precipitation increases with height and to the west and south. The majority of the rain falls between June and September. The temperatures in the lowlands can be exceedingly high. The highlands are almost entirely cultivated, but a few places at 1500-3000 m have still maintained some of the original mountain vegetation, which consisted of open, evergreen bush land, Acacia wooded grassland, and dry forest. Below ca. 1500 m, on the western side of the highlands there are deciduous woodlands, where the grass usually burns every year, and on the eastern side of the highland there is deciduous bush land, semi-deserts and desert, as in Somalia and northern Kenya. The Ethiopian flora is relatively rich in species because of the great variation in topography and climate, and ca. 5000 species of flowering plants are now known from the country.

The fast growing population on the Horn of Africa

[6] The diagrams have been constructed on population data from Worldometers.

Due to the fast growing population, particularly in Ethiopia, there is high and growing pressure on the natural vegetation on the Horn of Africa and, with a median age in Ethiopia 19 years, it is almost certain that it will take long time reduce the growth rate. This has already happened in the larger towns, not least in Addis ababa, but in the countryside the population is still growing with more than 2% per year. Farming of cereals with a simple type of plough has existed in Northern Ethiopia for more than two millennia, and the farmland in many parts of the Ethiopian highlands is intensely utilised up to the upper limits where hardy cereals such as barley can grow. 

New areas are being cultivated and the fallow period, during which wild plants were allowed to invade the fields and cattle could graze, is being steadily shortened. In the lowlands of Ethiopia, and in Eritrea, Djibouti and Somalia the population is largely nomadic, but also cattle grazing in the often very hot and dry lowlands may bring a heavy strain on the natural vegetation. Another factor is the burning charcoal from the wood of the indigenous trees. This charcoal is not only consumed locally, but also sold to highlanders via lorry drivers and owners of private cars. Politicians and scholars are aware of the problems caused by the rapidly growing Ethiopian population, and it is hoped that better knowledge of the environment will contribute towards the UN sustainable global, as stated in the section ‘Scientific Social Responsibility (SSR) and the UN Sustainable Development Goals’.

[9] Studies of grass fires in the deciduous woodland on the western of the Ethiopian highlands and of the previously unrecognized dry transition zone on the eastern side

During our fieldwork in 2013, 2014 and 2015 we focussed on the vegetation on western and eastern slopes of the Ethiopian highlands. From our earlier work we knew that fires were an important ecological factor on the western slopes, but we were not sure how well the distribution of fires agreed with the characteristic vegetation type, the Combretum-Terminalia deciduous woodland, normally associated with fires. 

In this study, we used our high-resolution map from 2010 of the potential natural vegetation and satellite data of recorded fires from MODIS to model and investigate the importance of fire as a driver of vegetation distribution patterns in Ethiopia. The red line in map A indicates the eastern border of the Combretum–Terminalia deciduous woodlands according to our vegetation map. The scale indicates the number of annual fires, from zero to 2.5. We employed statistical modelling techniques to estimate the distribution of fire and the vegetation types under current climatic conditions, and used the calibrated models to project distributions for different climate change scenarios. 

The results showed a clear congruence between distribution patterns of fire and the Combretum–Terminalia deciduous woodlands, although we observed a small number of striking incongruences, which we hope to study further, as explained above, particularly the decline in fire frequencies in the Northern and Southern part of the Combretum–Terminalia deciduous woodlands, and a surprisingly low fire frequency in the area marked with ”D” in Map A, where field studies have shown that the grass stratum is scarce. Fire-prone areas will increase under future climates, and the models predicted stronger expansion of Combretum–Terminalia deciduous woodlands and a more limited increase of moist Afromontane forests. The results were published in: 

Paulo van Breugel, Ib Friis, Sebsebe Demissew, Jens-Peter Barnekow Lillesø & Roeland Kindt. 2016. Current and future fire regimes and their influence on natural vegetation in Ethiopia. Ecosystems  19(2): 369-386.

Field studies during 2013-2015 on the Eastern slope of the Ethiopian highlands confirmed the existence of a floristically distinct, semi-evergreen bush land. Evergreen bush lands in Ethiopia have been inadequately studied and mapped. We based our study on a review of the recent descriptions of evergreen bush lands in Ethiopia and hypothesize that there was a distinct zone of natural semi-evergreen bush land, which is restricted to the Eastern and South-Eastern escarpment of the Ethiopian Highlands. In contrast, evergreen bush lands in other parts of Ethiopia are, according to our many field observations, of a secondary nature. To test this hypothesis, we classified 354 locations across Ethiopia from our previous and recent fieldwork on the occurrences of indicator species. We created predictive distribution models of the semi-evergreen bush land in Ethiopia using various environmental distribution models. 

The vegetation surveys confirmed the existence of a distinct type of evergreen bush land vegetation. The transitional semi-evergreen bush land forms a transitional zone between the Acacia–Commiphora woodland and bush land and the Afromontane forest on the Eastern and South-Eastern escarpments of Ethiopia, but is also characterized by a number of species that are unique to this zone. Our results highlighted the unique character of this part of the evergreen bush lands in Ethiopia, which we argue should be taken into account for future conservation planning. When mapped on our vegetation map from 2010 (see map B), we find that the transitional semi-evergreen bush land forms a zone which is prominent and easy to observe in the field on the South-Eastern slope of the Ethiopian highlands, but less discernible elsewhere. The results are published in Paulo van Breugel, Ib Friis & Sebsebe Demissew. 2016. The transitional semi-evergreen bush land in Ethiopia: Characterization and mapping its distribution using predictive modelling. Applied Vegetation Science 19(2): 355–367.

Scientific Social Responsibility (SSR) and the UN Sustainable Development Goals

We hope that a better knowledge of the environment will contribute to the UN sustainable global goals, particularly No. 15, ‘Improved Life on Land’. A better-balanced environment may also contribute to the achievement of some of the other goals, for example No. 2, ‘Zero Hunger’. The close collaboration between Ethiopian and Danish scholars must fall under goal No. 17, ‘Partnership for the Goals’.

Although the grants from the Carlsberg Foundation are formally given to Ib Friis, they also benefit the long standing scientific collaboration between him and Prof. Sebseb Demissew, Addis Ababa University. Since 2013, Prof. Sebsebe Demissew has served as a member of the Multidisciplinary Expert Panel (MEP) for the Intergovernmental Panel for Biodiversity and Ecosystem Services (IPBES), which provides scientific information about biodiversity and ecosystems to policy makers in 125 member-governments under the auspices of the United Nations Environment Programme (UNEP), the Food and Agriculture Organization (FAO), the United Nations Development Programme (UNDP), and the United Nations Educational, Scientific and Cultural Organization (UNESCO). 

In October 2013, Prof. Sebsebe Demissew was awarded the prestigious International Medal of the Royal Botanical Gardens, Kew, for his work on plant biodiversity in Ethiopia. Receiving the medal, Prof. Sebsebe Demissew acknowledged the support he has received from the Carlsberg Foundation for the joint research with Ib Friis, the recognition from the Royal Danish Academy and Sciences and Letters by electing him a foreign Academy member, and the support from the Academy by publishing the detailed vegetation atlas of Ethiopia in 2010 [3], authored by Ib Friis, Sebsebe Demissew and Paulo van Breugel. The Danish-Ethiopian collaboration also supports goal No. 4, ‘Quality Education’, as Ib Friis and Prof. Sebsebe Demissew are in contact with young scientists from a range of Ethiopian Universities. 

These young scientists follow our work via modern electronic media. Sometimes we get the opportunity to go with them into the field and compare ideas, observations and experiences. When working in a developing country and with the younger generations, it is obvious that concepts like quality education, and sustainable utilisation of land and partnership between North and South must play an important role. We are very grateful to the Carlsberg Foundation for the support, which allows us to do all this.

Papers published from our fieldwork in 2013-2015

  • Ib Friis & Odile Weber. 2014. Crotalaria trifoliolata (Leguminosae: Papilionoideae), a previously incompletely known Ethiopian endemic rediscovered after 120 years. Kew Bulletin 69: 9536, pp. 1-9. DOI 10.1007/S12225-014-9536-7.

  • Ib Friis , Michael G. Gilbert , Odile Weber & Sebsebe Demissew. 2016. Two distinctive new species of Commicarpus (Nyctaginaceae) from gypsum outcrops in Eastern Ethiopia. Kew Bulletin 71: 34, pp. 1-19. DOI 10.1007/S12225-016-9648-3.

  • Ib Friis, Odile Weber, Paulo van Breugel & Sebsebe Demissew. 2016. An endangered Ethiopian endemic, Crotalaria trifoliolata (Leguminosae: Papilionoidaeae), and its little-known habitat. Symbolae Botanicae Upsalienses 38: 19-39.

  • Paulo van Breugel, Ib Friis & Sebsebe Demissew. 2016. The transitional semi-evergreen bush land in Ethiopia: Characterization and mapping its distribution using predictive modelling. Applied Vegetation Science 19(2): 355–367.

  • Paulo van Breugel, Ib Friis, Sebsebe Demissew, Jens-Peter Barnekow Lillesø & Roeland Kindt. 2016. Current and future fire regimes and their influence on natural vegetation in Ethiopia. Ecosystems 19(2): 369-386.