2.1. Phytogeography of the country
2.2. Utilization patterns of forest species
2.3. Threats
The vegetation of Eritrea has been described in a number of different publications, including some historical Italian-accounts, which still contain useful information, especially with regard to rates of change of natural vegetation. Perhaps the most significant of these is Pichi-Sermolli (1957 cited in Eritrean Biodiversity Stocking Assessment, 1998), which recognized 24 vegetation types and provided detailed descriptions of the typical species composition and distribution of the vegetation types in Eritrea.
More recently, the forest vegetation of NE Africa, including Eritrea has been described by Friis (1992 cited in Eritrean Biodiversity Stocking Assessment, 1998). These vegetation descriptions agree generally with those presented in White's Vegetation Map of Africa.
Descriptions of the regional vegetation of Eritrea were produced by Bristow and, for the coast, by Hemming (1961 cited in Eritrean Biodiversity Stocking Assessment, 1998). “The useful trees and shrubs in Eritrea” by Bein et al. (1996) was published more recently. The descriptions below are derived largely from these accounts, supplemented by additional material drawn from other sources.
The country is classified into six agro- ecological zones based on altitude, vegetation, and other ecological and geographic factors. The classification is very appropriate, although open for modification, as additional information are made available. The zones are:
Central Highlands Zone
The Central and Northern Highlands cover an area of 2,672,000 ha. This area includes the plateaux of the highlands and the higher parts of the eastern and western escarpments. It receives an uneven rainfall throughout the year. In the highlands around Asmara, the wet season may extend from March to December, but July and August are the only months with relatively reliable rainfall.
About 30 km North-East of Asmara, on the escarpments facing to the east, the area is more or less evenly rain fed all the year round. It is clear that on the highlands, depending on the latitude and slope directions, fluctuations of the precipitation levels is sometimes considerable. Temperature varies from one site to another, depending on latitude, altitude and slope directions.
As result of the relatively high moisture and lower temperature in this area, a specific floristic composition has been evolved. The relatively high rainfall and moderate climate have attracted more people to establish their farming systems, in the Highlands, mostly at the expense of the natural vegetation. Widespread clearing has converted forests into cultivation fields; and timber and non-woody collections have significantly disturbed the remaining forests. Some remnants of mountain mixed evergreen forests still exist in small locations throughout the highlands.
The mixed natural forest of the highlands is composed mainly of Juniperus procera and Olea africana, which may also be found in pure stands. Juniperus procera tends to occupy the higher elevations.
At lower altitude, it is either mixed or totally replaced by Olea africana and a number of other species. This formation also includes Celtis africana, Carissa edulis, Teclea nobilis, Anogeissus leiocarpus, Dodonaea angustifolia, Combretum molle, Acacia etbaica, and Terminalia brownii. The height of the principal tree species encountered (Olea africana, Juniperus procera) ranges from bush-like vegetation to over 15 m. The forest stands are composed of more than layers with varying crown closures.
Western Escarpment Zone
The western aspect of the mountains is relatively dry and hot. Rainfall regime and mean daily temperature change drastically from the top of the mountains to the Western Lowlands and from the relatively wet south to the drier north. The western part of the country receives rainfall from the southwest monsoon from April to November, with the heaviest rains in July and August. With the decrease of moisture and increase of temperature as one goes towards the west, the natural vegetation changes in composition, structure and physiognomy. In general, except in restricted sites, vegetation is very poor, low and scattered, and is composed largely of scattered shrub-like acacia species. Closed canopy woodlands are very rare, but do occur occasionally in the southern part of the area and in depressions and flat terrain between the mountains.
Human pressure on natural vegetation is higher in the southern portion of the watershed, owing to the higher rainfall, and diminishes in the dry northern part. The floristic composition of the natural woody vegetation includes a number of tree species namely: Terminalia brownii, Albizia amara, Boswellia papyrifera, Euphorbia abyssinica, Faidherbia albida, Balanities aegyptiaca, Syzygium guineense, Tamarix aphylla, etc. The average height of trees in this vegetation ranges from 2 to 5 m, sometimes higher in very limited areas. Density varies from thicket-like in depression to scattered trees or shrubs.
The physiognomy of the woody vegetation suggests three main classes in the western escarpment namely: closed to medium closed canopy woodlands, open woodlands and shrublands/bushlands.
The Green Belt Zone
This zone, on the Eastern Escarpment, is one of the few remaining areas of relatively undisturbed natural woody vegetation in upland Eritrea. The area is unique as it is fortunate to have two rainy seasons providing more than 1,000 mm annually. This accounts for its relative luxuriance and high rates of growth and regeneration. The major rainy season is from June to August, with light rains November to December.
The Eastern Escarpment forests are located to the north east of Asmara and cover an extensive area from the top plateau, running down to the east through several ranges of hills to the beginning of the Eastern Lowlands. The entire area has been, in theory, ‘enclosed,’ although several hundred families have maintained cultivation rights in the area. There is a significant amount of grazing, particularly around the forest margins. Even so, virtually all the steeper hillsides are covered with woody vegetation. Canopy cover ranges from 20 percent on hillsides to 80 percent in gullies and valley bottoms. At the highest point of the forested area (2300 m) vegetation is dominated by scrubby Juniperus procera, seldom exceeding 8m in height (despite its potential for reaching 40 m). The species is slowly regenerating and young trees are relatively plentiful. Other species found in these forests include Acacia etbaica, A.abyssinica, Euclea schimperi and Carissa edulis. African olive is uncommon although it was once a dominant part of the flora. There is sparse under-storey of Aloe abyssinica and various grasses. Sisal (Agave sisalana) is found along roadsides, but is not under cultivation for fibre production.
A large range of species is found at lower elevations. These include Terminalia spp., Barbeya oleaides, Celtis africana, large numbers of Dodonaea viscosa (angustifolia), Rhus abyssinica, Croton macrostachys, and Diospyros mespiliformis (African Ebony) in well watered areas. The understory includes Jasmium sp., Rosa abyssinica, Asparagus africana and a range of shrubs and climbers. At the lowest elevations, the species range diminishes and is dominated by Acacia etbaica, Balanities aegyptiaca and Cadaba rotundifolia.
The Southwestern Lowland Zone
The South western Lowland Zone covers 20,000 km2 in the Gash Barka Zone with rolling hills to the east that level out into flat fertile plains in the west. These plains are comprised of mostly fertile vertisols and are as yet under-utilised for agriculture. They are however difficult to cultivate. The area comprises important surface water resources consisting of the perennial Tekeze/Setit river system, and the seasonal Mereb/Gash and Barka systems, which flow for 3 to 4 months annually.
Tree cover is characterised in the east by extensive savannah woodland covering rolling hills, and very large areas of annual grassland, with scattered Acacia/Capparis shrub and woodland. The vegetation shows very little signs of grazing or browsing. It is likely that species composition and density has not changed much in recent history, although larger trees have probably been taken for fuel wood and construction. Regeneration is patchy but evident for most species, with the exception of Adansonia digitata. The characteristic shapes of heavily browsed trees and shrubs, including those in populated settlements, are not apparent.
Extensive forests of Doum palm (Hyphaene thebaica) are found near major river systems, particularly the Barka and Gash. The trees are healthy and, although stems had been considerably exploited in the past for timber, they have been used minimally recently. The preference for mid-sized palm timbers for building purposes may explain the present two-storied forest structure, with tall old trees providing a scattered canopy over early vegetative phases of the palm comprising leaves without any significant stem. There are no intermediate size trees. These palms regenerate from the base when cut, so stem cutting is not as serious a threat as it can be to other species. Uncontrolled leaf cutting, however, is a major threat. Other parts of the riverine forest support Faidherbia albida, Ficus sp., and Ziziphus spina-christi. Prosopis chilensis and/or P. juliflora are making an appearance in the Tessenei area of the riverine forest, and this exotic is likely to spread, causing considerable difficulties to cultivators and herders.
The Northwestern Lowland Zone
The North Western Lowland covers 3,040,000 ha. There is little information about the vegetation of this zone. For the most part it consists of extensive sandy plains with Acacia scrub, Capparis decidua, Balanites aegyptiaca, Calotropis procera (where the water table is near the surface), Boscia senegalensis and a range of other semi-arid species. Extensive and fringing riverine forests can be found along seasonal watercourses, similar to those described in the previous section. Pastoralists following traditional migration routes frequently use this zone. There is very little rain-fed agricultural cultivation as rainfall is inadequate. Fuel wood reserves are extensive.
The Coastal Plains Zone
The total area of the Eastern Coastal Plain is 4,670,000 ha. The coastal plains average around 24-32 km in width and consist almost entirely of mixed sand and stony alluvium (river flow deposits) derived from the highlands to the west. Along much of the coast, a broken line of limestone hills running approximately parallel to the coast about 15 km inland interrupts the plains. Near Massawa, there is another range of hills about 35 km inland which result in the formation of a sub-coastal plain at around 1000 m. South of Massawa the coastal plain is interrupted by the hills forming Jebel Ghedem which rises to 1018 m.. South of Jebel Ghedem, the coastal plain is formed from lava baserock overlain in most places with silt and gravel alluvial deposit.
The vegetation of the coastal plains is, in general, sparse and forms only a thin, partial cover to the land. Much of the vegetation of the region is similar in species composition and is dominated by species assemblage tolerant of a combination of drought, grazing pressure and salinity. There are however significant differences in plant community composition in response to local variations in conditions. No recent, complete floristic inventory has been undertaken in this zone and the species identified for this zone are undoubtedly incomplete. However, Hemming (1961) provides the best available description of the plant ecology of the coastal area of northern Eritrea.
Around Zula, much of the natural vegetation has been cleared for agriculture. Uncultivated areas are mostly degraded and heavily grazed, consisting mostly of weedy species, such as Echinochloa colona, Datura metel, Solanum sp., Aerva javonica and Heliotropium petrocarpum. Nearer to the shoreline, saline areas not used for cultivation, are covered by patchy Suaeda monoica, with a ground layer of Cenchrus ciliaris grass and occasional Trianthema crystallina.
On the richer loamy sands near Hirgigo and Wangobo, the grass cover is more extensive, comprising mostly Cenchrus ciliaris and Cenchrus setigerus, Dactyloctenium scindicum and very occasional stunted Acacia tortilis trees. This area is under considerable grazing pressure.
The plains lying between Wadis Wakiro and Desset support scattered low Acacia tortilis bushland, mixed with Salsola spineseens bushes after good rains. The groundcover may contain Heliotropium pterocarpum, Euphorbia aegyptiaca, Dauerygium glaucum, Tribulus longipetalus, and small amounts of Aerva javonica and Aristida spp. Panicum turgidum, a grass, is predominant at areas with deep soil, while in more saline soils, this species is replaced by Eleusine coca and Dactyloctenium scindicum. However, Sueada monoica bushes with Dipterygium glaucum and Cenchrus ciliaris grass cover are found in areas with most saline soils.
The vegetation cover is heavily denuded. Fuel wood consumption is one of the serious detrimental demands on ecology. Wood fuel is the major source of household energy in the country. The national fuel wood consumption is estimated at 1.29 million metric tonnes annually. Rural communities and most urban households, including some commercial enterprises, depend on biomass fuel for energy, but the supply has dwindled. Hence, the rural people who used to enrich farmlands with animal manure and agricultural residues have minimised their traditional practice, not out of choice, but need. Instead, they are using such by-products for fuel due to the scarcity of fuel wood. This is causing environmental deterioration and reduction of soil fertility.
Significant non-wood products include gum arabic, gum olibanum and dried doum palm leaves. Both gum arabic and gum olibanum are traditional non-wood products of Eritrea. Although world prices exhibit fluctuations, there exists a preference in final markets for the natural products rather than synthetic substitutes. In 1996, 463 tonnes of gum olibanum and 117 tonnes of gum arabic and in 1997, 542.6 tonnes of gum olibanum and 49 tonnes of gum arabic were produced under MOA licence.
The country has no domestic supply of timber, importing 60,000 cubic metres annually. The landed value of timber is currently around US$ 332 per cubic metre (FAO, 1997). Transmission poles and scaffolding are also imported. Matches are produced in Asmara from imported splints. Undoubtedly Eritrea has an expanding need for construction grade softwood timber, spurred by an expanding population and expected rapid development.
Eritrea was once host to a wide variety of fauna and flora. However, due to mismanagement and natural calamities, these resources have dwindled greatly. As a matter of fact, many of them are either extinct or endangered. The main causes of forest destruction in Eritrea are: expansion of agriculture and unwise land use, overgrazing, traditional house construction, fuel wood and charcoal, and past excessive logging for timber production.
Endangered or rare species are those which have low numerical abundance compared with others (Schulze et al, 1994). Several tree and shrub species are endangered in Eritrea. Boswellia papyrifera, which is found in the western lowland is endangered owing to excessive and unwise exploitation for the production of gum olibanum. Adansonia digitata, the baobab tree, is also regarded as endangered owing to insufficient regeneration. Local people use the fruit for medicinal purposes and its bark, leaves and fruits as emergency food. In areas, where sesame oil crop is grown, people use the stem of Tamarindus indica for turning the mortar for the production of sesame oil. In addition to this, the fruit are used for food and medicine. As a result, the population has decreased significantly. Balanites aegyptiaca and Ximenia americana are also regarded as endangered species.
Effective study as to the number, type and distribution of flora has not yet been conducted in Eritrea. Due to this fact, the data and information, especially of herbs and succulent plants, which do have great importance in terms of bio-diversity, are very limited. Some of these may well be extinct.
The University of Asmara, the Ministry of Agriculture, and the Ministry of Marine Resources of the State of Eritrea are working co-operatively to identify the existing species, their distribution, their local uses, and propagation techniques. So far, some species have already been recognised as endangered. List of these species as per 1997 is tabulated in Table 2.
Table 2. Endangered trees and shrubs species
|
No |
Scientific Name |
Common Uses |
|
1 |
Albizzia anthelmintica |
Firewood, timber, fodder, tannin |
|
2 |
Adenosine digital |
Food source, medicinal |
|
3 |
Balanites aegyptiaca |
Food, fodder, building, medicinal, firewood |
|
4 |
Boscia angustifolia |
Food, building, artifact, medicinal |
|
5 |
Boscia salicifolia |
Food, fuel, fodder, tool, furniture |
|
6 |
Boswellia papyrifera |
Incense, fodder, medicine, building |
|
7 |
Buddleja polystachya |
Fodder, construction, medicine |
|
8 |
Commiphora africana |
Food, fodder, incense, medicinal |
|
9 |
Cordia africana |
Food, building, shade, bee forage, |
|
10 |
Croton macrostachys |
Firewood, building, tools, drum |
|
11 |
Diospyros mespiliformis |
Fodder, firewood, building |
|
12 |
Diospyros abyssinica |
Firewood, charcoal, timber |
|
13 |
Dobera glabra |
Firewood, timber, food, fodder |
|
14 |
Entada abyssinica |
Firewood, timber, fodder, medicine |
|
15 |
Erythrina abyssinica |
Firewood, soil conservation, nitrogen fixation |
|
16 |
Ficus sycomorus |
Food, bee hive, shade |
|
17 |
Ficus vasta |
Food, bee hive, shade |
|
18 |
Juniperus procera |
Pole, timber, firewood, medicinal |
|
15 |
Kigelia africana |
Firewood, timber, fodder, bee forage, dye |
|
19 |
Maesa lanceolata |
Firewood, medicine, live fence |
|
20 |
Maerus crassifolia |
Firewood, |
|
21 |
Mimusops kummel |
firewood, fodder, for tool handle |
|
22 |
Nuxia congesta |
food, firewood, furniture |
|
23 |
Oncoba spinosa |
Firewood, timber, medicine, fodder |
|
24 |
Ozoroa insignis |
Firewood, timber, medicine, fodder |
|
25 |
Pappea capensis |
Timber, medicine, gum |
|
26 |
Piliostigma thonningii |
Firewood, timber, fodder |
|
27 |
Prunus africana |
Firewood, timber, medicine, fodder |
|
28 |
Rhus abyssinica |
food, firewood, construction |
|
29 |
Rhus natalensis |
food, firewood, construction |
|
30 |
Syzygium guineense |
firewood, construction wood |
|
31 |
Securidaca longipedunculata |
Poles, medicine, bee forage |
|
32 |
Tamarindus indica |
firewood, food, furniture, mortar |
|
33 |
Ximenia americana |
food, fodder, firewood, oil |
Source: Report of the Ministry of Agriculture (1997).* Due to limited information, the extinct and endangered herbs as well as the extinct tree species are not included in the above table.
