These are found in mountain zones with an annual rainfall of 150 to 350 mm, but also along wadis where a high closed jujube-based (Ziziphus spp.) association is found. The most typical are the forests on Mount Goda (the Day Forest, lying between 1 300 and 1 800 m) and Mount Mabla (whose highest peak is 1 247 m). In both areas, the density of the vegetation and the height of the trees increase with altitude, especially on south- and east-facing slopes, which are exposed to the wind. Differences in forests are thus related both to local natural conditions and the state of degradation of the forest in question.
Closed dry forest, with an average cover of 50%, contains a rich flora and many endemic species. The last specimens of a palm over 30 m in height (Wismania carinensis) are found here (the only other place being Somalia, where it occurs only sporadically). Closed dry forest takes two forms:
- closed dry Terminalia brownii forest is found exclusively on Mounts Goda and Mabla at altitudes between 400 and 900 m, growing to heights of 6 to 10 m, with a patchy dominant storey but a thick undergrowth covering 30 to 60%. It contains a large number of woody species. Buxus hildebrandtii is also plentiful here;
- the mountain forest on Mount Mabla is not as thick as the Day Forest (see the Junipera procera forest below), but more extensive. It contains various associations, depending on altitude; thus, associations of Acacia spirocarpa, A. mellifera, A. glaucophylla, Tamarindus indica, Cordia ovalis, etc., predominate below 800 m. Between 800 and 1 000 m, the forest contains many species in association, including Buxus hildebrandtii, Commiphora africana, Combretum molle and Acacia seyal. Above 1 000 m, Acacia etbaica, Lavandula coronopifolia, Grewia bicolor and Nuxia oppositifolia are found; acacias and jujubes are more numerous in the valleys.
Closed, tall riparian forest is composed primarily of figs (Ficus vasta, F. salicifolia, F. sycomorensis) and jujubes (Ziziphus spina, Z. mucronata, Z. mauritiaca). Mangroves cover a small area in the Obock plain in the northwest of the country (150 m in altitude, with annual rainfall of 100 mm). They form an 80% cover on average, and combine Avicennia marina and Rhizophora mucromata.
Closed dry juniper (Juniperus procera) forests are found between 800 and 1 600 m and represent the structure of the Day Forest prior to degradation.
Juniperus procera is predominant, constituting the high tree storey (15-20 m), associated with box (Nuxus hildebrandtii), constituting the lower tree storey (3-4 m). The juniper crowns are almost touching (giving a cover of 60-80%), and the other main species in the dominant storey are fig (Ficus vasta) and olive (Olea africana). In addition to box, the shrub layer includes Turyops arabicus, Psidia arabica and Solanum incarnum. There is practically no herbaceous cover. About 350 botanical species have been identified in this type of formation.
When it becomes highly degraded as a result of grazing, this forest takes on a much more open aspect (with a cover to 20-25%), with, for example, shrub associations containing camphor (Tarchonanthus camphoratus), which grows to 2 m.
These forests are found on high pebble-covered land (500-800 m), and also in irrigated plains and along large wadis.
Lannia triphylla, Cissus adenocaulis and Tarenna graveoleus associations predominate on high ground. When conditions are damper, various acacias are found (Acacia flava, A. nubica and A. mellifera), in association with other species such as Cadaba longifolia and Gyrocarpus hababensis.
Vegetation composed mainly of Acacia spirocarpa and Tamarix articulata is found in irrigated plains and wadis, as well as Cadaba rotundifolia, Panicum turgidum, Salvadora persica, Acacia asak, etc.
Flood-prone Acacia nilotica forests are usually pure stands 4 to 10 m high with a cover of about 20%.
Other wooded land
These formations constitute the largest part of the country's woody cover, taking various forms depending on local natural conditions and altitude.
The Acacia tortilis and Balanites racemosa association is found on soils of alluvial origin in the sandy zones of the coastal plain. This formation contains two layers, Acacia tortilis reaching over 2 m in height and Balanites racemosa reaching about 60 cm. This type of formation provides a forest cover of 10 to 30%.
Tree savannah is found on basaltic sands and alluvial plains between 100 and 300 m with an average annual rainfall of 150 mm. Acacia arabica, A. flava, A. tortilis and Balanites racemosa are abundant in these formations.
Acacia mellifera tree steppes are found at medium altitudes (250-700 m) with an annual rainfall of 150 mm, and are the most widespread formations in Djibouti. Acacia mellifera is often found in combination with A. tortilis and sometimes with A. asak. This type of formation provides a forest cover of 10 to 30%.
Rhigozum somalense bush steppes are typically found on escarpments and low dry hills (250-500 m). Rhigozum somalense is the dominant species in these stony regions and grows in very pure stands, with a minimal herbaceous layer. They are low formations (1-2 m), providing a maximum of 40% forest cover.
Acacia ortilis and A. asak shrub steppes are found on coastal plains and in low valleys at altitudes between 300 and 500 m.
Acacia etbaica tree steppes are found in the Day Forest. Juniperus procera forest turns into a shrub association with camphor when it becomes degraded. Exploitation for wood and grazing then turn this formation into an open tree steppe where only A. etbaica survives. With further degradation, all that remains is an Aizoon canariense bush steppe.
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