Saturday, June 25, 2011

Climate and vegetation of Air Mountains

Because of its altitude (on average between 500 and 900 m) and despite its low rainfall (50 to 160 mm/year on the lower plateau), the Aïr forms a green region in comparison with the surrounding deserts, especially after the August-September seasonal rains. The climate is classified as Sahel, like that of the regions well to its south. While the mountains are largely bare of vegetation, the dry wadi river valleys (known by the Hausa term "Kori") channel and hold rainwater in gueltas (stone pools, such as that near the town of Timia), creating oases which provide forage for animals, and in some areas, farming. The high Bagzane plateau of the central Aïr in particular provides adequate rainfall for intensive agriculture. Other, vast, areas of the region are entirely devoid of plant life and with their volcanic protrusions and rock fields present an otherworldly appearance.

More than 430 vascular species has been recorded so far in the Aïr mountains. The location of the Aïr as a southern extension of the Hoggar mountain range makes it a connection between the Saharan Flora and the Sahelian Flora. However, the presence of mountains up to 2000 m a.s.l. generates locally favourable conditions for several species of the Sudanian zone and the Mediterranean zone.

During the 20th Century a series of scientific missions in the Aïr has permitted to identify the majority of plant species developing in the Aïr. Acacia tortilis subsp. raddiana (Afagag) and Balanites aegyptiaca (Aborak) are among the most frequent tree species in the intermountain zone. In the vicinity of temporary rivers named koris, species like Acacia nilotica, Faidherbia albida and the palm Hyphaene thebaica coexist with planted date palms Phoenix dactylifera. Severe droughts and high aridity have made the intermountain zone of the Aïr a particularly harsh place for plants to develop. The additional presence of domestic herbivores has led to a severe deficit in tree regeneration, which has been cited as a major ecological concern. Tree regeneration has been observed enhanced as soon as tree seedlings are protected by large tussocks of the frequent grass Panicum turgidum. This positive interaction between plants represents a promising restoration tool to be used by local inhabitants.

In comparison, mountainous areas are even less documented. Tropical tree species less resistant to drought have been described in the highlands, among which the Fabaceae Acacia laeta and Acacia seyal. Quezel has observed the remnant presence of a rare endemic taxa related to the Olive tree in the Northern sector of the Aïr range. Recently, this taxa, Olea europaea subsp. laperrinei, has been found in other mountains of the Aïr: these very isolated, small populations represent the Southern limit of the species distribution. A study led on the slopes of the highest summit in the Aïr, Idoukal’N’Taghes (2022 m a.s.l.), identified plant species that had never been inventoried in Niger before. Among them, Pachycymbium decaisneanum, Cleome aculeata, Echinops mildbraedii and Indigofera nummularia are tropical species with relatively low resistance to water stress, whereas Silene lynesii, Tephrosia elegans, and Echinops mildbraedii have a Saharan-Mediterranean distribution. Interestingly, three ferns were found for the first time in the Aïr recently, Cheilanthes coriacea, Actiniopteris radiata, and Ophioglossum polyphyllum, suggesting that ferns may be more prone to develop in arid environments than commonly proposed. All these data evidence a marked mountain climatic specificity in the Aïr, with a positive impact on species richness and species diversity. Because of their strong geographic isolation within a Saharan matrix, these species have a high conservation value


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