Walia Special Edition on the Bale Mountains (2011) - Zoologische ...

There might be some evidence for the existence of such stream in the studied section
of the Harenna escarpment. First, is the presence of a potential source of highly acidic litter at
high altitudes, such as the Erica arborea forest belt (~3000 m a.s.l.), or of decaying roots from
the grasslands in the plateau (~4000 m a.s.l.). Second, is the report from local inhabitants of the
existence of a lake called locally “black lake” (haro gurratti) and of a river called “black river” (laga
gurratti), although their position is unclear and it is not known if they are so named for the colour
of their waters of for other geographical of historical reasons. Third, are the observed properties of
the water springing from the Tabala Sof Omar hot spring, nearby to S-1850 (~1900 m a.s.l.). Here,
there is evident flocculation of black material, phenomenon which might be analogue to the one
described by Chauvel et al. (1996) in reference to the “black mud” observed in the Rio Negro basin.
The fourth is in the black coatings and stratification patterns found in the 2m soil profile in Arrawa
(S1, 2390 m a.s.l.).
Although the parent material in the Harenna forest is predominantly trachitic and basaltic,
hence basic, several granitoids outcrops have been observed in areas of reduced vegetation. Presence
of acidic bedrock could certainly contribute locally to water oligotrophy and also explain the
occurrence of glades, or patches of reduced vegetation density, on hills and slopes as it was found in
Totani glade.
Finally, there is conspicuous hydrothermal activity in the Harenna forest, resulting in one
hot-spring and several mineral springs which properties vary greatly. More detailed knowledge on
the properties of this heterogeneous hydrological basin are needed in order to explain correctly how
it affects vegetation.
Vegetation in the glades varies with altitudinal position but also with grazing intensity.
Permanent grazing affects species composition by reducing both the number of species and the
abundance of herbs, though local perceptions are the process is reversible, at least in terracing
glades (S-2300). The establishment of (traditional) permanent settlements with permanent grazing
is favoured in poorly drained sites due to low abundance of fossorial mammals, where there is less
tunnelling and bioturbation, resulting in less superficial mud.
Human use of the glades has varied over the past century but the glades continue to be
an important grazing resource for permanent and temporary settlers in the forest. The spatial
organization of settlements is clearly shaped by the landscape, with glades playing a central role as
common grazing land.
In the Harenna, there is a network of settlements (such as Ogate, Addeye, Gabicho) established
alongside the archipelago of glades at lower altitudes, with the village/glade of Hawo and Angetu
(SW) and Delo-Mena (SE) as important socio-economic reference points. The strength of the social
network represented amongst the settlements is indicated by the presence of infrastructures (e.g.,
suspended bridge) and regional markets.
A similar network of settlements was reported to exist in glades at mid and higher altitudes in
the Harenna before the villagization (30 y bp). Historical religious and communal infrastructures can
be found in the vicinity, such as a circular stone wall surrounding the Tabala Sof Omar hot-spring.
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 141