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

Total aboveground biomass ranged from 565 g m -2 in Ordoba (S-1850) to 316 g m -2 in
Arrawa (S-2400), without an association with altitude or grazing regime, however dicot biomass
was positively associated with altitude (0.79, P=0.2, N=8).
Unfortunately, our experimental design does not allow us to separate the factors of grazing
and altitude because all of the glades sampled at the lower altitudes were permanently grazed, and
all of those sampled at higher altitudes were seasonally grazed (Table 3). However, our results
support the hypotheses that species richness is greater under seasonal than under permanent grazing,
and that monocot biomass is greater where grazing is permanent than where it is seasonal.
Vegetation and soil properties
Plant species richness in the glades was negatively associated with pH, total exchangeable bases
and base saturation. Dicot aboveground biomass showed a similar pattern of associations, whereas
total aboveground biomass and monocot biomass did not show any significant correlations with soil
properties. Monocot biomass (g m-2 ) was much greater in the glades with permanent grazing than
those with seasonal grazing (permanent, mean = 160 (SE=30); seasonal , mean = 87 (SE=44)).
Discussion
The results from this study indicate that the soils found in glades are distinct from those in adjacent
forests, being more acidic and having lower base saturation. Some soil properties recorded in the
glades are consistent with a hypothesis of an anthropogenic origin. For example, the dark organic
coatings found deep in the soil profile in Arrawa could indicate historical burning and the removal of
tree vegetation on a freely drained terrain would enhance leaching and loss of cations. Alternately,
historical grazing practices should have reduced water infiltration rates thus decreasing leaching of
cations. Overall, the collective evidence suggests that the origins of the glades examined in this
study are more related to natural biophysical factors than to anthropogenic factors.
Leaching is the mobilization and transportation of soluble soil particles, such as cations and
clays, through vertical or lateral water movement through the soil. Intense leaching will impoverish
the soil of exchangeable bases (TEB), particularly under acidic conditions. Leaching can be
promoted by local topography. Either excessive drainage or waterlogging are likely to be important
to the formation of the glades in the Harenna.
Glades originate on flatter topographic units than the adjacent forest and emerge as terrace
formation at higher altitudes. In this case, a low slope angle would (1) increase the intensity of
vertical leaching and, importantly, (2) cut off the glades’ higher soil profile from lateral supplies of
leachates from upper sites. Overall, this is likely to create heavily leached patches, in accordance
with the results of this study. At lower altitudes, instead, the flat topography results in waterlogging,
due to the more gentle overall slope. It is important to gain further information about the nature
and properties of the ground water, since this affects the vegetation response to poor drainage. The
poorly drained glades studied showed more acidic and nutrient poor soils compared to adjacent
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 139