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This study analyzed the ecological factors influencing soil degradation in the Atacora Mountains in northern Benin, which harbor two endemic species,Thunbergia atacorensis and Ipomoea beninensis. Data were collected along line transects from plain to summit within 22 plots of 30 m 330 m. Indicators of physical soil degradation (extent of organic layer, color of topsoil, compactness of soil, presence and extent of rills, and occurrence of sheet erosion) and factors (canopy and ground cover, topography, occurrence of flooding, and slope) were assessed. Cluster analysis identified 4 soil degradation classes: light, moderate, high, and extreme. Discriminant and multivariate variance analyses identified canopy and ground cover as the 2 main ecological drivers of soil degradation. Plant, litter, and stone cover were found to decrease as soil degradation increased. The parts of the Atacora Mountains with high elevation and steep slope were found to be less degraded than areas with low slopes, which are easily accessible for human activities. Policies to mitigate soil degradation should prioritize practices with low impact on vegetation cover and promote soil protection practices such as tree planting and mulching.
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Ecological Factors Influencing Physical Soil Degradation in the Atacora Mountain
Chain in Benin, West Africa
Author(s): Farris A. Y. Okou, Achille E. Assogbadjo, Yvonne Bachmann, and Brice Sinsin
Source: Mountain Research and Development, 34(2):157-166. 2014.
Published By: International Mountain Society
DOI: http://dx.doi.org/10.1659/MRD-JOURNAL-D-13-00030.1
URL: http://www.bioone.org/doi/full/10.1659/MRD-JOURNAL-D-13-00030.1
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Ecological Factors Influencing Physical Soil
Degradation in the Atacora Mountain Chain in
Benin, West Africa
Farris A. Y. Okou
1
*, Achille E. Assogbadjo
1
, Yvonne Bachmann
2
, and Brice Sinsin
1
* Corresponding author: farrisy@yahoo.fr
1
Laboratory of Applied Ecology, Faculty of Agronomic Sciences, University of Abomey-Calavi, 01 BP 526, Cotonou, Benin
2
Institute of Ecology, Evolution and Diversity, J. W. Goethe University, Max-von-Laue-Straße 13, 60438 Frankfurt am Main, Germany
Open access article: please credit the authors and the full source.
This study analyzed the
ecological factors
influencing soil
degradation in the Atacora
Mountains in northern
Benin, which harbor two
endemic species,
Thunbergia atacorensis
and Ipomoea beninensis.
Data were collected along
line transects from plain to summit within 22 plots of 30 m 3
30 m. Indicators of physical soil degradation (extent of organic
layer, color of topsoil, compactness of soil, presence and
extent of rills, and occurrence of sheet erosion) and
environmental factors (canopy and ground cover, topography,
occurrence of flooding, and slope) were assessed. Cluster
analysis identified 4 soil degradation classes: light, moderate,
high, and extreme. Discriminant and multivariate variance
analyses identified canopy and ground cover as the 2 main
ecological drivers of soil degradation. Plant, litter, and stone
cover were found to decrease as soil degradation increased.
The parts of the Atacora Mountains with high elevation and
steep slope were found to be less degraded than areas with low
slopes, which are easily accessible for human activities.
Policies to mitigate soil degradation should prioritize practices
with low impact on vegetation cover and promote soil protection
practices such as tree planting and mulching.
Keywords: Soil degradation; degradation classes; ecological
factors; Atacora Mountains; Republic of Benin; West Africa.
Peer-reviewed: February 2014 Accepted: April 2014
Introduction
Soil degradation, recognized as a major process of land
degradation (Stocking and Murnaghan 2001), can be
physical, chemical, or biological and includes erosion
and loss of fertility (Snakin et al 1996). Erosive processes
transform soils into a mosaic of land surfaces (Chartier
and Rostagno 2006). According to Pickup (1985), land
surfaces represent different states of soil loss or soil gain,
each characterized by particular vegetation types and
soil characteristics. Soil degradation is a worldwide
threat that is increasing in extent and severity (Adams
and Eswaran 2000; Bai et al 2008; Kapalanga 2008). It is
seen as the outcome of the complex interplay of both
natural processes and socioeconomic factors
(particularly land use) (Blaikie and Brookfield 1987;
Stocking and Murnaghan 2001; Reynolds and Stafford
Smith 2002; Boardmana et al 2003; Geist and Lambin
2004; Foley et al 2005; Reynolds et al 2007; Haines-Young
2009).
The severity of soil degradation depends on the
sensitivity of individual elements in the ecosystem to
anthropogenic and natural disturbances. The design of
decision support systems to reverse land and soil
degradation requires, above all, sound assessments of
these systemic interdependencies (Kapalanga 2008).
Efforts must be based on a profound understanding of
soil degradation—its causes, risks, impacts, and severity,
their relationships with socioeconomic factors, and the
key ecological factors affected. However, soil degradation
processes are difficult to grasp in their totality. One of the
most common assessment methods at the field or farm
level is the use of indicators of soil degradation combined
with a classification of soils based on their erodibility
(Stocking and Murnaghan 2001, Kapalanga 2008).
Comparisons of soil classes can provide a good
understanding of ongoing processes and the factors
influencing them (Berry et al 2003; Majule 2003; De Bie
2005).
An important mountain chain in West Africa extends
from Ghana (Akwakpim Hills) to Togo (Mount Togo) and
Republic of Benin (Atacora Mountains). In Benin, the
chain reaches altitudes of 400–600 m; it is of a high
socioeconomic importance (Avohou and Sinsin 2009) and
ecological specificity, with the occurrence of 2 endemic
Beninese plant species, Thunbergia atacorensis
(Acanthaceae) and Ipomoea beninensis (Convolvulaceae)
(Akoe` gninou and Lisowski 2004). The Beninese part of
the Atacora mountain chain is undergoing soil
degradation (Adegbidi et al 1999; Mulder 2000). Soil
nutrients are lost due to, among other causes, the
hilly relief, with steep slopes.
MountainResearch
Systems knowledge
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An international, peer-reviewed open access journal
published by the International Mountain Society (IMS)
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Mountain Research and Development Vol 34 No 2 May 2014: 157–166 http://dx.doi.org/10.1659/MRD-JOURNAL-D-13-00030.1 ß2014 by the authors157
There have been few studies of land degradation in the
Atacora Mountains. Saı
¨dou et al (2004) investigated
farmers’ perceptions of the causes and consequences of
land degradation at the field level and found that farmers
assessed the soil fertility on the basis of dicotyledonous
weeds, soil texture, soil color, and fauna (earthworm
casts). Tente and Sinsin (2005) found that the loss of soil
in the Atacora Mountains is more severe upslope than
downslope and greater in ploughed areas. However, much
remains to be learned about soil degradation processes in
the Atacora mountain chain, and this knowledge is
needed for effective monitoring and management. The
aim of this study is to contribute to that knowledge by (1)
analyzing physical soil degradation processes in the
Atacora Mountains using indicators and a classification
approach and (2) highlighting the main ecological factors
influencing these processes.
Study area
The study area covered the district of Toucountouna
(10u219360Nto10u44934.80N; 1u129360Eto1u38927.60E).
Four main ethnic groups dwell in the Atacora range: the
Be´tamaribe´, Wama, Natemba, and Gourmantse´ (Wala
2005). The four ethnic groups mingle, and all of them
carry out activities that both affect and depend on the
mountains, primarily farming and animal breeding, but
also fishing, hunting, and small industry including stone
crushing (INSAE 2002).
The Atacora mountain chain (Figure 1) is located in
the Sudanian West African climate zone, which is
characterized by a mean annual precipitation range of
800–1000 mm and two seasons (a dry season from
November to March and a rainy season from April to
October). However, the plains areas surrounding the
mountain chain, under the orographic influence of
mountains, show a higher annual rainfall of up to
1300 mm per year instead of the range of 800–1000 mm
observed in the common Sudanian zone (Houndenou and
Hernandez 1998; Heinrich and Moldenhauer 2002).
Average temperature and relative humidity during the
last 35 years have ranged between 25.3uC and 30.5uC and
between 26.8% (dry season) and 80.5% (wet season),
respectively. The major characteristic of the topography
is steep (30–60%) slopes, but there are also hilltops,
plateaus, and valleys. In general, hills face east or west.
East-facing hillsides are mainly exposed to the Harmattan,
a dry northeast wind from the Sahara Desert blowing
during the dry season (Jenik and Hall 1966). West-facing
hillsides are exposed to a moist monsoon from the
Atlantic Ocean (Le Barbe et al 2002).
Rocky and shallow soils are dominant throughout the
mountain chain. Sandy and clayey soils with moderate
stone content are found in seasonally wet or inundated
valleys. The combination of ecological factors explains
the diversity of vegetation patterns, consisting of shrubs,
trees, and woodland savannas dominated by Isoberlinia
doka,Daniellia oliveri,Vitex spp., Terminalia glaucescens, and
Parinari polyandra (Sieglstetter and Wittig 2002; Tente and
Sinsin 2002; Wala 2005).
Material and methods
Data collection
Two sampling sites were identified, with the help of
extension service agents and local residents. The sites
met the following criteria: ferricrete areas close to
hillsides, and accessibility during the rainy season.
Transect lines for data collection were identified based
on local knowledge about soil degradation. Within each
site, transect lines were established from plain to summit,
aiming to cover the ecological diversity of the sites. At
each topographical position (plain, downslope, hillside,
and summit) along the transect line, we installed 5 or 6
nested sample plots on the grass, shrub, and tree savanna
encountered (30 m 330 m for the woody layer and 10 m
310 m for the herbaceous layer). The vegetation
structure (number of layers, their canopy, and basal
cover) and the ground cover (litter and stone cover) were
estimated using Braun-Blanquet’s (1932) cover/
abundance scale: +5rare (less than 1%), 1 51–5%, 2 5
5–25%, 3 525–50%, 4 550–75%, and 5 575–100%.
Canopy cover is the area of ground covered by the
vertical projection of the outermost perimeter of the
natural spread of foliage of plants; basal cover is the area
of ground surface occupied by the basal portion of the
plants; ground cover is the cover litter, rocks, and gravel
on a site. In total, 22 plots were sampled. Heights of
herbaceous and shrub layers were assessed with a
decameter, and we used a clinometer for the height of
the tree layer. Visual indicators of erosion were
estimated and coded as follows:
NExtent of organic layer was assessed by vertically dropping
a metal rod on the ground and noting what kind of soil
layer (organic layer or other) it touched (Daget and
Poissonnet 1971). The rod was dropped every meter in
8 directions, until a distance of 15 m (for 4 half-median
directions) and 21 m (for 4 half-diagonal directions)
from the center of the plot was reached (Figure 2).
NColor of topsoil layer was assessed visually and coded as
15red (which indicates a mineral layer of soil) or
25black (which indicates an organic layer of soil).
NCompactness of topsoil was assessed visually and coded as
follows: 1 5loose, 2 5compact, 3 5very compact, and
45hard.
NSheet erosion was assessed visually based on the presence
of sand sedimentation and coded as 1 5not visible or
25visible.
NPresence and extent of rills were visually assessed and
coded using the Braun-Blanquet (1932) cover/abun-
dance scale as described above.
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FIGURE 1 Extent of the Atacora mountain chain in Benin and location of the sampling plots in the district of Toucountouna.
(Map by authors)
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Ecological factors were assessed and coded as follows:
NTopography was assessed visually and coded as
15valley, 2 5plain, 3 5downslope, 4 5hillside,
and 5 5plateau or summit.
NFlooding during the rainy season was assessed visually
and based on local knowledge of the site status during
dry season. It was coded as 1 5dry or 2 5flooded.
NSlope angle was assessed with a clinometer and recorded
as number of degrees.
NCanopy cover and ground cover factors were derived from
the height and cover of each vegetation layer estimated
using Braun-Blanquet’s (1932) cover/abundance scale.
Data analysis
Data on 5 indicators of erosion from 22 plots were
submitted to a hierarchical cluster analysis with Ward’s
linkage method, using the software SAS 9.2 (SAS 2009) to
produce clusters of plots based on the degree of soil
degradation. Four clusters or soil degradation classes
were defined.
Next, a discriminant analysis was carried out to
determine whether and how ecological factors driving soil
degradation varied along the gradient of degradation.
The factors addressed in the analysis were topography,
flooding, slope, canopy cover factor, and ground cover
factor.
In order to identify the main ecological drivers of
soil degradation, a multivariate analysis of variance
(MANOVA) was performed. The average of measured
ecological drivers was assessed according to the
degradation stage (Azihou 2008; Azarnivand et al 2010).
As variables, canopy cover factor, ground cover factor,
and slope were used (topography and flooding were not
included in the analysis because they were qualitative
variables).
The following sections explain how canopy cover
factor and ground cover factor were computed. It is
important to know that lower values of these factors
indicate good canopy and ground cover, which are
associated with lower soil loss (De Bie 2005).
Canopy cover factor: A canopy cover protects bare soil
against erosion, but the foliage protection decreases in
the direction of higher layers in the canopy (Wischmeier
and Smith 1978; Kooiman 1987). Canopy cover factor
quantifies the degree of protection offered by the canopy
of a vegetation layer. Canopy cover factor is computed for
each vegetation layer as
CCf,x~S1{½1{(37:13H{½½4:02|EH 2
z½½0:146|EH3)=100|ECT,
ð1Þ
where EC is the effective canopy cover (%), and EH is the
effective canopy height (m) (Wischmeier and Smith 1978;
Kooiman 1987; Palmer 1989; Figure 3).
The EC of the vegetation layer could be defined as the
part of the remaining bare soil not covered by the canopy
of vegetation layer below and that which is covered by the
FIGURE 2 Sampling pattern for assessing the extent of the organic layer.
FIGURE 3 Canopy cover factor assessment results. The plain lines are the
representation of Equation 1. They could be used for determining the canopy
cover factor without computation of Equation 1. The dotted lines show the steps
to be followed to determine the canopy cover factor based on EC and EH.
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canopy of the vegetation layer concerned. Assuming a
random distribution of basal cover and a nonrandom
positioning of successive canopy layers, effective canopy
cover was computed according to De Bie (2005) as:
1. Canopy cover of herbaceous layer (%) 2basal cover
(%)
2. Canopy cover of shrub layer (%) 3(1 2canopy cover
of herbaceous layer [%])
3. Canopy cover of tree layer 3(1 2canopy cover of
shrub layer [%]) 3(1 2canopy cover of herbaceous
layer [%]).
EH could be defined as the average distance traveled
by a raindrop from intercept by a plant before falling
from this plant (De Bie 2005; Jakubı´kova
´et al 2006;
Figure 4). It was computed as
EH~2=3(Ht{H1), ð2Þ
where H
t
5total height, and H
l
5height of the lower layer.
The canopy cover factor of one plot (C
Cf
) was
computed by summing all layer-specific C
Cf,x
as follows:
(C
Cf,1
+C
Cf,2
++C
Cf,X
)2(X21), where Xis the number
of layers.
Ground cover factor: A high ground cover fraction (litter,
basal cover, and stones) provides a protection against
eroding rainfall and reduces runoff velocity while
increasing infiltration (Wischmeier and Smith 1978). As
shown in Figure 5, canopy cover factor decreases sharply
as the ground cover increases. Ground cover factor (C
Gf
)
values were computed for each plot according to the
ground surface roughness as
CGf ~½exp{(b|ground cover(%),ð3Þ
where bis ground surface roughness, with a value of 0.05
recorded for rough field surfaces and 0.04 for smooth
field surfaces (De Bie 2005). Ground cover percentage was
computed as the sum of the percentages of litter, basal
cover, and stones (De Bie 2005).
Results
Soil degradation classes
Cluster analysis of the 22 study plots, based on the severity
of the erosion indicators, resulted in 4 clusters. An R-
squared value of 96.8% was sufficient to obtain distinct
clusters.
Cluster 1 was composed of 10 plots. They were
characterized by a low level of soil compaction. Soil
compaction limits water infiltration (few rills, no visible
sheet erosion). On the topsoils of these plots, a black
organic layer covered the entire surface; no reddish soils
were observed. Based on these characteristics, these soils
were characterized as having a light degree of erosion (to
allow for the possibility of other types of physical soil
degradation not observed during this study).
Cluster 2 contained 6 plots showing a great
disturbance of the soil structure. The topsoil of these
plots was ferricrete, rich in iron, and hard (Duchaufour
1952). The organic layer remained only on small patches
(around 40% of rod contacts). Exposure of red soils was
obvious. The presence of this ferricrete layer reduced the
depth to which roots could grow. The thickness of the
organic layer rarely exceeded 10 cm. There was no visible
evidence of sheet erosion, and the mean rill cover was
very low (0.5%) because of the high level of compaction.
Due to the degree of surface sealing, the level of
degradation assigned to this cluster of plots was extreme.
FIGURE 4 Determination of the effective canopy height of a tree. (Based on
Jakubı
´kova
´et al 2006)
FIGURE 5 Ground cover factor assessment results (bis ground surface
roughness). The plain lines are the representation of Equation 3. They could be
used for determining the ground cover factor without computati on of Equation 3.
The dotted lines show the steps to be followed to determine the ground cover
factor based on the ground cover value.
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Clusters 3 and 4 contained 3 plots each. The plots of
cluster 3 were characterized by compact red soils, with a
thin clay crust on the surface. Sheet erosion occurred on
these soils, and rills covered on average 7% of the surface.
Organic layers remained as thin patches (around 48% of
rod contacts). The soils of cluster 4 were red and very
compact. They looked like ferricrete but remained
friable. Sheet erosion occurred here too. Rills covered a
larger surface than on the other soils (around 45%),
whereas the thin patches (never more than 5 cm thick) of
organic layer were less extended (only 23% of rod
contacts). The degree of degradation of cluster 3 was
considered moderate, and that of cluster 4 was high.
Table 1 summarizes the erosion indicator values for
each plot cluster.
Ecological drivers of soil degradation
The main result of discriminant analysis was the
projection of soil degradation classes in the canonical
system axes based on ecological factors. Discriminant
analysis summarizes all the information relative to the
ecological drivers of degradation on many axes. In our
case, the first 2 axes were significant and explained
97.12% of the initial information relative to the
ecological drivers of degradation for each plot.
Correlation between ecological drivers and axes 1 and 2
(Table 2) showed that the canopy cover (0.87) and the
ground cover (0.99) were highly positively correlated with
the first axis. Slope (20.90) and topography (20.93) were
highly negatively correlated with the first axis. The only
occurrence of flooding was positively correlated with the
second axis (0.75). In other words, axis 1 corresponded to
the axis of high values of canopy and ground cover
factors, low slope values, and low elevation values. Axis 2
represented the gradient of occurrence of flooding.
Extremely, highly, and moderately degraded soils were
highly positively correlated with axis 1, while lightly
degraded soils were highly negatively correlated with the
same axis (Figure 6). Information on axis 1 indicated that
extremely, highly, and moderately degraded soils were
found on low slopes at low elevations. They were
characterized by high values for C
Cf
and C
Gf
, which
correspond to poor soil protection and high soil loss.
Lightly degraded soils were found on steep slopes at high
elevations. They were characterized by dense canopy
cover and extensive ground cover.
Extremely degraded soils were highly positively
correlated with axis 2. However, highly, moderately, and
lightly degraded soils were negatively correlated with axis
2. Based on the ecological factors explained by the axis 2,
we concluded that extremely degraded soils were often
flooded during the rainy season, while lightly, highly, and
moderately degraded soils were not.
Table 3 summarizes the ecological characteristics of
the 4 degradation classes represented by the plot clusters.
An analysis of variance was conducted between the mean
values of some studied factors to find the reason for the
differences in severity of soil degradation. The results
provided by an F-test were significant (p,0.01) for all of
the studied factors except slope. Canopy cover and
ground cover factors were the 2 main ecological drivers of
soil degradation.
Discussion
This study identified 4 soil degradation classes in the
Atacora mountain chain in northern Benin. They
represent the different steps of forest soil degradation
identified by Duchaufour (1952). According to
Duchaufour, 2 factors—organic layer removal due to
erosion, and hardening and desiccation due to solar
radiation—explain all of the differences observed along
TABLE 1 Erosion indicator values for the 4 plot clusters.
Clusters
Erosion indicator
Cluster 1 Cluster 2 Cluster 3 Cluster 4
Mean SD Mean SD Mean SD Mean SD
Extent of organic layer (%
%)100.00 0 40.42 10.6 48.89 2.93 23.61 10.48
Presence of rills (%
%of area) 1.95 4.59 0.50 0 7.00 6.92 45.83 14.43
Color of topsoil Black Red Red Red
Compactness of soil Loose Hard Compact Very compact
Occurrence of sheet
erosion
Not visible Not visible Visible Visible
TABLE 2 Correlation between ecological factors and the 2 axes.
Axis 1 Axis 2
Canopy cover factor 0.87 20.14
Ground cover factor 0.99 0.02
Slope angle (%) 20.90 20.05
Topography 20.93 0.26
Flooding during rainy season 0.65 0.75
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the gradient of tropical forest soil degradation. The 4 soil
types identified were:
1. Lightly degraded soils: These are used as control plots
for the observation of degradation processes because
they present no obvious signs of soil degradation.
2. Moderately degraded soils: These appear when ero-
sion, due to anthropogenic and ecological drivers,
removes the organic layer, exposing the red layer. This
layer is more compact and often forms a thin layer of
curled plates called still depositional crust (Valentin
and Bresson 1992).
3. Highly degraded soils or lateritic carapace: These
occur when the red layer is hardened by desiccation.
Great variations in temperature (17.4–37.1uC) and
relative humidity (23–82%) in the study area facilitate
the desiccation process.
4. A hard surface called ferricrete occurs when iron and
aluminum oxides are dissolved during the rainy season
and migrate to the surface during the dry season
(Duchaufour 1952; Bockheim and Gennadiyev 2009;
Yuangen et al 2009). Compaction and surface sealing
reduce infiltration and hydraulic conductivity, which
increases surface water runoff (Frisby and Pfost 1993;
Horton et al 1994; Radcliffe and Rasmussen 2000) and
lengthens runoff periods (Hinckley et al 1983). The
presence of water on extremely degraded soils during
the rainy season can be explained by soil surface
sealing. According to Bilong et al (1992), the erosive
force of water could break up ferricrete into loose
soil and then facilitate regeneration/reversal of the
soil degradation process and recolonization by
vegetation.
This study often found lightly degraded soils at high
elevations with high slope angles. Similar results were
found by Majule (2003) on the slopes of Mount
Kilimanjaro, where soils in the lower zones were more
FIGURE 6 Projection of soil degradation classes in the system axes based on ecological factors.
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degraded (in physical and chemical terms) than soils in the
middle and upper zones. This is an interesting result, as
slope and elevation have also been recognized as increasing
runoff velocity and therefore posing a greater erosion risk
(Stocking and Murnaghan 2001; Saı
¨dou et al 2004; Tente
and Sinsin 2005; Inkpen and Stephenson 2006). Bonan
(2002) and Molinar et al (2001), on the other hand, found
that slope can act as a buffer, depending on aspect and
initial stability of the soil. The presence of an organic layer
rich in humus improves the soil structure and enhances the
ability of the soil to hold moisture (Pidwirny and Jones
2010), and the lightly degraded soils of the study area are
characterized by extensive coverage of organic layer. This
explains the low level of degradation of these areas despite
the fact that they are found at high elevations with high
slope angles.
The increase of canopy and ground cover factors
indicates the decrease in plant, litter, and stone cover as
soil degradation worsens. Chapman (1948) established
that vegetation canopies change the drop size and
distribution of rain and that splash detachment under
dense canopy is different from that under sparse canopy.
Plant, litter, and stone cover protect the soil from
raindrop impact. Splash tends to slow the movement of
surface runoff and increase the infiltration rate and water-
holding capacity of the soil (Molinar et al 2001; Bonan
2002). According to Bonan (2002), the erosion factor used
to estimate total erosionlosses in forests drops from 0.36 in
soil with no litter cover to 0.003 with 100% litter cover.
Chartier and Rostagno (2006) found that the rate of soil
erosion increased in northeastern Patagonian rangelands
when plant and litter cover decreased. They identified 90%
plant and litter cover as a site conservation/soil erosion
threshold. We found that in the Atacora Mountains, C
Cf
and C
Gf
values above 0.5 indicated degraded soils or soils
with high risk of erosion. Plant and litter also reduce soil
evapotranspiration rates and reduce soil insolation
(Molinar et al 2001; Kohutiar 2008).
We examined only ecological drivers because they
are the key elements of an environment threatened by
anthropogenic activities and other natural processes
that lead to soil degradation. The identification of the
most sensitive ecological factors will help decision
makers to identify activities that have the greatest
impact on these factors, thus contributing most to soil
degradation. For instance, we found that plant, litter,
and stone cover, if removed, are main ecological drivers
of soil degradation processes. Activities such as shifting
cultivation, overgrazing, and lopping of trees for fodder
during the dry season, which remove the vegetation or
vegetation cover and leave the soil bare, are threatening
activities. Moreover, the local population should be
trained to identify the different soil degradation classes
and their characteristics in order to enhance their
capacity to monitor soil degradation closely on their
own.
Elevation and slope angle can also be seen as
protecting soil because the higher and steeper the site,
the more difficult it is for human beings to reach it
and disturb it by their activities. Nevertheless, the
TABLE 3 Ecological characteristics of the 4 degradation classes represented by the plot clusters.
Ecological
factors
Degree of degradation
Light Moderate High Extreme
FvalueMean SD Mean SD Mean SD Mean SD
Canopy
cover
factor
0.49 0.09 0.51 0.03 0.72 0.09 0.61 0.10 8.98 ***
Ground
cover
factor
0.09 0.05 0.45 0.03 0.73 0.09 0.71 0.08 142.02 ***
Slope angle
(%
%)
3.85 2.50 0.80 0.29 2 0 0.75 0.27 4.96 ns
Topography Plain,
downslope,
hillside,
plateau, or
summit
Valley or plain Valley or plain Plain
Flooding
during rainy
season
Dry Dry Dry Dry or flooded
*** P#0.001; ns 5no significant difference.
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Mountain Research and Development http://dx.doi.org/10.1659/MRD-JOURNAL-D-13-00030.1164
protection of steep hillsides and summits should be
prioritized in management policies by tree planting
and other management practices based on local
knowledge.
Conclusion
Physical soil degradation processes in the Atacora
Mountains are very complex. This study examined various
indicators of land degradation at the field level in
northern Benin and found that:
NSoils of the Atacora mountain chain can be classified
in 4 soil degradation categories (light, moderate, high,
and extreme).
NSoils at high elevations with high slope angles were less
degraded than those in lower zones.
NPlant, litter, and stone cover are the main ecological
factors affecting soil degradation; they protect the soil,
and their decrease leads to an increase in degradation.
For management purposes, plant, litter, and stone
cover are indicators that could be used for monitoring of
land degradation. Policies to mitigate soil degradation
have to prioritize practices with low impact on vegetation
cover and promote soil protection practices such as tree
planting and mulching. Large-scale studies integrating
remote-sensing data and geographic information systems
are needed to investigate in detail the erosion risk status
of the entire mountain chain.
ACKNOWLEDGMENTS
The authors are indebted to the UNDESERT project (EU FP7 243906)
‘‘Understanding and Combating Desertification to Mitigate Its Impact on
Ecosystem Services,’’ funded by the European Commission, Directorate
General for Research and Innovation, Environment Program, for financial
support. Our acknowledgment also goes to Amanda Morgan, An ne
Zimmermann, Belarmain Fandohan, Romain Gle
`le
`Kakaı
¨, and two anonymous
reviewers, whose useful comments helped improve the quality of this
document.
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Virtually every element of the Earth’s landscape is to some extent the product of erosion. Denudation and sedimentation are inevitable geologic phenomena that man may regard as either benevolent or pernicious depending on the place and time. Erosion of upstream fertile lands, for example, leads to enrichment of the prolific Nile and Mesopotamian floodplains, nuturing the first agriculturalists (Moss and Walker, 1978). From another point of view, modern agricultural researchers commonly estimate that only 0.1 to 0.8 mm year−1 of soil removal can be sustained indefinitely without loss of productivity on croplands (U.S. Department of Agriculture, 1975). Yet erosion of thin topsoils, rapid sedimentation of streams and lakes, and gullying of landscapes are in some areas esthetic and economic problems of massive proportions. Figure 5–1, for example, illustrates a Mojave Desert hill slope where erosion has become a serious problem following slope modifications by vehicles. The problem, however, is not erosion per se, but rather erosion rates, and the adjective “accelerated” is commonly applied to erosion rates considered to be significantly greater than “natural.”
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In this paper, the literature is reviewed in order to present data and relationships showing the effects of compaction on soil hydraulic properties and water flow, as well as numerically modeled water flow for some management systems. The objectives are to: review some fundamental principles of water retention and transmission; highlight observations of soil hydraulic properties and hydrologic components for various compactive and tillage situations; report results of modeling efforts for predicting the hydrology of soils; and suggest future research directions. -from Authors