Journal of Sustainable Development in Africa (Volume 11, No.1, 2009)
Journal of Sustainable Development in Africa (Volume 11, No.1, 2009)
Clarion University of Pennsylvania, Clarion, Pennsylvania
BUSH ENCROACHMENT IN ZIMBABWE:
A PRELIMINARY OBSERVATION ON SOIL PROPERTIES
Department of Soil Science, University of Venda, South Africa
Department of Environmental Science, Bindura University of Science Education, Zimbabwe
Vegetation structure and composition are closely linked to soil type in the savannas. A study was carried
out to compare soil characteristics associated with bush encroachment and those of the adjacent
savanna woodland. The objective of this study was to determine if bush encroachment is related to
particular soil properties. Soil profiles were described and samples collected by major morphologic
horizons. Bulk density and infiltration rates were determined. Samples were analyzed for soil organic
carbon (C), cation exchange capacity (CEC), and total exchangeable bases (TEB). Results show very
little differences in soil properties between bush encroachment and savanna woodland sites. Soil erosion
was, however, more evident on bush encroachment sites compared to the savanna woodlands sites.
From the results of this study we could not conclusively link soil properties to bush encroachment
Keywords: Zimbabwe, bush encroachment, soil survey, savanna woodland, soil properties, vegetation
The phenomenon of increasing woody plant abundance in the savannas, with accompanying changes in
herbaceous cover and composition, is termed bush encroachment (Smit, 2004; Britz and Ward, 2007).
Bush encroachment is accompanied by changes in tree to grass ratio, which is important in semi-arid
savannas (Britz and Ward, 2007). Characteristically, bush encroachment leads to an increase in density
of woody plants, often unpalatable to domestic livestock (Kraaij and Ward, 2006; Wiegand et al.,
2006). The reasons for an increase in the abundance of woody plants in any vegetation type are diverse
and complex (Smit, 2004). Increase in tree cover leads to reduced productivity and profitability of
rangelands (Jacobs, 2000; Smit, 2004; Britz and Ward, 2007). Bush encroachment has been linked to
ecosystem disturbances, such as by fire, overgrazing (Kraaij and Ward, 2006), and soil degradation
(Dougill and Cox, 2007).
Bush encroachment in pasturelands is a distinctive type of secondary vegetation succession, which leads
to an increase in woody biomass of the savanna ecosystem (Bond and van Wilgen, 1996; Scholes and
Archer, 1997; Skowno et al., 1999). The problem of bush encroachment is of global concern because it
lowers carrying capacity and this reduces livestock production (Dean and Macdonald, 1994; Jacobs,
2000; Smit, 2004). In Africa, the main encroaching species are thorn trees (e.g. Acacia karroo, A.
reficiens, A. tortilis, A. mellifera, and Dichrostachys cinera) (Kraaij and Ward, 2006). These species
also tend to have very high levels of phenolic compounds (e.g. tannins) in their leaves, which reduce
their digestibility to livestock and wildlife. The combination of thorniness and low digestibility of
Acacia trees reduces their accessibility and natural value to consumers (Jacobs, 2000).
Understanding the ecology and functions of savanna ecosystems is critically important. Tropical
savannas occupy approximately 65% of Africa, 60% of Australia, and 45% of South America, but still
they are among the least understood terrestrial ecosystems (Huntley and Walker, 1982). Woody species
significantly affect tropical grasslands and savannas in a number of ways. Several hypotheses have been
proposed to explain the causes of bush encroachment but none of them have been found to be
universally applicable (Dougill and Cox, 2007). A two-layer model, called Walter’s two-layer
hypothesis proposed by Walter (1954), cited in Britz and Ward (2007), is mainly used to explain the
bush encroachment phenomenon. According to Walter’s two-layer hypothesis, water is the limiting
factor for both grassy and woody plants. Grasses use only top soil moisture, while woody plants use sub-
soil moisture (Wiegand et al., 2006). In this model the balance between grass and bush production is
determined by the relative availability of soil water and nutrients in different rooting zones (Dougill and
Cox, 2007). Savanna grasses out-compete bush species for water and nutrients in the top soil layers,
while woody species have the competitive advantage in the sub-soil. According to the two-layer model,
cattle’s grazing affects this balance by suppressing grass growth and promoting leaching to greater
depth. This provides the opportunity for woody species to increase and for certain species to encroach
(Britz and Ward, 2007) and, hence, the conclusion that bush encroachment is caused by the replacement
of indigenous browsing animals by cattle and heavy livestock grazing (Skarpe; 1990, Hoffman and
Ashwell, 2001; cited in Britz and Ward, 2007). Recent studies reveal that Walter’s model is no longer
sufficient to explain the bush encroachment process (Ward, 2005; Wiegand et al., 2006; Dougill and
Cox, 2007). Ward (2005) argues that rooting niche separation cannot be an explanation for the initiation
of bush encroachment because young trees use the same subsurface soil layer as grasses in the early
stages of growth. He further states that overgrazing in combination with rooting niche separation is not a
prerequisite for bush encroachment because bush encroachment sometimes occurs on soils too shallow
to allow for root separation. Simulation and field studies have also disputed that rooting-niche separation
is the sole mechanism explaining tree-grass coexistence (Wiegand et al., 2006).
Several new hypotheses have been proposed as a consequence of the inadequacy of the two-layer model
and the hypothesis that overgrazing is the cause of woody plant encroachment. Alternative hypotheses
were proposed by Ward (2005) and Wiegand et al., (2007). In his review, Ward (2005) cited disturbance
models developed by Higgins et al. (2000) that can be used to explain tree-grass coexistence in the
savannas. Higgins et al. (2000) hypothesized that grass-tree coexistence is driven by the limited
opportunities for tree seedlings to escape both drought and the flame zone into the adult stage. By this
hypothesis bush encroachment occurs due to increased tree recruitment caused by reductions in grass
standing crop and, hence, fire intensity. They predict that rainfall-driven variation in recruitment is more
important in arid savannas, where fires are less intense and more frequent. Ward (2005) further cited
another disturbance-based model that focuses on the role of fire (and its interaction with herbivory). The
model by Langevelde et al. (2003) proposed that there is a positive feedback between fuel load (grass
biomass) and fire intensity. Increased levels of grazing reduce fuel load, making fires less intense and,
thus, less damaging to trees. This leads to an increase in woody vegetation and a switch from an open
savanna to woodland. Browsers may enhance the effect of fire on trees because they reduce woody
biomass. Indirectly, grass biomass is increased (assuming that there is strong competition between trees
As a consequence of increased fuel load (because there is more grass), fires are more intense and,
consequently, more biomass is removed by fire. The ecosystem then switches from one dominated by
trees to a mixture of trees and grasses. This model, like any other has its weaknesses (see Ward, 2005).
Wiegand et al. (2006) have hypothesized that bush encroachment in many semi-arid environments is a
natural phenomenon occurring in ecological systems governed by patch-dynamic processes. Wiegand et
al. (2007) hypothesized that any forms of disturbance (e.g. grazing or fire) create space, making water
and nutrients available for tree germination. Under low soil nitrogen conditions, the nitrogen-fixing trees
have a competitive advantage over other plants and, given enough rainfall, may germinate en masse in
these patches created by the disturbances. Ward (2005) argues that in order to understand the causes of
bush encroachment, we need mechanistic models to guide us and multi-factorial experiments in order to
determine the interactions among causal factors.
Topography, soil structural properties, soil moisture, and nutrients all contribute to the tree-grass
dynamics within savanna systems, but the specific factors involved in determining the tree-to-grass ratio
and bush encroachment are not well understood (Ward, 2005; Britz and Ward, 2007). Scholes (1991)
cited in Smit (2004) reported that nutrients, such as nitrates, phosphorus, a series of anions and cations,
and various trace elements, are essential to the nutrition of plants, and acts as determinants of the
composition, structure, and productivity of vegetation. Britz and Ward (2007) reported that soil texture
is a crucial determinant of the tree-to-grass ratio due to its effects on plant growth, soil moisture, nutrient
presence, and availability. Kraaij and Ward (2006) also noted that, in addition to nutrients and moisture,
fire and herbivory play important roles in tree-grass dynamics in the savannas. The relationships
between bush encroachment and soil properties have not been established in Zimbabwe.
This study was formulated as part of a major research project designed to study the ecology of the
Shangani Ranches in Zimbabwe (Mzezewa et al., 2003). The objective of this study was to determine if
bush encroachment is related to particular soil properties. To do this, we determined bulk density and
infiltration rates and analyzed soil organic carbon (C), cation exchange capacity (CEC), and total
exchangeable bases (TEB) on two sites experiencing bush encroachment and compared these properties
from adjacent undisturbed savanna woodlands. The results from this investigation will contribute to a
better overall understanding of the bush encroachment process.
MATERIALS AND METHODS
The study was conducted in the Bulawayo Syndicate Block and Shangani Farm North of the Shangani
Ranches (19˚30’S-20˚00’S, 29˚00’E-29˚30’E), situated about 340 km southwest of Harare, Zimbabwe.
The altitude is about 1340 m asl. Other geographical details of the site are described in Mzezewa et al.,
(2003). Two sites affected by bush encroachment were selected for this study. Immediately adjacent to
these sites, relatively undisturbed savanna woodlands were selected as controls. Site I is located in the
Shangani Farm North, while site II, in the Bulawayo Syndicate Block, is 12 km to the northwest. The
study areas are used as paddocks for cattle and wildlife ranching.
The study area lies in agro-ecological Region IV, characterized by a semi-arid climate with a mean
annual rainfall of 400-500 mm (Vincent and Thomas, 1960). The area experiences high temperatures,
high evaporation rates, and moisture deficits (Mzezewa et al., 2000). Soils on study sites are developed
on gneissic granite parent material (Nyamwanza and Mzezewa, 1997). The soils are generally shallow
with a sandy texture on the surface and medium textures at depth. Gravelly soil layers and quartz
stonelines are common features.
Vegetation on sites affected by bush encroachment was dominated by thorny thickets of A. karroo with
scattered Albizia amara subsp. sericocephala and occasional Euclea divinorum. Both sites were virtually
devoid of grasses and signs of severe sheet erosion were evident. Sites free of bush encroachment
consisted of an open mixed woodland of Colophospermum mopane, Sclerocarya birrea subsp. caffra,
Combretum apiculatum and some tall Hyperrhenia grass species.
A series of augering was done along transects in encroached and control areas on both sites to establish
representative (modal) soil types. Soil profiles were dug to the parent rock on representative auger holes
and described by standard methods (Soil Survey Staff, 1990). Samples were collected by morphologic
horizon following international standards (Soil Survey Staff, 1990). Soil color was classified according
to the Munsell color notation (Munsell Color, 2000). Soil samples for bulk density analysis were taken
using cores of 7 cm diameter and 5.2 cm depth (200cm3), following the method of Blake (1965).
Triplicate core samples were taken from the middle of demarcated horizons.
Infiltration measurements were conducted on representative soil profiles from sites experiencing bush
encroachment and in adjacent savanna woodland (sites I and II). The method used was adapted from
Landon (1984). Standard double ring infiltrometers with three diameter combinations (inner/outer =
28cm/53cm, 30cm/55cm, or 32cm/57cm) and a height of 25 cm were used. The infiltrometers were
simultaneously driven (to a depth of about 10 cm) into the soil using a soft nylon hammer. The rings
were simultaneously filled with water (to a height of 12 cm) and water intake readings were taken from
the inner ring at set time intervals. For the first 5 minutes, readings were recorded every 30 seconds.
After the first 5 to 10 minutes of infiltration, readings were taken at 1 minute intervals. Readings were
later taken every 5 or 10 minute intervals depending on the rate of the infiltration. The experiment was
stopped after 4 hours when the infiltration process had stabilized. Cumulative infiltrations over the entire
measurement period were calculated from the depth recordings.
Cores were used for the determination of bulk density (Blake and Hartge, 1986). A sharpened, open-
ended cylindrical metal container was carefully pushed into the soil until the level of the ring was flush
with the soil surface. The core was then carefully extracted, the soil trimmed away until flush with the
ends of the container. The ends of the container were covered with lids and taken back to the laboratory.
In the laboratory, the core plus soil were oven-dried at 110 oC for 24 hours and the soil was then re-
weighed. The volume of the core was determined. The bulk density was calculated by dividing the oven
dry mass of soil by the total volume of the core.
Soil pH and Texture
All data refer to the fine-earth fraction passing a 2 mm round hole sieve. Each 15-g soil sample was
weighed into a 200-ml jar to which 75 ml of 0.1M CaCl2 was added. The mixture was mechanically
shaken for 30 min and pH was determined using a digital pH meter (Orion 701). Clay (
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