African Journal of Range & Forage Science 2005, 22(2): 101–105
Printed in South Africa — All rights reserved
Copyright © NISC Pty Ltd
AFRICAN JOURNAL OF
RANGE & FORAGE SCIENCE
ISSN 1727–9380
Do we understand the causes of bush encroachment in African savannas?
D Ward
Department of Conservation Ecology, University of Stellenbosch, Private. Bag X01, Matieland 7602, South Africa
Current address: School of Biological and Conservation Sciences, University of KwaZulu-Natal, Scottsville 3209, South
Africa e-mail: ward@ukzn.ac.za
Received 14 October 2003, accepted 6 April 2005
Bush encroachment affects the agricultural productivity and biodiversity of 10–20 million ha of South Africa. Many people believe
that we understand the causes of bush encroachment. We do not. Many people believe that either fire or heavy grazing by
domestic livestock is the sole cause of bush encroachment. This is wrong. Bush encroachment occurs in many arid regions
where fuel loads are insufficient for fires to be an important causal factor. Belief in grazing as the sole cause of bush encroachment
stems from Walter’s two-layer model. This model states that grasses typically outcompete trees in open savannas by growing
fast and intercepting moisture from the upper soil layers, thereby preventing trees from gaining access to precipitation in the
lower soil layers where their roots are mostly found. When heavy grazing occurs, grasses are removed and soil moisture then
becomes available to the trees, allowing them to recruit en masse. The fact that many bush-encroached areas are heavily grazed
means neither that grazing causes encroachment nor that Walter’s model is correct. Bush encroachment is widespread in areas
where there is a single soil layer and where grazing is infrequent and light. We need to move away from observational studies
and these single-factor explanations. If we are to understand the causes of bush encroachment, we need mechanistic models
to guide us and multi-factorial experiments to tease out the interactions among causal factors. Current disturbance-based models
have many of the right elements necessary to make mechanistic predictions but need to be appropriately parameterised. Some
patch dynamic models also appear to hold great promise in this regard. Field experiments carried out to date show that support
for factors conventionally claimed to cause bush encroachment is underwhelming, and that rainfall amount and frequency,
coupled with specific soil nutrient levels, may drive this phenomenon.
Keywords: disturbance, fire, grazing, plant-herbivore interactions, rangelands, shrub encroachment
In the past 50 years, evidence has accumulated suggesting
that savannas throughout the world are being altered by a
phenomenon known as ‘bush encroachment’ (reviewed by
Archer et al. 1995). Bush encroachment is the suppression
of palatable grasses and herbs by encroaching woody
species often unpalatable to domestic livestock. Therefore,
bush encroachment reduces the carrying capacity for
livestock. The reduction in carrying capacity is of great
significance because savannas in southern and central Africa
contain a large and rapidly growing proportion of the world’s
human population, including many pastoralists whose
livelihood is threatened by this process (Lamprey 1983,
Scholes and Archer 1997).
Many people believe that we understand the causes of
bush encroachment. We do not. Many people believe that
either heavy grazing by domestic livestock or fire is the
sole cause of bush encroachment. This is wrong. Belief in
grazing as the sole cause of bush encroachment stems
from Walter’s (1939) two-layer model. This model states
that grasses typically outcompete trees in open savannas
by growing fast and intercepting moisture from the upper
soil layers, thereby preventing trees from gaining access
to precipitation in the lower soil layers where their roots are
mostly found. When heavy grazing occurs, grasses are
removed and soil moisture then becomes available to the
trees, allowing them to recruit en masse. The fact that many
bush-encroached areas are heavily grazed means neither
that grazing causes encroachment nor that Walter’s model
is correct. Bush encroachment is widespread in areas where
there is a single soil layer and where grazing is infrequent
and light (Brown and Archer 1999). Similarly, while some
models have shown that the two-layer hypothesis may
indeed lead to tree-grass coexistence (Walker et al. 1981,
Walker and Noy-Meir 1982), a spatially-explicit simulation
model by Jeltsch et al. (1996) showed that rooting niche
separation might not be sufficient to warrant coexistence
under a range of climatic situations. However, Jeltsch et al.
(1998a) found that introducing safe sites for seedling
establishment by simulating the effects of various small-
scale heterogeneities allowed coexistence.
Clearly, 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
sensitive early stages of growth. Furthermore, overgrazing
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