The role of fire in bush encroachment in Ithala Game Reserve.
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Date
2010
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Abstract
The increase of woody vegetation (also known as bush or shrub encroachment) in savannas
has become of global concern to conservationists and rangeland managers alike. Bush
encroachment has been associated with a decrease in rageland palatability. In addition, the increase
in woody biomass has consequences for climate change, carbon sequestration, rangeland hydrology
and nutrient cycling. As a result of these large changes in ecosystem functioning with bush
encroachment, biodiversity may be threatened. Fire is considered to be one of the most important
management tools used to control woody biomass in savannas. However, despite the use of fire in
Ithala Game Reserve, areas have become encroached. This thesis assesses the role of fire in bush
encroachment in Ithala Game Reserve.
I start this thesis with a discussion of the bottom-up (water, nutrients, and light) and topdown
(fire and herbivory) ecosystem components in the literature review. This sets the foundation
for an understanding of the factors that affect savanna tree:grass ratios for the rest of this thesis. In
addition the review discusses the potential effects of climate change on savanna tree:grass ratios.
Recently, it has been proposed that increasing atmospheric carbon dioxide concentrations result in
an increased competitive ability for C3 woody plants against C4 grasses. Many models have been
produced to explain savanna dynamics. By assessing the role of fire in Ithala Game Reserve, its
functioning is assessed in light of the current issues of bush encroachment.
Textural analysis is a remote sensing technique that has been used to detect changes in
woody vegetation using aerial photographs. Textural analysis was used to assess changes in woody
vegetation cover and density from 1943 (earliest period for which aerial photographs were available
for the study area) to 1969, 1990 and 2007 in Ithala Game Reserve (IGR). Field surveys were
performed to assess the effects of the fire regime in IGR on woody vegetation structure and
composition. Transects were performed in areas with different fire frequencies. The effects of fire
frequency were compared between similar vegetation communities.
Textural analysis showed that woody vegetation cover (+32.5%) and density
(657.9 indiv. ha-1) increased from 1943 to 2007. Importantly, in some areas of IGR, the suppression
of fire led to the rapid invasion of woody plants from 1990 to 2007. Field studies demonstrated the
importance of fire in controlling woody vegetation in IGR. The densities of the encroachers,
Dichrostachys cinerea and Acacia karroo were resistant to annual burns. However, the height of
these deciduous microphyllous woody encroachers was reduced by more frequent fires. Following
the suppression of fire, these trees grew taller and their negative impact on the herbaceous layer
increased. Consequently, fuel loads (grassy biomass) declined and prevented the use of frequent
and intense fires by management. The reduction in fire frequency allowed the invasion of woody
evergreen macrophyllous species. Continued development of fire-resistant patches of evergreen
macrophyllous vegetation will further reduce the effectiveness of fire in controlling bush
encroachment.
To control bush encroachment in IGR and the consequential loss of biodiversity, an
intermediate fire frequency (one burn every 2 to 4 years) is required. Burns also need to be hot
enough to increase the current rate of topkill. Management should act to optimize the accumulation
of grassy biomass to fuel fires.
Description
Thesis (M.Sc.)-Unversity of KwaZulu-Natal, Pietermaritzburg, 2010.
Keywords
Woody plants--Effect of fires on--KwaZulu-Natal--Ithala Game Reserve., Savanna ecology--KwaZulu-Natal--Ithala Game Reserve., Rangelands--Fire management--KwaZulu-Natal--Ithala Game Reserve., Vegetation management--KwaZulu-Natal--Ithala Game Reserve., Grasses--KwaZulu-Natal--Ithala Game Reserve., Plant communities--KwaZulu-Natal--Ithala Game Reserve., Theses--Botany.