Controlling woody plant encroachment in a southern African savanna.
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Date
2021
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Abstract
Woody plant encroachment is considered one of the most extensive forms of degradation
affecting savannas in arid and semi-arid ecosystems. Thus, reactive interventions such as
chemical and mechanical controls, as well as fire application remains the norm in management
of woody plant encroachment. I conducted a series of woody plant control experiments at the
Agricultural Research Council’s Roodeplaat experimental ranch, situated in Gauteng Province,
South Africa. The first experiment was a tree-thinning study at two savanna sites that differ in
soil texture and woody species. Site 1 was on previously cultivated clay-dominated soils
characterized by severe soil erosion and was encroached by Vachellia tortilis. Site 2 has never
been cultivated and was on sandy soils with several woody species. At each site, 24 30 m × 30
m plots separated by 5 m wide fire breaks were established. Trees were removed to the
approximate equivalents of 0% (control-no removal), 10%, 20%, 50%, 75% and 100%
(complete removal of trees), followed by herbicide application on half of the stumps for each
plot. I also investigated the effectiveness of Tree Poppers® (weed wrench) as a low-cost
mechanical control tool to physically uproot seedlings and saplings of woody plants. To
examine the effectiveness of the Tree Popper®, I used eight dominant tree species that were
grouped into three height classes (0-49 cm, 50-99 cm, 100-150 cm) of ten seedlings and saplings
per species per height class. In addition, investigated the effects of five years of annual burning
on vegetation dynamics in a Vachellia karroo woodland. To determine the effects of annual
burning on vegetation dynamics, plots (0.25 ha) established in 2013 were used. These studies
are summarized below:
(1) I determined the combined effects of tree species, tree thinning, stump diameter and
herbicide application on resprouting patterns of woody plant species (Dichrostachys cinerea,
Ehretia crispa, E. rigida, Gymnosporia buxifolia, Pappea capensis, Searsia lancea, S. caffra,
Vachellia karroo, V. nilotica, V. robusta, V. tortilis and Ziziphus mucronata) that encroach
study site 1. All the tree species in this study resprouted after cutting. Herbicide application
significantly reduced the resprouting ability of D. cinerea, E. rigida, V. robusta and Z.
mucronata. Tree removal positively influenced the resprouting ability and vigour of E. crispa
only. The diameter of stumps was an important factor in determining resprouting ability, with
shoot production decreasing with increasing stump diameter. The findings from this study suggest that woody plants are more likely to resprout and survive as juveniles than as adults
after cutting.
(2) I found no significant differences in the number of seedlings and saplings uprooted
by Tree Popper®. However, there were significant differences in the number of juveniles
uprooted using a Tree Popper® with a few individuals of Vachellia species uprooted. The
effectiveness of the Tree Popper® may be due to differences in plant morphological structure,
particularly the root system. The Tree Popper® is not an effective tool for controlling the
Vachellia species used in this study. However, communal ranchers may mechanically control
shallow-rooted tree seedlings with the Tree Popper® but not deep-rooted ones, specifically
Vachellia species.
(3) In the tree thinning experiment, I determined the effects of different tree thinningintensities
on grass species-richness, composition, cover, β diversity, and soil fertility. I found
that tree thinning did not have any significant effects on grass species-richness in either study
site. However, we found a clear separation of different grass species among the treatments over
the study period. Different levels of tree thinning increased the abundance of two dominant
grass species (i.e. Digitaria eriantha and Panicum maximum) in both study sites, particularly
in moderate (50%) and high removal (75% and 100%) treatments. However, the nitrophilous
grass (i.e. P. maximum) will likely decline in abundance with time, particularly in the 100%
thinning treatment because the ecological process that is responsible for N-fixation is no longer
existent. Contrastingly, I found no evidence that tree thinning affects the amunt of soil cover.
In addition, tree thinning did not have a significant impact on soil fertility in either study site. I
recommend maintaining a stand density of 50% in rangeland affected by woody plant
encroachment. In this study, 50% thinning created an opportunity for different palatable grass
species to increase in abundance, which may help to increase forage production.
(4) I determined the effects of different tree removal-intensities on grass production,
tree-seedling establishment and growth, and the growth of the remaining large trees. In site 1,
tree-removal treatments (i.e. 75 and 100%) significantly reduced grass biomass production after
the first growing season, with no effect after the second season. In site 2, tree removal
significantly increased grass biomass production. I found no significant effect of tree removal
on tree seedling establishment in site 1. In site 2, tree removal had a significantly negative effect
on overall tree seedling establishment. In both sites, there were no significant differences in tree
seedling growth. Moderate (50%) to high (75%) removal of trees had a positive effect on the growth of remaining large trees in both study sites. I found that increased and/or diminished
grass biomass production plays a vital role on tree seedling recruitment. Reduced tree
competition facilitates the growth of the remaining large trees. An implication of these findings
is that regardless of the substantial costs of woody plant control, the recovery of key ecosystem
services such as an increased forage production may not be realised. However, this may be
system-specific. In other systems, the absence of management interventions such as tree
removal may compromise provision of ecosystem services and ecosystem functioning.
(5) In the fire experiment, I investigated the effects of five years of annual burning on
the density of young and adult Vachellia karroo plants. This study also aimed to investigate the
effects of annual burning on tree growth (i.e. height, stem diameter and canopy size). The results
supported the “fire-trap” paradigm by demonstrating substantially higher densities of young
plants in the burned plots than in the unburned plots. In addition, the recruitment of young
plants and saplings into adult trees was significantly higher in the unburned plots than in the
burned plots. V. karroo populations substantially increased in growth (height and basal
diameter) in the unburned plots. Different grass species changed in abundance in response to
annual burning. However, I found no significant changes in grass species diversity and richness
between the treatments. I found that the removal of the grass layer by fire and repeated topkill
increased the number of young V. karroo individuals. Annual burning limited V. karroo
juveniles and saplings from reaching an adult size class that may have detrimental effects on
the herbaceous layer. I demonstrated that grass species composition is more prone to fireinduced
changes than species diversity and richness in our study area. In conclusion, I show
that managers of savanna rangelands may use annual burning to achieve specific vegetation
structural objectives.
This thesis demonstrated that mechanical- and chemical -control, as well as fire
application influences the structure and functioning of savannas. By creating gaps that promote
grass production, these management practices may assist increase the economic viability of
savanna ecosystems. However, despite the popular belief that reduced tree densities promote
ecosystem functions, this thesis demonstrates that the impact of control techniques (especially
tree thinning) on forage production vary across savanna sites. This thesis also shows that
management with prescribed annual fire reduced woody plant encroachment across the 5-year
study, suggesting that fire management can be beneficial and should be explored as a
management method.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.