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Controlling woody plant encroachment in a southern African savanna.
Get PDFDoctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Larger trees facilitate understory herbaceous biomass but not diversity in a South African savanna
No full textLarge single-standing trees contribute to the structural diversity of savannas as they strongly influence their immediate surroundings including soils and understory plant communities. The influence of woody vegetation at a stand level on the understory vegetation has been extensively studied; however, the understanding of the role of single large trees is limited. The objectives of the study were to 1) evaluate the impact of large trees on understory plant species diversity and composition, herbaceous cover and grass standing biomass, and 2) to establish if plant size and functional qualities such as N-fixing ability modulate understory vegetation responses to overstory trees over two growing seasons (January 2022 and 2023). Vachellia tortilis (a leguminous tree) and non-leguminous woody species (Searsia lancea and Ziziphus mucronata) were studied. We systematically selected 30 trees for each woody species and divided them into two size classes (i.e. small and large trees). Understory plant vegetation was assessed using quadrats under and outside the tree canopies. Plant species diversity and abundance were highest under small tree canopies and outside tree canopies, compared to large trees, likely due to the dominance of Megathyrsus maximus under large trees. Megathyrsus maximus dominated under large trees regardless of N-fixing ability. Grass cover increased under large V. tortilis and Z. mucronata, suggesting that evergreen species like S. lancea negatively impact grass cover. Standing grass biomass was higher under large tree canopies than under small trees or outside canopies, with N-fixing ability having no significant effect (p > 0.05), thus, indicating that large trees of both leguminous and non-leguminous woody plants improve grass productivity. The current findings imply that in agrosilvopastoral systems and game farming, where ecological conservation is a priority for farmers, it is essential to have an overstorey composed of both small and large trees to sustain understory diversity and biomass
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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