Browsing by Author "Ward, David Mercer."

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The biology and systematic relationships of Crowned, Blackwinged and Lesser Blackwinged Plovers.
(1987) Ward, David Mercer.; Maclean, Gordon Lindsay.
Plovers was compared with particular reference to breeding biology, feeding ecology and habitat preference. Crowned and Blackwinged Plovers coexisted in mixed flocks in grasslands which aided predator detect ion and defence against predators. Lesser Blackwinged Plovers were opportunistic in their use of habitat and were found in habitats ranging from the muddy edges of pans to woodlands. Body size was found to have an important effect on the breeding biology of these birds, affecting incubation behaviour and reproductive pattern. The behaviour of Lesser Blackwinged Plovers differed considerably from that of Crowned and Blackwinged Plovers. Crowned and Blackwinged Plovers appear to constitute a superspecies on account of their behavioural and morphological similarities while the Lesser Blackwinged Plover is possibly closely related to the ancestral stock of the genus Vanellus.
Changes in adult female white rhino seasonal home ranges in relation to variation in food quality and availability.
(2013) Hebbelmann, Lisa.; Shrader, Adrian Morgan.; Ward, David Mercer.
As the dry season progresses across southern Africa, the availability and quality of food declines for large herbivores. Female white rhinos compensate for these declines by expanding and/or shifting their home ranges. These changes may be to incorporate habitat types that contain high quality food or quite simply more food. To determine the factors that drive these seasonal changes in home ranges, I focused on dry season changes in the availability and quality of grass in habitats utilised by white rhinos in the Ithala Game Reserve, South Africa. I expected that if food quality was the main driver, white rhinos would follow optimal foraging principles and incorporate habitat types with the highest nutritional quality into their dry season home ranges. Alternatively, due to their large body size (>1000 kg) and thus ability to survive on low quality food, they may rather incorporate habitat types with high food availability. In contrast to previous studies, I found that during the dry season female white rhinos did not increase the size of their home ranges, but rather shifted their home range boundaries. This resulted in individuals increasing the amount of Bushveld and decreasing the amount of Wooded Grasslands within their dry season home ranges. When I explored the different factors that could explain these patterns, I found that changes in the crude protein content of grass was the key factor driving the incorporation and exclusion of habitat types in the home ranges. During the dry season, white rhinos incorporated the habitat that had the smallest seasonal reduction in crude protein content, while excluding the one with the largest decrease in crude protein. As a result, my results suggest that the search for high quality best explains the seasonal home range shifts of female white rhinos in the Ithala Game Reserve.
Competitive interactions between savanna trees.
(2011) Pillay, Tiffany Prileeni.; Ward, David Mercer.
Savannas are socio-economically important ecosystems, which support high floral and faunal diversity. This biome covers large areas of Africa, Australia, South America, India and parts of North America, and is comprised of a mixture of grasses and woody plant biomass. Most empirical studies of savanna ecology have focused primarily on the interactions between trees and grasses, especially at the seedling stage where recruitment is regarded as a key driver of savanna dynamics. However, studies of interactions between woody savanna plants, such as competition and facilitation, are less common in the literature. Considering the increasingly negative effects of woody plant encroachment and global climate change, interactions between woody plants need to be closely monitored and evaluated. In this thesis, I investigated the effects of neighbourhood competition on four dominant tree species from humid savannas (receiving more than 1000 mm of mean annual rainfall, hereafter referred to as “humid species”) and four dominant tree species from mesic savannas (receiving around 650 mm of mean annual rainfall, hereafter referred to as “mesic species”). I employed a greenhouse-based study to examine the effects of neighbour density on the growth, survival and biomass of savanna tree seedling species. I quantified two aspects of competitive ability (competitive effect and response), and compiled competitive hierarchies for both groups. In addition, I correlated competitive ability with several plant traits. Using field surveys of natural stands of Acacia karroo from humid savanna sites across KwaZulu-Natal, South Africa, I examined the spatial patterns and competitive interactions between trees. A greenhouse experiment revealed that mesic species suffered high levels of mortality when exposed to increasing neighbour density, while humid species were relatively unaffected in terms of survival. However, mesic species were able to maintain constant relative growth rates (RGR) despite increasing neighbourhood competition while the RGR of humid species decreased as neighbour density increased. The total biomass of both humid and mesic species also declined as the neighbourhood competition increased. In terms of competitive effect and response, we found that these two aspects of competitive ability were not concordant (i.e. good effect competitors were not necessarily good response competitors). Lastly, we found that plant traits such as specific leaf area and above-ground features (e.g. shoot biomass and leaf number) were significantly related to the competitive response or effect of savanna tree seedlings. Spatial distribution patterns of a dominant humid savanna species, Acacia karroo, revealed that juvenile plants are aggregated, as expected due to facilitation, seed dispersal and vegetative reproduction. However, the regular spacing of larger individuals due to competition and density-dependant mortality were not detected. We found, using nearest neighbour analysis, that trees with closer neighbours had smaller canopy diameters. This suggests that while competitive interactions are present, they may be weak and insufficient to cause mortality, rather resulting in decreased plant performance. Overall, I found that, at the seedling stage, neighbourhood competition was particularly important for both humid and mesic savanna trees. Competitive interactions between mesic seedlings resulted in significantly higher mortality rates, greatly reducing the recruitment of these species. Humid species, although able to successfully recruit, experienced reduced growth rates under dense neighbourhood competition. In the field, patterns of competitive interactions were difficult to detect using spatial statistics alone. However, we did find evidence of weak competitive interactions among humid savanna trees. In summary, competitive interactions were important for all savanna species at the crucial seedling stage. However, field comparisons showed that competitive interactions were relatively weak in A. karroo and resulted in reduced performance rather than differential mortality.
Controlling woody plant encroachment in a southern African savanna.
(2021) Monegi, Piet.; Tsvuura, Zivanai.; Ward, David Mercer.; Tjelele, Julius Tlou.
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.
Do herbivores facilitate seed germination and seedling recruitment of woody plants?.
(2014) Tjelele, Tlou Julius.; Ward, David Mercer.; Dziba, Luthando.
Woody plant encroachment is a phenomenon whereby trees and shrubs invade grasslands or increase in an already wooded area, resulting in lower yields of herbaceous plants and a reduction in the carrying capacity of rangelands. It is not only the extent of woody plant encroachment, but also the rate at which it occurs, that is a major concern for livestock ranchers interested in herbaceous production. The question of what causes woody plant encroachment still remains unanswered. Animals consume a considerable amount of woody plant seeds during the dry season and could be responsible for spreading undesirable species to new areas or they could enhance the dispersal of species already established. However, under natural conditions, seed dispersal does not guarantee seed germination, which depends on appropriate environmental and seed survival conditions. To better understand the mechanisms involved in woody plant encroachment, we studied the effects of livestock (goats, sheep and cattle), diet quality (high-quality (Medicago sativa hay) vs. low-quality (Digtaria eriantha hay)), seed characteristics (size, shape and hardness), grass competition and fire on germination and seedling recruitment of Dichrostachys cinerea and Acacia nilotica seeds. These objectives were achieved by conducting the following trials: a) recovery and germination of D. cinerea seeds fed to goats, b) diet quality on germination of Dichrostachys cinerea and Acacia nilotica seeds fed to ruminants, c) the effects of gut passage and dung fertilization on seedling establishment of Dichrostachys cinerea and Acacia nilotica seeds and d) the effects of gut passage, dung fertilization, trampling, grass competition and fire on seedling recruitment of the two woody plant species. During the first trial, seed recovery rate of D. cinerea seeds consumed by goats, either voluntarily after mixing them with feed pellets (mixed), or by force-feeding (gavaged) and germination percentage were measured. Seed recovery for the gavaged treatment (32.7%) was significantly higher than for the mixed treatments (9.9%; P < 0.001). Seeds that passed through the digestive tract (mixed (35.5%)) and gavaged (31.2%) treatments) had a significantly higher germination percentage than untreated seeds (19.0%). A non-negligible proportion of D. cinerea seeds remained intact after chewing and passage through the digestive system, and their germination percentage increased. In the second trial, I tested the effects of animal species (goats and sheep, goats and cattle), diet quality (Medicago sativa hay and Digitaria eriantha hay), and seed characteristics (size, shape and hardness) on the effectiveness of animal seed recovery and germination of D. cinerea and A. nilotica seeds. The trial was divided into two experiments. In the first experiment, a significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay) and seed species (A. nilotica seeds, D. cinerea seeds) was found on percentage germination (P < 0.0001). There was also a higher percentage seed recovery (P < 0.009) when animals were offered high-quality hay (47.4% + 4.65) compared to low-quality hay (30.2% + 3.24). In goats fed D. eriantha hay, A. nilotica seed germination (9.38% + 3.66) was higher (P < 0.05) than goats fed D. eriantha hay and D. cinerea seeds (6.78% + 1.13). A greater germination percentage was observed in goats fed M. sativa hay with D. cinerea seeds (6.71% + 1.53) than goats fed M. sativa hay with A. nilotica seeds (2.50% + 0.97) (P < 0.05). In the second experiment, animal species had a positive impact, both on seed recovery (P < 0.0325; goats 32.0% + 6.44; cattle 50.3% + 4.27) and germination percentage (P < 0.055; goats 14.1% + 1.48; cattle 9.3% + 0.94; control: D. cinerea 0.64 + 0.06; A. nilotica 0.59 + 0.07). Animal species in experiment one (goats and sheep) and two (goats and cattle) was most important for seed recovery and germination. However, diet (M. sativa hay and D. eriantha hay) and seed species (D. cinerea, A. nilotica seeds) also had important effects on germination of seeds retrieved from experiment one. The interaction of animal species and size, diet quality, and seed characteristics (size, hardness) all played a major role in recovery of viable and scarified seeds either alone or in combination. I also studied the effects of seedling emergence, seedling establishment and recruitment of dispersed A. nilotica and D. cinerea seeds by goats and cattle under natural conditions. The interaction effect of animal species, seed recovery day and seed germination treatment/planting method was significant on seedling recruitment. Seeds retrieved from goats in the last four days and planted 2 cm in the soil with dung (25.85% ± 0.46) and seeds planted 2 cm in the soil with no dung (24.77% ± 0.35), recruited significantly better than seeds planted on top of the soil (16.98% ± 0.46). The results also indicated significant differences in percentage seedling recruitment among goats, cattle and control, with goats and cattle having the highest percentage recruitment than controls or untreated seeds. Overall, seeds can potentially germinate and recruit following passage through the gut, thereby facilitating woody plant encroachment. The results of the last trial showed that seed passage through the digestive tract of goats and cattle compared to untreated seeds (i.e. not ingested) played an important role in improving germination through seed scarification. However, seed recovery by livestock does not guarantee seedling establishment survival, survival and recruitment. Fire and grass mowing treatments affected seedling emergence, seedling survival and recruitment, most probably because of reduced grass competition, thereby reducing competition for resources (light, water and nutrients) between grasses and seedlings. In conclusion, this study indicated that animal species (goats, sheep and cattle), associated diet (low-quality vs. high-quality), seed species (D. cinerea and A. nilotica) and seed characteristics (size, shape, hardness) all played an important role in seed germination. The interactions of animal species, fire, dung, and season either directly or indirectly were pivotal in the emergence, survival and recruitment of D. cinerea and A. nilotica seedlings. Thus, acid scarification in the gut of herbivores in combination with their indirect effects (dung fertilization) and removal of grass competition (either by fire or mowing) can facilitate seedling emergence, seedling survival and recruitment of woody plant species, which may lead to woody plant encroachment.
Ecophysiology of encroaching Acacia mellifera in intra- and inter-specific interactions.
(2010) Kambatuku, Jack Ratjindua.; Ward, David Mercer.; Cramer, Michael D.
The long-term economic viability and ecological integrity of savanna rangelands is being undermined by increasingly dense woody thickets at the expense of palatable herbaceous cover. This process is known as shrub- or bush-encroachment. Bush encroachment is a subset of a broader ecological riddle underlying the coexistence of woody and herbaceous vegetation that has been the subject of many ecological models. The ecophysiological mechanisms and interactions between trees and grasses on which most assumptions of ecological models are premised have seldom been tested empirically. This document synthesises the results of greenhouse and field-based investigations of the underlying ecological mechanisms and ecophysiological interactions between encroaching Acacia mellifera trees and grasses in a semi-arid environment. In a greenhouse study, I determined the contribution of N2 fixation to the N-budget of Acacia mellifera under conditions of both varying N availability and competition from grass. Tree seedlings had longer shoots and greater total dry mass in the absence of grass. The leaf δ15N values were lower with grass than without grasses. Thus, trees were more reliant on N2 fixation in the presence of grasses. N2 fixation may enable the tree seedlings to survive competition with grass at critical and vulnerable developmental stages of germination and establishment. In a field removal experiment, I monitored the growth rates, water relations and mortalities of shrubs around which neighbouring woody plants were removed (target) and control shrubs over three years. Results showed target trees to have benefitted from removal of neighbours, which was manifested in significantly faster growth rates, less negative predawn water potential and a relatively small degree of canopy die-back. Nonetheless, neighbouring trees appeared to prevent the whole plant mortalities resulting from severe environmental stress. Growing in close proximity with neighbours could therefore yield positive and negative ecophysiological effects. In another greenhouse experiment, I tested the effects of the separation of moisture uptake with depth between tree seedlings and grasses on two common substrate types. I also examined the influence of repeated grass clipping on the persistence of soil moisture. Results indicated a three-tier rooting pattern with a top layer exclusively exploited by grasses, an intermediate layer occupied by both grass and tree roots and deeper layers exclusively tapped by trees. Tree seedling biomass was negatively affected by grass competition although the biomass of grass was enhanced in the presence of tree seedlings on sandy substrates only. The repeated clipping of grass benefitted tree seedlings on rocky substrate more than it did on sandy substrate. The effects of heavy grazing on soil moisture availability to woody shrubs and thus bush encroachment may be contingent on substrate type, being more acute on rocky terrains. Grass competition suppresses tree seedlings but the removal of grass by grazing weakens this suppressive effect, particularly on rocky substrates. The insufficiency of space and soil resources on rocky substrates may necessitate increased investment in root biomass by plants. It is not known why grasses have lower densities on rocky substrates than on sandy substrates, but the obstruction by rocks disadvantages grasses against tree seedlings, leaving grasses vulnerable to grazing pressure. This may allow the woody plants on rocky substrates to benefit more from grass removal than on sandy substrates. Root restriction by rock barriers and, perhaps, sparse soil volume further lead to small tree sizes on the rocky substrate. Small shrubs are less likely to compete intensively for resources and cause density-dependent mortality. Intraspecific competition may maintain shrub sizes within the threshold that can be supported by available resource pools. I conclude from my results that the two-layer hypothesis of niche separation between savanna vegetation is valid although there is an overlap in the grass and tree rooting depth/moisture uptake. An additional factor that affects the success of A. mellifera is the substrate.Trees are more dense on rocky substrates but grow larger on sand. I have further shown that A. mellifera trees fix nitrogen when competing with grasses but do not do so when grasses are absent. A mechanistic model of savanna dynamics will need to integrate water use patterns, substrate and nutrients to make effective predictions about encroachment patterns.
Ecosystem carbon change of shrub-encroached grasslands across a precipitation gradient in South Africa.
(2015) Mureva, Admore.; Ward, David Mercer.
Abstract available in PDF file.
Factors affecting savanna tree sapling recruitment.
(2013) Vadigi, Snehalatha.; Ward, David Mercer.
Savannas are globally important ecosystems characterized by the coexistence of trees and grasses. Woody plants, which are slow-growing dominant life forms, influence the physiognomic structure and function of savanna ecosystems. Their density and distribution provides sustenance to a vast and unique savanna biodiversity, by forming a major source of food material to large mammalian herbivores, sheltering them and through their facilitation of diverse plant species. Savanna tree existence is strongly affected by factors that determine their sapling recruitment. We defined „sapling‟ as a young tree, in the first season of its growth, which does not depend on cotyledonary reserves (=seedling stage) and relies on external resources to grow further. Sapling recruitment may strictly be defined as the progression of a young plant from seedling to sapling stage. However, we believe that savanna tree saplings, present within the grass layer in the initial years of their growth, are equally vulnerable to environmental stresses. This study examines the factors affecting tree sapling establishment in a humid savanna (1250 mm mean annual precipitation). Additionally, the effects of fire were tested in a greenhouse experiment. Dominant species from humid savannas (> 1000 mm MAP), Acacia karroo, Acacia sieberiana, Schotia brachypetala and Strychnos spinosa, and mesic savannas (approx. 750 mm MAP), Acacia nigrescens, Acacia tortilis, Colophospermum mopane and Combretum apiculatum, were studied. In this thesis I examined the effects of resource availability (water, nutrients and light), disturbances (fire and herbivory) and competition (grass) on the sapling ecology of these species. Sapling recruitment and growth were assessed in terms of survival and aboveground growth responses, i.e. total biomass, stem growth rates (used as proxy measures for assessing persistence) and leaf biomass proportion (important for producing root reserves necessary to resprout). I studied the effects of fire and a nutrient gradient on survival and growth of four Acacia species in the presence of grass competition, in a controlled greenhouse experiment. Generally, Acacias invest in defenses after herbivory. I also determined their physical and chemical defense investments in this experiment. Sapling survival was not influenced by nutrients but highly varied among the species due to fire, indicating that fires may have a differential effect on species composition at a landscape scale. Intermediate levels of nutrients were found to be beneficial for sapling growth than high and low levels. This may be due to an increase in grass competition at higher levels of nutrients. Fires did not have a positive influence on sapling defence investment. To evaluate the relative importance of resource availability on sapling tree recruitment and its interactions with grass competition, I tested the effects of water (frequent irrigation vs. rainfall), shade (presence vs. absence), nutrients (addition vs. no addition) and grass competition (presence vs. absence) on sapling survival and growth under controlled field conditions in a humid South African savanna. Treatments did not have an effect on sapling survival, indicating that mortality is not defined by resource availability and grass competition in humid savannas. Shade had the greatest negative effect on sapling growth, suppressing the beneficial effects of nutrients and absence of grass competition. Nutrient limitation and grass competition had a relatively small influence on savanna sapling growth. Frequency of water availability had no effect on sapling growth, perhaps owing to high rainfall experienced over the experimental period. Therefore, canopy shade can be considered to be an important driver of tree dynamics in humid savannas with some degree of influence by nutrient availability and grass competition. The effects of clipping (i.e. simulated herbivory of grass and tree saplings) as influenced by nutrient availability and grass competition were examined on sapling survival and growth of all study species in a humid savanna. None of the treatments had an effect on sapling survival. This signifies that herbivory alone cannot significantly decrease plant density in humid savannas. However, tree saplings grew taller with a reduction in diameter and overall biomass, implying that saplings may become more susceptible to fires after herbivory. Nutrient addition and grass competition in general had a positive and negative effect, respectively, on sapling growth. This response was prominent in the stem length growth rates of defoliated saplings of one humid and two mesic species. These results imply that clipping (or herbivory) is the major factor reducing sapling vigour to establish, but is affected by both grass competition and nutrient availability. This study shows that fire has a differential effect on sapling survival of different species, particularly between humid savanna species. Light interception among all other resources limits the recruitment of saplings into adult size classes. Clipping, nutrient availability and grass competition had a relatively small direct effect, but may interact with other factors to alter sapling establishment dynamics. Wet-season droughts in humid savannas are not a hindrance to tree establishment because sapling survival was not dependent on frequency of rainfall. Thus, in humid savannas, fires can have a major impact on tree species density and composition while canopy shade has a very high potential to alter tree distribution.
Host specificity in South African mistletoes.
(2013) Okubamichael, Desale Yosief.; Ward, David Mercer.; Griffiths, Megan Elizabeth.
Mistletoes intimately connect to their host trees with a haustorium that allows them to access nutrients and water. Mistletoes in South Africa vary greatly in their degree of host specificity. Most species occur on a wide range of host families, while others are restricted to a single host family or—at the extreme—to a single host species. Mistletoes that are host generalists at a larger spatial scale may become host-specific at a local scale. One of the challenges in mistletoe biology is determining the factors that maintain local host specificity. Birds potentially reinforce the mistletoe–host interactions by direct dispersal. However, many mistletoe species coexist while parasitising different co-occurring host species. This suggests that host trees may impose more selection pressure than birds in determining host specificity. Thus, my thesis examines the role of host trees as ecological and physiological filters that influence the infection patterns and determine host specificity of mistletoes in South Africa. The second chapter of this thesis synthesises the literature on host specificity in mistletoes. I then present the results of four field and laboratory experiments that were used examine the features affecting host specificity in representatives of two families of mistletoes (Loranthaceae and Viscaceae) in South Africa. My main research objectives focus on host abundance and morphology, host compatibility, host water and nutrient content, abiotic influences on mistletoe seedling survival and growth and mistletoe–host stomatal morphology in relation to water potential that affect nutrient acquisition by mistletoes from their host trees. The geographic mosaic approach was explored as a potential explanation for the mistletoe–host interactions that direct host specificity in mistletoes. I synthesised the available literature on the mechanisms and factors that direct mistletoe host specificity. This was supported by data analysed from South African herbarium collections, books describing the South African flora and field observations in South Africa. I suggest that host abundance (host availability through time and space) and host compatibility (as determined by genetic, morphological, physiological and chemical factors) play a primary role in determining host specificity in South African mistletoes, while differential bird dispersal strengthens or weakens mistletoe–host interactions. Analysis of the network structure of mistletoe–host interactions at different levels (e.g., at the level of population, species and genus) followed by genetic and reciprocal germination experiments may reveal the patterns and mechanisms of host specificity in mistletoes. I quantified the mistletoe–host composition, height of potential host trees and nutrient and water content of mistletoes and their hosts at Pniel Estates. Surveys of the study site revealed a single mistletoe species, Viscum rotundifolium, parasitising only Ziziphus mucronata and Ehretia rigida. Both parasitised host species were not the most abundant trees, were not the tallest trees and did not have the highest water or nutrient content of trees in the area, although these factors have been found to be good predictors for mistletoe parasitism in other studies. Subsequently, I tested mistletoe–host compatibility by conducting a germination experiment in the greenhouse by inculcating seeds of V. rotundifolium on freshly cut branches of nine available potential host trees. I found that mistletoe seeds had a greater chance of attachment and subsequent survival on branches of E. rigida and Z. mucronata as compared with seeds on co-occurring Acacia and other potential host species. This suggests that host compatibility plays a role in directing the host specificity of V. rotundifolium at Pniel Estates. I found that individuals of V. rotundifolium had more negative water potentials than their host trees and, by doing so, they passively maintain the flow of nutrients. In addition, I found evidence that the mistletoe uses active uptake to access nutrients from host phloem because the leaf tissue of a mistletoe had a nitrogen-to-calcium ratio (N:Ca) >1. Conventionally, a high N:Ca ratio (>1) in the leaf tissue of a mistletoe is taken as evidence of active uptake from host phloem because N is highly phloem-mobile while Ca is a large molecule and is phloem-immobile. This method has shortcomings discussed at greater length in the chapter but my findings suggest that the mistletoe V. rotundifolium uses a combination of passive and active nutrient uptake. I quantified the mistletoe–host community composition and host physical features (height and diameter at breast height) in two sites in KwaZulu-Natal, South Africa—Highover and Mtontwane. The mistletoe Agelanthus natalitius (Loranthaceae) is common at both sites, parasitising the most abundant host species—Acacia karroo—and the second most abundant host tree—Acacia caffra. Prevalence of mistletoe infection (percentage of trees parasitised) was positively correlated with tree size (height and diameter at breast height). The two host species did not differ significantly in height. At Highover the host species A. caffra and A. karroo had a similar prevalence of mistletoe infection but at Mtontwane a significantly higher percentage of A. caffra trees was parasitised in comparison with A. karroo. However, the intensity of mistletoe infection (mean number of mistletoes per tree) was lower for A. caffra (Highover: 0.66 ± 0.01, Mtontwane: 0.89 ± 0.04) than for A. karroo (Highover: 0.73 ± 0.04, Mtontwane: 1.03 ± 0.64). There were two highly infected big trees in Highover and one in Mtontwane where many mistletoe-dispersing birds were nesting which inflated the numbers for intensity of mistletoe infection in A. caffra, however. I tested mistletoe–host compatibility by conducting a reciprocal transplant experiment in the two study sites. I applied a paired design, using one local and one non-local mistletoe seed in each pair, with seed pairs placed on the two main host species at the different sites. Except in Highover where an unidentified pathogen retarded growth and survival, mistletoe seeds placed on the same substrate and in the same site as their source host grew a longer hypocotyl and had greater survival. Regardless of source, mistletoes placed on A. karroo had longer hypocotyls and greater survival than mistletoes on A. caffra. These results suggest that there may be adaptation of the mistletoe Agelanthus natalitius to the most frequently encountered host species, Acacia karroo. To simulate the conditions encountered by mistletoes during the dry and cold South African winter, mistletoe seedlings were monitored at different levels of microclimate (light, temperature and moisture) in a growth chamber. I found that higher light availability (20% and 40% shade versus 80% shade), cool temperatures (15°C and 20°C versus 25°C) and continuous moisture availability improved seedling development and subsequent survival of two mistletoe species (Viscum rotundifolium and Agelanthus natalitius). I studied the leaf stomata of two host–mistletoe pairs (Acacia karroo–Agelanthus natalitius and Vitex obovata–Erianthemum dregei) using a scanning electron microscope to investigate some of the underlying mechanisms that enable mistletoes to maintain more negative water potentials than their host trees and at the same time control water loss. In addition, I examined the response of mistletoes to the application of abscisic acid (ABA), a plant growth regulator that controls stomatal closure. I found that the mistletoes had a higher density of stomata and had larger stomata than their host trees. In addition, both mistletoe and host leaves closed their stomata during midday and in response to exogenous ABA. The ability of mistletoes to control water loss in this way may be one reason why mistletoes rarely kill their host trees, which would be maladaptive. The mistletoes used in my studies are known to be host generalists at a larger spatial scale but I found that they were host specific at a local scale. The results of my research suggest that host abundance and compatibility play a role in directing host specificity, while host nutrient and water status have little effect on host specificity at this local scale. The interactions between the generalist mistletoes used in my studies and their hosts are likely to vary over the geographic ranges of the mistletoe and alternate among different hosts. This may create multiple locally host–specific mistletoe populations and produce a complex geographic mosaic of mistletoe–host combinations across space and time. I suggest that mistletoe populations in South Africa may comprise numerous lineages incapable of parasitising the full range of host species, which could potentially lead to the formation of distinct host races over time. In the future, it would be interesting to document the infection patterns of these generalist mistletoe species across their entire geographic ranges in southern Africa, with particular focus on the patterns of mistletoe infection in places where the host abundance changes among sites. Host preferences may vary with changes in host frequency and host community composition. This could be paired with reciprocal transplant germination experiments in several sites to ascertain whether the mistletoe species have higher fitness on the most locally abundant hosts.
Host specificity of the hemiparasitic mistletoe, Agelanthus natalitius.
(2009) Okubamichael, Desale Yosief.; Ward, David Mercer.; Griffiths, Megan Elizabeth.
Mistletoes are a group of hemiparasitic plants that grow on a wide variety of host trees and differ in their degree of host specificity, ranging from specialists to generalists. Mistletoes can also be locally host specific where host preference varies geographically, i.e. at a given location a mistletoe species may infect only part of its overall host set. The mistletoe Agelanthus natalitius parasitises at least 11 tree genera distributed throughout South Africa. However, there is geographic variation in infection patterns over the parasite’s range, suggesting that A. natalitius may be locally host specific. We quantified the degree of host specificity and tested the mechanisms that direct host specialisation in two distinct mistletoe populations at Highover and Mtontwane (about 110 km apart) in KwaZulu-Natal, South Africa. We investigated the distribution, abundance and community composition of woody species that host the mistletoe. We also assessed the effect of light on germination and early survival of the mistletoes in a greenhouse experiment. We conducted field reciprocal transplant experiments at both sites to investigate the compatibility of these mistletoes with their hosts Acacia karroo and A. caffra during early development. We then analysed the nutrient and water contents of the mistletoe-host pairs to investigate the role of nutrient and water status in directing host specificity in mistletoes. We further studied avian dispersal in the field and in captivity to investigate optimal dispersal distance and germination success, and evaluated their role in determining mistletoe host specificity. At both study sites, five host species were recorded as being parasitised by the mistletoe A. natalitius. A. karroo and A. caffra appear to be the two most common host species in the region; both grow abundantly at the study sites and were recorded with high infection by A. natalitius. However, A. karroo is the most abundant host species and the mistletoe showed a high degree of host specificity on A. karroo. Infection by mistletoes was positively correlated with tree size, and was highly aggregated, both individually and locally. Field observations and greenhouse shade experiments showed that light can influence mistletoe distribution. Germination of mistletoe seeds was independent of host species and site. However, hypocotyls (the structures that develop into haustoria) grew longer when placed on their source host species within their locality. Additionally, they showed preference for the most abundant host species, A. karroo. Water and nutrient status of the host species A. karroo and A. caffra had no significant effect. Thus, host nutrient and water content may not account for host specificity in this mistletoe species. Mistletoes accumulated more nutrients and maintained more negative than their host trees. We also investigated the mistletoes’ use of passive nutrient uptake (from host xylem) and active nutrient uptake (from host phloem) by using the N:Ca ratio as an index of nutrient access. Mistletoes growing on A. caffra had a ratio > 1, i.e. the mistletoe actively accessed nutrients from the phloem of host trees. However, mistletoes on A. karroo had a N:Ca ratio < 1, which implies that they passively accessed nutrients from the xylem. The difference in mechanism of nutrient acquisition on different host species may reflect the level of compatibility between mistletoe and host. Several bird species were frequently observed to feed on mistletoes, many of which were used in our captivity studies. Although birds did not consume mistletoe fruits in captivity as they do in the field, they were effective in removing the pulp cover of mistletoe fruits and exposing seeds in germinable condition. In captivity, the Red-winged Starling ingested whole fruits and regurgitated seeds, deliberately wiping their bills on twigs to remove the sticky seeds. As a result, germination success of mistletoes processed by Red-winged Starlings was higher than any other bird species tested in captivity. Overall, there appears to be host specificity in morphologically identical mistletoes. Understanding the mechanisms that result in host race evolution are potentially important to the process of speciation in hemiparasitic mistletoes. We need to take into account genotypic matching in conserving these different forms of mistletoes and their host Acacia genotypes. Further research into the mechanisms of host specificity and patterns of genotypic matching is warranted.
The influences of plant secondary metabolites on the foraging behaviour and carrying capacities of the african elephant, loxodonta africana.
(2017) Schmitt, Melissa Holbrook.; Shrader, Adrian Morgan.; Ward, David Mercer.
Herbivore diet selection is influenced by plant quality and availability. For browsing and mixed-feeding herbivores, such as elephants, which consume trees and shrubs, plant quality is influenced by the concentrations of crude protein, fibre, and plant secondary metabolites (PSMs). To date, the driving factors behind elephant diet selection have not been well understood. The broad aim of this study was to better understand how pre-ingestive cues such as volatile organic compounds (VOCs) and postingestive cues such as the effects of plant secondary metabolites (PSMs) influenced the foraging behaviour of African elephants. I also examined the salivary tannin-binding proteins of elephants and how these influenced carrying capacities. I found that elephants were able to differentiate between plant odours in the form of VOCs across multiple spatial scales. This could help elephants to reduce their search time while foraging across a landscape. I also found that these pre-ingestive cues were a better indicator of diet preference than the tested postingestive cues such as digestibility, crude protein, tannins (a type of total polyphenol) and the ratio of palatable: unpalatable indices. The VOCs in preferred plant species had significantly lower concentrations and diversity of PSMs than avoided species, particularly with respect to monoterpenes, a known anti-herbivory VOC. In contrast, avoided plant species were more digestible and had lower levels of polyphenols and tannins. Ultimately, I found that terpenes play a stronger role in elephant diet choice than phenolics (such as tannins) and other postingestive feedback measures. This is likely because terpenes are a pre-ingestive cue that also incur postingestive costs, which the elephants can use prior to making foraging decisions, unlike other postingestive feedback measures. While it seems counterintuitive that preferred plants have high concentrations of tannins, which reduces the amount of crude protein available, I found that elephants have salivary tannin-binding proteins, which neutralize the influences of tannins by an average of 75%. This neutralization influences the amount of available crude protein, which would, in turn, influence the carrying-capacity for elephants. I constructed an optimal-foraging model for carrying capacity that incorporated the negative effects of tannins and the neutralization of these chemicals by the tannin-binding affinity of elephant saliva. I also included diet breadth and dietary contribution of browsed species, browse quality, and the available standing crop of browsed species. As a result, my model produced more plausible estimates of elephant carrying capacity than conventional standing-crop models.
Recovery, resilience and stability of piospere systems in the Kruger National Park.
(2010) Matchett, Katherine Jean.; Kirkman, Kevin Peter.; Ward, David Mercer.; Peel, Michael John Stephen.; Morris, Craig Duncan.
Water provision is an important tool in the management of savanna ecosystems. Artificial water sources are a potential focus for degradation (biodiversity and loss of ecosystem resilience at a range of spatio-temporal scales), because they alter plant-animal interactions and soil function and stability, through the creation of piospheres. This study was undertaken as part of a drive by the Kruger National Park (KNP) to enable managers to integrate artificial waterhole management (e.g. waterhole closure or rotation) when setting goals for heterogeneity and biodiversity conservation in the park. The over-arching goal was to quantify the relationship between water provision and different attributes of heterogeneity, as part of a broader initiative to place water provision and piospheres within an ecosystem threshold framework. Herbivore utilisation gradients (piospheres) around artificial waterholes in the KNP, described in 1990, were resurveyed in 2006-2007, against a backdrop of waterhole closure in the KNP, to contribute to an understanding of the factors governing recovery and resilience in grazing systems. The responses of the plant community and soil parameters to a relaxation of herbivore utilisation pressure around closed waterholes (recovering piospheres) were examined, as were changes in the same parameters at sites that have remained open (active piospheres). These ecosystem properties were considered in relation to structural and functional ecosystem thresholds, and the piospheres surveyed incorporated a range of rainfall and edaphic gradients in the KNP. Herbaceous basal cover and soil infiltration capacity both increased significantly between 1990 and 2006/7, regardless of waterhole status. This was linked to higher rainfall in 2006/7, compared to 1990. The only vegetation variables to respond consistently to distance from water were the remote-sensed Normalized Difference Vegetation Index (NDVI) and herbaceous species composition. NDVI increased with distance from water, and annual grasses and forbs were most abundant close to water. Perennial, disturbance-sensitive climax species increased in abundance further from water. Soil analyses (N, P, pH, organic matter, and texture) and field measurements (infiltration, compaction) revealed no systematic piosphere patterns. Waterhole closure did not result in soil or vegetation recovery, but piosphere intermittency and the increases of basal cover and infiltration rate indicated that ecosystem resilience has not been compromised vii by long-term artificial water provision in the KNP. This study has shown that the traditional piosphere model is of limited use in sub-humid savanna ecosystems like the KNP during above-average rainfall periods.
Regrowth patterns, defences and allocation of stored energy reserves in Acacia seedlings following herbivory and fire.
(2012) Hean, Jeffrey William.; Ward, David Mercer.
Disturbances, such as herbivory and fire are commonplace in savanna ecosystems. The effects of herbivory and fire on growth and defences of adult trees is a much studied filed in plant ecology. However, there are comparatively few studies that have investigated the effects of herbivory and fire on seedling growth, defences and establishment. African Acacia trees are common and widespread, and are suggested to be keystone species in savanna ecosystems. They have been shown to significantly positively increase soil characteristics, such as soil moisture and infiltration, while also increasing spatial heterogeneity of savannas. These trees are distinguishable by their array of physical and chemical defences. Physical defences are either in the form of spines (physiologically costly to produce, thus considered inducible) or prickles (physiologically cheap to produce, thus considered constitutive), while condensed tannins are the most common form of chemical defences. Adult Acacia trees have been shown on several occasions to be highly resilient to disturbance events, primarily due to their large size. However, the effects of herbivory and fire on Acacia seedlings have been little studied despite their apparent importance for our understanding of African savanna ecosystem functioning. In two separate experiments, this thesis aimed to investigate the individual and combined effects of simulated herbivory and fire on the regrowth, defences, and total non-structural carbohydrates (TNC) of the seedlings of several Acacia species, while also testing the Resource Availability Hypothesis (RAH) (14 Acacia species) and the Expanded Growth-Differentiation Balance Hypothesis (GDBH) (three Acacia species). The RAH (Coley, Bryant & Chapin 1985) and the GDBH (Loomis 1932) both assume that arid-adapted (resource-poor) species are slow growing, with low tissue turnover rates. Contrastingly, humid-adapted (resource-rich) species have fast growth rates and thus a high tissue turnover rate. Therefore, arid-adapted species are predicted to invest more carbohydrate reserves in defence after a disturbance event, in order to defend new photosynthetic material. Conversely, humid-adapted (resource-rich) species are predicted to invest carbohydrate reserves into increased growth after a disturbance event, in order to compensate for tissue loss. The first greenhouse experiment found that, in accordance with the RAH, humid-adapted species displayed elevated growth rates compared to arid-adapted species. Overall, defences significantly increased after herbivory, but significantly decreased after fire. Herbivory was also shown to significantly reduce TNC stores in roots more than fire. We found that arid-adapted species did indeed invest more carbohydrate stores (TNC) into defence, and displaying an increase in spine and prickle abundance, spine and prickle length, and condensed tannin concentration. Humid-adapted species displayed an increase in growth rather than in defence, with the majority of species only increasing one defensive trait. Humid-adapted species also displayed significant trade-offs (negative correlations) between new stem growth and defence traits, while arid-adapted species overall did not display any significant trade-offs between stem growth and increased defences. The majority of arid-adapted species also displayed an allometric effect for spine abundance and length, with an increase in one trait led to an increase in the other. Prickles were found to be inducible, despite the assumption that due to low physiological cost, they are no inducible. Herbivory and fire were shown to not be substitutable in their effects on Acacia seedlings in a controlled experimental setting. The second greenhouse experiment tested the Expanded Growth-Differentiation Balance Hypothesis (GDBH) in the seedlings of A. erioloba, A. karroo, and A. nilotica using five levels of nutrient availability. Overall, spine abundance and spine length displayed a unimodal trend in all three species, with spine abundance and spine length being greatest at a nutrient availability of 800 mg/ ℓ. Spine abundance, spine length and condensed tannins increased significantly after herbivory, but were shown to significantly decrease after fire. We found that with an increase in nutrient availability, the growth of stems and roots, along with the production of TNC in roots and stems of all species significantly increased. Our data provide mixed support for the assumptions and predictions of the expanded GDBH. The regrowth of stems and physical defences were consistent with the GDBH. Chemical defences (i.e. condensed tannins) were however, inconsistent with the predictions of the GDBH. We have shown that Acacia seedlings are highly resilient to disturbance events, while the growth responses of Acacia seedlings are underpinned by TNC stores in roots. Simulated herbivory and fire are often substituted for one another in controlled experiments. However, we have shown that the effects of herbivory and fire have significantly different effects on regrowth patterns of Acacia seedlings.
The role of fire in bush encroachment in Ithala Game Reserve.
(2010) Gordijn, Paul Jan.; Ward, David Mercer.
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.
Understanding the effects of changing climate and land use on woody plant encroachment in South African grasslands and savannas.
(2017) Pillay, Tiffany Prileeni.; Ward, David Mercer.
The increase in dominance of woody species, observed in many grasslands and savannas worldwide, highlights the sensitivity of woody herbaceous biomass ratios to changed environmental conditions. While the major factors driving the tree–grass continuum have been identified as precipitation, nutrients, increased CO2 concentrations, fire and herbivory, the interaction of these driving forces determines the tree: grass ratio, and ultimately the occurrence/ absence of woody plant encroachment. Furthermore, with forecasted alterations to the global climate (such as an increase in the frequency and magnitude of climate extremes), and increased nutrient availability (either through extensive use of fertilizer or increased nitrogen deposition), more knowledge is required on the factors which promote woody plant performance. We conducted a series of greenhouse experiments to manipulate these factors, and a two-year field trial to disentangle the effects of varying climate and land use on woody plant encroachment. These studies are summarized below: (1) We investigated the effects of manipulated water availability and passive warming on the productivity of the commonly occurring grass species Eragrostis curvula, and the emergence and performance of seedlings of the woody encroacher, Acacia sieberiana. We simulated a typical savanna microhabitat by planting tree species within a grass matrix. Watering treatments altered to represent drought (minus 40% of the average), normal (same amount as the long-term average of the area), and excess (addition of 40% of the long term average) precipitation. Passive warming was achieved using open-top chambers. Tree seedling emergence was monitored in the first two weeks. Thereafter, weekly tree growth rate measurements were recorded. The final above-ground grass biomass, tree root: shoot ratio and specific leaf area were quantified. We found that water availability was the main limiting factor for grass productivity, with low grass biomass attained under the drought treatments. Warming had no significant effect on grass productivity, but had a large positive effect on tree growth and performance when coupled with excess or normal water treatments. Tree seedling root:shoot ratios were highest in the drought treatment, possibly as an adaptation to low water supply. Higher specific leaf area was recorded with warming, further elucidating the positive effect of increased temperature for tree growth. (2) Using a completely randomized experimental design in the greenhouse, we investigated the effects of fertilizer, fire and grazing on the survival, growth and biomass of Acacia sieberiana. We simulated a typical savanna microhabitat by planting a common savanna grass species, Eragrostis curvula, together with saplings of A. sieberiana. Treatments of fertilizer, simulated grazing and fire were applied. Weekly measurements of relative growth rate (RGRheight and RGRdiameter) were recorded. At the end of 24 weeks, survival, total above- and below-ground biomass, and key functional traits (specific leaf area and average thorn length) were recorded to assess differences in tree performance. We found that fertilization was beneficial to grasses only, increasing their total biomass. Consequently, we observed increased tree sapling mortality with high grass biomass. Fire increased the saplings’ investment in shoot growth and stem diameter. However, we found no evidence of differential post-fire allocation to roots or shoots when grass competition was absent. Grass biomass was also found to decrease the specific leaf area of tree saplings. (3) We conducted a two-year field fertilizer trial, using pairs of open grassland and encroached plots at four sites across a precipitation gradient, ranging from 300–1500 mm mean annual precipitation (MAP). We assessed the effects of encroachment, N, P and N+P addition on soil N stocks and soil P, litter % N and C:N ratio (index of decomposition), plant productivity and species richness. We found that soil N stocks were on average 15 times higher in the higher precipitation sites regardless of nutrient addition or encroachment. Larger variation in soil P was noted at the high precipitation sites, with some evidence of P-limitation in the encroached area with the highest precipitation. We found significant positive effects of encroachment on litter % N (quality), C:N ratios and forb cover across the precipitation gradient. Fertilization increased grass biomass and reduced species richness in the high precipitation sites only. The purportedly beneficial effects of encroachment on litter and plant species richness was more pronounced at the high-precipitation sites. Overall, increased nutrient availability was found to be of importance in high precipitation sites only, possibly due to water scarcity at the low-precipitation sites. (4) To elucidate the effects of fertilizer addition, simulated fire and grazing on the rate of nitrogen (N) fixation and soil respiration we conducted a greenhouse experiment using a woody encroaching species, Acacia sieberiana, and the commonly occurring grass species Eragrostis curvula. Treatments of fertilizer, simulated grazing and fire were applied. Thereafter the Acetylene Reduction Assay was used to determine the rate of biological Nitrogen fixation (BNF). We found a significant decrease in BNF with fertilizer addition, and increases in BNF after fire application. Soil respiration increased with fertilizer addition and decreased after fire application. Grazing had no independent effect on any of the response variables. However, decreased grass biomass resulted in increased BNF across all treatments. Furthermore we found that larger saplings achieved a higher rate of BNF, with a positive correlation between the rate of BNF and both the number and weight of root nodules. The implications of these studies are: (1) Future climate-change predictions of increased drought may constrain grass biomass, thereby promoting woody plant success. Predicted warming is likely to further enhance woody plant performance. (2) Increased nutrient availability, whether as fertilizer addition or increased nitrogen deposition, may promote the competitive ability of the grass component, thereby limiting woody plant invasion. This is based on the assumption that grass productivity is not limited by frequent fire or intensive grazing. (3) Encroachment of leguminous woody plants is purportedly beneficial in higher precipitation areas, due to increased soil nitrogen, higher grass foliar quality under trees, greater forb cover and increased decomposition. Hence, encroachment control should be prioritized in low precipitation sites, particularly in rangelands where woody plants are undesirable. (4) Low intensity or infrequent fire is ineffective at controlling woody plant proliferation, particularly if the invader is able to resprout and is capable of biological nitrogen fixation.
Vegetation change in Northern KwaZulu-Natal since the Anglo-Zulu war of 1879.
(2013) Russell, Jennifer Mary.; Ward, David Mercer.
Historic photographs have been successfully used to compare landscape change over time. I used photographs taken of the grassland biome during and just after the Anglo-Zulu War (1879) in KwaZulu-Natal (South Africa), which are some of the earliest known available landscape photographs. The study area encompassed Fugitives’ Drift, Isandlwana and Rorke’s Drift and included communal and commercial rangelands, as well as conservation areas. These fixed-point photographs showed a dramatic increase in woody cover (< 82.5%) since the Anglo-Zulu War in all three land-use types. Floristic sampling showed that while vegetation structure did not differ significantly, plant species diversity and richness differed significantly for each land-use type. I also used a set of aerial photographs to give a much wider perspective of the landscape changes for the study area from 1944 to 2005. These images indicated that the increase in woody cover was progressive, with most of the woody plant recruitment occurring prior to 1964. Thereafter, the increase in woody plant cover was due to bush-clump thickening rather than recruitment into grasslands. This pattern did not occur, however, in the commercial rangeland, where recruitment into open grassland commenced in the 1980s. Although the theory of patch dynamics is cyclical in nature, this model may fit the patterns observed in the study area. Analysis of rainfall and temperature data showed that there has been a decrease in average annual rainfall since 1902 and an increase in minimum daily temperature since 1973. However, the decrease in mean annual rainfall is not consistent with woody plant encroachment. While the increase in mean annual daily temperature appears consistent with a shift to an environment typical of savannas, woody plant encroachment started before the increase in daily temperature. A survey of long-term residents in the study area with regard to livestock numbers, grazing patterns, fire and wood harvesting was also inconclusive. I, therefore, speculate that increasing levels of atmospheric CO2 is driving bush encroachment, with the other drivers such as rainfall, temperature, absence of intense fires, grazing patterns and land-use, playing a modifying role.
What drives the seasonal movements of african elephants (Loxodonta africana) in Ithala Game Reserve?
(2013) Muller, Kayleigh.; Ward, David Mercer.; Shrader, Adrian Morgan.
The changes in plant quality and availability in space and time present a substantial problem to mammalian herbivores. As a result, these herbivores need to alter their foraging behaviour to maximize their energy gain at both small (plant level) and large (landscape level) scales. A megaherbivore, the African elephant (Loxodonta africana), has been shown to be selective in its foraging choices at both of these scales. Furthermore, the ratio of palatability:defences (e.g. fibre and total polyphenols) has been highlighted as an important determinant of habitat selection in elephants. The elephants in Ithala Game Reserve (IGR) frequently leave IGR during the wet season and forage outside the reserve. However, they predominantly feed on the low-nutrient granite soils of the reserve and return to a high-nutrient area with dolerite soils during the dry season. In an attempt to understand these seasonal movements, I focused on how the small-scale foraging decisions of the elephants lead to large-scale seasonal movements in IGR, KwaZulu-Natal. Plant availability was determined seasonally for seven target species across four areas in the reserve. Crude protein, fibre, energy and total polyphenols as well as the ratios of palatability:digestion-reducing substances were analysed in the wet and dry seasons. All factors and their interactions were significant in a MANOVA. Consequently, I employed a dimension-reducing Principal Components Analysis (PCA) to better understand the factors of greatest importance. The PCA highlighted four of the six most important factors to be the ratios of palatability:digestion-reducing substances. The other two important variables were total polyphenols (negative effect) and crude protein (positive effect). At small spatial scales, I found that the elephants were selective in their decisions, especially during the dry season. For example, the increased inclusion of the principal tree species Acacia nilotica from 2.9% in the wet season to 39.3% during the dry season appears to be a result of a decline in total polyphenols and fibre during the dry season. At large spatial scales, the elephants moved back into IGR from the low-nutrient granite soils in the east in response to an increase in forage quality in the west as the quality declined in the east at the same time. However, it is unclear as to why the elephants are leaving the reserve during the wet season. Some possible explanations for this are discussed.Key-words: acid detergent fibre, crude protein, Loxodonta africana, neutral detergent fibre, total polyphenols, plant part quality.