Tim O'Connor’s research while affiliated with University of the Witwatersrand and other places


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Publications (7)


Dominant species of mid-elevation grasslands of the uKhahlamba- Drakensberg Park are predicted to be largely immune to climate change
  • Article

October 2023

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97 Reads

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2 Citations

Journal of Mountain Science

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Tim G O'connor

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Anticipating global change impact on natural vegetation, especially in poorly researched systems, requires an understanding of the environmental factors that most strongly influence the organisation of plant communities at different scales. The environmental relations of plant communities in the Drakensberg mountains of South Africa remain poorly researched. This study investigated the influence of selected environmental determinants on the dominant species of plant community organisation in this area. In order to provide an overview of the influence of the environment on botanical composition and plant diversity, this study, conducted as four discrete exercises, investigated the interplay of altitude, aspect and slope on the abundance of selected species, and to examine the influence of latitude, geological substrate and local topography on species composition. Plant diversity showed no pattern of response to investigated environmental variables. Species composition was related to temperature and solar-radiation variables. High altitude sites facing east were distinct for non-graminoid species. Altitude and latitude exerted a pronounced effect on species composition, confounded by longitude and precipitation gradients. Geological substrate, in concert with slope steepness and orientation, influenced composition, restricting the abundance but not distribution of some grasses. Highly individualistic responses were found for C3, C4, and non-graminoid species in relation to altitude, slope and aspect. Slope and aspect mediate the effects of altitude and hence temperature for some species. This study has demonstrated the complexity of drivers influencing species and compositional distribution in the Drakensberg and permits a preliminary consideration of potential changes in the composition and dominance structure of mid-elevation grasslands in response to climate change. The dominant grass species were widespread across environmental Dominant species of mid-elevation grasslands of the uKhahlamba-Drakensberg Park are predicted to be largely immune to climate change 2469 gradients, occupying currently warm, cool, moist and dry habitats. Hence, we predict that these mid-elevation C4 grasslands will persist in the face of current patterns of climate change provided they are not unduly impacted by other global change threats such as altered fire regimes and nutrient deposition. This prediction is, however, unlikely to apply to the herbaceous dicotyledons and non-graminoid monocotyledons, a component which contributes 78% towards species richness. This study could not make a critical assessment of the non-graminoid species owing to their low frequency of occurrence and low abundance in our plot-based sampling approach. Further monitoring and a targeted sampling approach is is required in future.


Figure 2: Standardised Precipitation and Evaporation Index (SPEI) for the Van Wyksvlei area from January 1950 to August 2020. 36
Developing an environmental research platform in the Karoo at the Square Kilometre Array
  • Article
  • Full-text available

November 2021

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165 Reads

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4 Citations

South African Journal of Science

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W. Richard J. Dean

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A part of the Square Kilometre Array (SKA) will be constructed in the northern Karoo of South Africa on approximately 135 000 ha of land. This land is formerly privately owned rangelands (farms) that were purchased by the South African National Research Foundation (NRF), on which the South African Radio Astronomy Observatory, as part of the global SKA project, will erect the SKA infrastructure. Additionally, a long-term environmental research programme will be established to investigate various dryland ecosystem components at a landscape scale. Livestock has been removed from the farms, and the area is now managed by the South African National Parks (SANParks) as the Meerkat National Park. The land-use and land cover changes present an unprecedented opportunity to study ecosystem dynamics. The property will be established as an NRF science park, incorporating an SKA research platform for radio astronomy and an environmental research platform of the South African Environmental Observation Network, with additional environmental research conducted by SANParks and their collaborators. We briefly describe current knowledge of the area’s environment, and report on past and contemporary changes in this part of the Karoo. We present a conceptual model for the larger landscape which considers possible future land-use scenarios, the projected trajectories of change under these scenarios, and factors influencing these trajectories. These deliberations represent the foundation for future research in this landscape and the development of an environmental observation research platform in the Karoo at SKA. Significance: • We summarise an extensive environmental baseline report on the SKA property and surrounding areas. • Withdrawal of livestock and other changes – such as clearing of alien invasive plants, reduced predator control and reduction in water-point maintenance – are expected to bring about changes in ecological processes and plant and animal communities. • We present a conceptual model of scenarios to test possible future trajectories as a first step towards an earth system science research platform in the NRF science park.

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Response of three mesic South African perennial grassland forbs to defoliation and competition

December 2019

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104 Reads

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4 Citations

African Journal of Range and Forage Science

The importance of maintaining biodiversity in natural grasslands under livestock production systems has been recognised. Forbs contribute more to species richness than grasses in South African grasslands. However, little is known about the impact of grazing and grass competition on the population dynamics of forbs. The aim of this study was to determine the response of three mesic grassland perennial forb species to simulated intensive defoliation and interspecific competition from a neighbouring dominant grass, Themeda triandra, in a field experiment. Two of the selected forb species are sensitive to defoliation (Afroaster hispida and Gerbera ambigua) and one is potentially resistant to defoliation (Hypoxis hemerocallidea). Defoliation resulted in smaller plants of all three species following regrowth. Only A. hispida showed an interaction response to defoliation and competition. Thus, a competitive release was observed for A. hispida in that the height of defoliated plants was reduced by 45% compared with undefoliated plants under full competition, but there was no difference under partial or no competition. Gerbera ambigua was unaffected by competition. Hypoxis hemerocallidea was sensitive to competition irrespective of being defoliated or undefoliated. Findings of this study highlighted an individual response such that a general response cannot be predicted.



Figure 2: Plot of species on the first two axes of a partial redundancy analysis (RDA) of forb species composition data from a fence-line study between high-density, short-duration stocking (HDG) and low-density stocking (LDG) systems at (a) Kokstad and (b) Cedarville (n = 40 for each site. Only species with at least 10% of their variation explained are shown. Key to species: ACAN = Acalypha angustata, ACSC = Acalypha schinzii, ANST = Anthospermum streyi, COAF = Commelina africana, COAS = Corchorus asplenifolius, CRHI = Crabbea hirsuta, ERCO = Eriosema cordatum, ERDI = Eriosema distinctum, ERAB = Eriospermum abyssinicum, GEAM = Gerbera ambigua, HEAU = Helichrysum aureonitens, HEDE = Hermannia depressa, HEGL = Helichrysum glomeratum, HEHE = Helichrysum herbaceum, HEMI = Helichrysum miconiifolium, HENU = Helichrysum nudifolium, HIAE = Hibiscus aethiopicus, LESP = Ledebouria sp., PASP = Pachycarpus sp., PEPR = Pentanisia prunelloides, RHCO = Rhynchosia cooperi, SEBU = Senecio bupleuroides, SCCO = Scabiosa columbaria, SESP = Senecio speciosa, STNA = Stachys natalensis, TAOF = Taraxacum officinale, VENA = Vernonia natalensis 
Figure 3: A partial redundancy analysis (RDA) of the effect of grazing system (triangles), after accounting for spatial variability, on the proportion of forb species with different growth habits (represented by arrows) at Kokstad (n = 40). Grazing systems were: high-density, short-duration stocking (HDG) and low-density stocking (LDG), and their difference accounted for 21.9% of the cumulative variance. Growth habits categories were: CE = cauline erect, CP = cauline prostrate, RE = radical erect, RP = radical prostrate 
Table 3 ).
. Shannon-Weiner diversity and species richness of the forb species was not affected by grazing system but the forb evenness was
Litter and ground cover variables (mean ± SE) of mesic grassland under high-density, short-duration stocking (HDG) and low-density stocking (LDG) at Kokstad and Cedarville, South Africa, assessed using a paired t-test. Values in bold are significant at p ≤ 0.05
What are the long-term effects of high density, short duration stocking on the soils and vegetation of mesic grassland in South Africa?

High density, short duration stocking (aka HDG) is currently gaining popularity amongst farmers in the South African mesic grasslands, but little is known about its potential impact on the soils, vegetation composition and diversity, particularly the forbs. The aim of this study was to assess the effect of HDG with cattle on soil properties, vegetation structure, composition and forb growth habits of a mesic grassland in South Africa. This study was conducted on a fence-line contrast at a landscape scale between two properties at two study sites, Kokstad and Cedarville. At each study site, one property had been stocked with cattle in a short duration rotational system at a high density (HDG) and the other had a rotational grazing system at a much lower density (LDG) for >15 years. At Kokstad, HDG had zero burning for 19 years and LDG had a burning frequency of 2 to 4 years, whereas at Cedarville both HDG and LDG had a burning frequency of 10 to 15 years. Soils from Kokstad and Cedarville were 31% and 19% more compacted under HDG than LDG, respectively, but did not differ in any of their chemical properties (total nitrogen, total carbon, available phosphorus and pH) at both sites. HDG resulted in higher litter mass accumulation which reduced irradiance at both study sites. At Kokstad, palatable, grazing-sensitive grass species (e.g. Themeda trianda) decreased and some unpalatable species (e.g. Eragrostis curvula) increased under HDG. HDG increased abundance of four forb species including the alien weed, Taraxacum officinale, whereas under LDG, the abundance of ten forb species was increased. The intense grazing and trampling under HDG resulted in the replacement of forbs with a cauline erect growth habit by those with prostrate growth. At Cedarville, only a few grass and forb species were responsive to HDG and LDG, possibly because of the overriding influence of the absence of burning in both grazing systems. These results indicate that HDG potentially has negative impact on a South African mesic grassland.


Does high-density stocking affect perennial forbs in mesic grassland?

May 2017

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117 Reads

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21 Citations

African Journal of Range and Forage Science

Livestock production is an appropriate land use for mainstreaming biodiversity conservation, but little is known about the impact of grazing strategies on forbs that contribute most species, in grasslands. This study compared the effects of high-density, short-duration stocking (HDG) with no grazing (control) on vegetation structure, composition and demography of selected perennial forbs in a South African mesic grassland in a three-year experiment in KwaZulu-Natal, South Africa. HDG resulted in four-fold more and deeper litter accumulation, which reduced irradiance. Cover of three forb species, including the alien Bidens pilosa, was promoted and cover of another three forb species was reduced under HDG. There were no short-term effects of HDG on diversity, evenness and species richness. Ninety percent of forb species were damaged by HDG; type of damage varied widely from leaves and stems grazed to tearing and shredding by the hooves of cattle. Study of demography revealed that HDG threatened future populations of the grazing-sensitive species Afroaster hispida, Agathisanthemum chlorophyllum and Gerbera ambigua through increased mortality or reduction in the recruitment of large from small individuals. This study has revealed that HDG potentially has negative direct and indirect effects on indigenous perennial forbs in a South African mesic grassland.

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Citations (4)


... Specificity has always been a hot topic in bio-geography research [15]. In a particular region, its geographical distribution pattern and richness are not only directly related to the current terrain, climate, land use and other factors [16,17], but also closely related to the geological evolution and climate change in the historical period [18,19]. Studying the differentiation pattern of endemic species in a certain region is the key to studying the nature, origin, formation and evolution of flora. ...

Reference:

Distribution Pattern of Species Richness of Endemic Genera in Mountainous Areas of Southwest China and Its Influencing Factors
Dominant species of mid-elevation grasslands of the uKhahlamba- Drakensberg Park are predicted to be largely immune to climate change
  • Citing Article
  • October 2023

Journal of Mountain Science

... Mountains are distinct and conspicuous parts of the landscape, characterized by extreme topography and complex slopes with steep gradients (Hirmas and Graham, 2011). Specifically, mountainous grasslands are characterized by high elevations and undulating topography where the mixture of grasses and other herbaceous plants predominate (O'Connor, 2005;Sala et al., 2013;Ralph Clark et al., 2021). These grasslands provide a range of material and non-material benefits to human livelihoods (Bardgett et al., 2021). ...

Montane Rangelands in a Changing World - Guest Editorial

African Journal of Range and Forage Science

... Eighty root samples (down to 150 mm) from 20 tufts of each of the four species were collected from a remnant mesic grassland in Pietermaritburg, South Africa (675 m asl). This ungrazed tufted, closed-canopy grassland (KwaZulu-Natal Hinterland Thornveld; Mucina and Rutherford 2006) on sandy clay loams derved from shale (Chamane et al. 2019) is rich in plant species (130 forb, 31 grass, and 7 sedge species; P Wragg, unpublished data). Each half of the study field has been burned biennially for at least the last three decades. ...

Response of three mesic South African perennial grassland forbs to defoliation and competition
  • Citing Article
  • December 2019

African Journal of Range and Forage Science

... The plants rooted within the quadrat were identified to species level and recorded. For grasses, the percentage cover of species in the quadrat was visually estimated to the nearest 5%, and species composition was expressed as the mean cover percentage of each species per plot [45]. For forbs, the abundance of each species was quantified by counting the number of individual plants based on a cluster of stems rooted in each quadrat [46] and was expressed as mean density per plot. ...

Does high-density stocking affect perennial forbs in mesic grassland?
  • Citing Article
  • May 2017

African Journal of Range and Forage Science