OpenUCT :: Browsing by Author "Slingsby, Jasper"

Browsing by Author "Slingsby, Jasper"

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Open Access
Branch junction constriction and hydraulic limitation in two species in the Cape Proteaceae : a mechanism explaining the trade-off between longevity and degree of ramification in the Cape Proteaceae
(2004) Slingsby, Jasper; February, Edmund C
The purpose of this study was to establish if hydraulic limitation and branch junction constriction explain the trade-off between longevity and increased ramification in species of the Cape Proteaceae. This was done by establishing if branch nodes are regions of greater resistance to xylem sap flow in the study species; attempting to quantify nodal resistance for a plant as a whole; and by determining if there is any evidence for hydraulic limitation or compensation in Leucadendron laureolum (Lam.) Fourc. and Leucospermum oleifolium (Berg.) R. Br. individuals. The extent of branch junction constriction was established by comparing the ratio of the xylem sap flow rates of adjacent branch segments of equal length. The ratio of a proximal intemodal to a distal intemodal segment was compared against the ratio of a proximal intemodal segment to a distal node. Branch nodes were found to create significant resistance to xylem sap flow rates in both species (p < .005 for L. laureolum, and p < .05 for L. oleifolium). Total nodal resistance in L. laureolum was modelled as equivalent extra branch length, and as loss of sap flow rate. Equivalent extra branch length increased exponentially with increased branching order, and increased much faster in more ramified plants. Loss of flow rate increased linearly with increased branching order and was unaffected by differences in ramification. This was because the nodes in more ramified plants had smaller basal sapwood areas, and the resistance imposed by nodes decreased with decreased basal sapwood area. As more ramified plants tend to branch earlier, and/or with greater temporal frequency, they will still accumulate greater nodal hydraulic resistance faster than less ramified plants, and thus may be limited to smaller size and younger maximum age. Branch specific conductivities (kₛ), leaf specific conductivities (LSC) and transpiration rates were investigated in higher (11 nodes) and lower (10 nodes) order branches of L. laureolum. All were found to be significantly lower in higher order branches (11 nodes), which can be related to greater nodal resistance to sap flow limiting the amount of water available to the area of developing xylem, causing the vessels to have narrower diameters. There was no significant difference in leaf to sapwood area ratios (Aₗ/As) between the two branching orders. Previous studies have suggested that changes in Aₗ/Aₛratios occur to compensate for hydraulic limitation. Reductions in Aₗ/Aₛin plants with greater hydraulic restriction allow LSC values, and thus transpiration rates, to stay as high as plants with less hydraulic restriction. As there was no such compensation, LSC values, and thus transpiration rates, were much lower in branches of higher order. Transpiration rate is a good indication of rates of photosynthesis. Lower transpiration rate in higher order branches is thus a good indication of hydraulic limitation on rates of photosynthesis with increased branching order. Hydraulic resistance in nodes thus imposes an important limitation on size and/or age of L. laureolum individuals, and explains the apparent trade-off between longevity and degree of ramification in the Cape Proteaceae.
Open Access
Ecological differentiation and the evolution and maintenance of fynbos diversity
(2011) Slingsby, Jasper; Verboom, George Anthony; Cramer, Michael D; Midgley, Jeremy J
Preserving the structure and functioning of ecosystems and the benefits they provide to society depends on our understanding of how biodiversity influences ecosystem function and which processes determine the composition and diversity of biological assemblages. Ecological, evolutionary and biogeographical processes interact to determine the genesis, maintenance and spatio-temporal arrangement of biodiversity. Here I investigate these processes in the Cape fynbos of South Africa by examining the diversification, biogeography and community assembly of a clade of reticulate-sheathed Tetraria species (Cyperaceae:Schoeneae).
Open Access
Examining patterns of coexistence in the Cape genus Tetraria (Cyperaceae) from a phylogenetic perspective : tracing the history of community assembly processes
(2004) Slingsby, Jasper; Verboom, George Anthony
The coexistence of species is fundamentally important in maintaining high species diversity in a defined area, and is partly responsible for the remarkable diversity of the Cape Floristic Region. The ecological attributes that determine the community assembly processes of competitive interactions and ecological sorting are subject to phylogenetic constraint and niche conservatism, suggesting that patterns of coexistence should be phylogenetically structured. This study compares patterns of coexistence against phylogenetic and ecological divergence in the genus Tetraria (Cyperaceae), and related sedges in the tribe Schoeneae, at different spatial and phylogenetic scales in multiple communities across the Cape Fynbos Biome. The investigation is based on coexistence data inferred from plot data from 13 phytosociological studies, ecological distances based on plant functional traits, and phylogenetic distances based on a molecular phylogeny of the species in question. Species coexisting in plots are significantly less related than expected on the basis of chance, and plots of phylogenetic distance against coexistence show triangular relationships, implying coexistence between closely related species is restricted, but that coexistence levels between more distantly related species may vary greatly. Quantification of these triangular relationships was problematic due to the small sample sizes and the low power of the nonparametric tests used. The pattern is more pronounced when a closely related subset of the species was used in a separate analysis, suggesting that phylogenetic scale is important. Coexisting species are significantly more closely related at the study than at the plot scale, implying relaxation of the effects of competitive interactions at coarser spatial scales. Significant positive correlation between ecological and phylogenetic divergence implies that phylogenetic constraint and niche conservatism has a strong effect on the ecological attributes of the study species. These results provide support for the hypothesis that the patterns of coexistence among species' in the genus Tetraria, and related sedges in the tribe Schoeneae, are influenced by phylogenetic constraint and niche conservatism on ecological attributes.
Open Access
Hydraulic trait variation in Protea repens with change in climate in space and time
(2016) Kellermann, Jacobus; February, Edmund C; Slingsby, Jasper
Global climate change and atmospheric CO₂ concentrations are affecting all levels of biodiversity in a number of ways. For example, the unique vegetation of the Cape Floristic Region (CFR) is expected to experience increased temperatures while rainfall becomes more seasonal, resulting in stronger summer drought with greater hydraulic stress in plants. Increased CO₂ concentrations, on the other hand, are expected to relieve hydraulic stress in plants that utilize the C3 pathway for photosynthesis (most fynbos species), by reducing the amount of time they have to keep their stomata open to take up the CO₂ they require. Observed weather data suggest that rainfall has remained relatively stable over the last 21 years, while temperatures for the region have increased marginally. Here I explore variation in the hydraulic traits (leaf and xylem anatomy) of Protea repens (L.) across a spatial climatic gradient in the CFR relative to a common garden experiment. I then compare the contemporary trait-climate relationships with a 21 year old xylem anatomy dataset. In the common garden experiment I explore xylem and leaf trait variation in P. repens from thirteen populations representing a gradient in temperature and mean annual precipitation. Because trait-gradient relationships can be confounded by genetic differences between populations along the gradient, I used a common garden experiment to test the degree to which trait variation was genetically constrained among populations. My results show that xylem vessel diameters and an estimate of hydraulic conductance increased with increases in maximum temperature and soil moisture days across the spatial gradient. My results for the common garden experiment does however show genetically constrained intra-specific differences in xylem vessel morphology between populations. Despite this, differences in xylem vessel and leaf morphology between plants in the common garden and their source locality demonstrate that P. repens has some ability to respond to changes in the environment through phenotypic plasticity. To determine the response of P. repens to changes in climate over the past 21 years, I compared contemporary xylem anatomy to an existing dataset collected from the same sites in 1994. My results show no significant change in vessel diameters since 1994 even though temperatures and atmospheric CO₂ have increased, with no change in rainfall amount. These results suggest that either P. repens is not experiencing increased drought stress under current climate conditions, or that xylem vessel anatomy is not a good proxy for small changes in drought stress in this species. The effect of increased drought stress due to higher temperatures and associated evaporative demand may be alleviated by increased atmospheric CO₂ reducing the amount of time the plants have to keep their stomata open to take up the CO₂ they require. Similarly, it is possible that drought stress has not changed substantially over this period, because a reduction in wind run across the CFR may have balanced the increase in evaporative demand created by higher temperatures. At a plant level, P. repens may not respond to small increases in drought stress by utilising deep water. In addition, P. repens is potentially able to reduce stomatal conductance thereby alleviating xylem anatomical responses to the small change in temperature since 1994.
Restricted
Radiation and Repeated Transoceanic Dispersal of Schoeneae (Cyperaceae) Through the Southern Hemisphere
(Botanical Society of America, 2013) Viljoen, Jan-Adriaan; Muasya, A Muthama; Barrett, Russell; Bruhl, Jeremy; Gibbs, Adele; Slingsby, Jasper; Wilson, Karen; Verboom, Anthony
• Premise of the study: The broad austral distribution of Schoeneae is almost certainly a product of long-distance dispersal. Owing to the inadequacies of existing phylogenetic data and a lack of rigorous biogeographic analysis, relationships within the tribe remain poorly resolved and its pattern of radiation and dispersal uncertain. We employed an expanded sampling of taxa and markers and a rigorous analytic approach to address these limitations. We evaluated the roles of geography and ecology in stimulating the initial radiation of the group and its subsequent dispersal across the southern hemisphere. • Methods: A dated tree was reconstructed using reversible-jump Markov chain Monte Carlo (MCMC) with a polytomy prior and molecular dating, applied to data from two nuclear and three cpDNA regions. Ancestral areas and habitats were inferred using dispersal–extinction–cladogenesis models. • Key results: Schoeneae originated in Australia in the Paleocene. The existence of a “hard” polytomy at the base of the clade reflects the rapid divergence of six principal lineages ca. 50 Ma, within Australia. From this ancestral area, Schoeneae have traversed the austral oceans with remarkable frequency, a total of 29 distinct dispersal events being reported here. Dispersal rates between landmasses are not explicable in terms of the geographical distances separating them. Transoceanic dispersal generally involved habitat stasis. • Conclusions: Although the role of dispersal in explaining global distribution patterns is now widely accepted, the apparent ease with which such dispersal may occur has perhaps been under-appreciated. In Schoeneae, transoceanic dispersal has been remarkably frequent, with ecological opportunity, rather than geography, being most important in dictating dispersal patterns.
Open Access
Satellite change detection in the albany thicket biome
(2025) Mahlasi, Craig; Moncrief, Glenn; Visser, Vernon; Altwegg, Andreas; Slingsby, Jasper
The Albany Thicket Biome has been subject to widespread transformation, with as much as 63% of the biome being severely degraded. The primary land use activity responsible for much of the transformation of the biome has been pastoralism and commercial agriculture land expansion (Mills et al., 2005; Powell, 2008; Stickler and Shackleton, 2015). There are primarily four traditional remote sensing based change detection frameworks: algebra, transformations, classification, and advanced models (van Oort, 2007, Asokan and Anitha, 2019). While these methods are able to detect changes in bi-temporal datasets they are inherently limited in that they are based on the assumption that each pixel's spectral signature is a linear combination of the features on the corresponding physical surface (Salih et al., 2017, Sun et al., 2015). These methods also suffer from the propensity for false positives resulting from differences in atmospheric conditions, viewing angles and illumination and soil moistures between the two images; another limitation of these methods is the observation interval between the initial and post-change or successive observations are often weeks or up to years apart. which makes the detection of transient changes difficult. Finally they are unable to provide information on changes in land cover that allows for timely intervention by the authorities. Continuous change detection on the other hand uses all available and usable observations to detect changes. Continuous change detection classifiers allocate pixels throughout a time series to predefined classes using labelled training data. The majority of tools that seek to perform continuous land cover change detection have been developed for forests and thus tend to perform poorly in thicket ecosystems. This study aims to use multi-temporal satellite imagery to detect transformation of Albany Thicket in near-real time. The first chapter seeks to generate a Thicket transformation map documenting the changes in the Thicket biome between 2016 and 2019 and to produce an online application to visualise and interpret these changes. Chapter two focuses on developing a change detection protocol for identifying clearings of Thickets using Temporal Convolution Neural Networks and comparing it against the Continuous Change Detection and Classification (CCDC) algorithm. Finally chapter three sets out to develop a Domain adaptive Temporal Convolution Neural Network for continuous change detection in the Albany Thicket biome. The study concluded that using medium resolution satellite imagery changes in Albany Thicket vegetation can be reliably detected and discerned from changes in other land cover types. The ability to continuously detect changes using TempCNNs was shown to outperform a state of art algorithm namely, CCDC. Albany Thicket cover dynamics were shown to be embedded within geographical contexts and that geographical gradients in biophysical variables influence the contextual representations learned by the TempCNNs and therefore fusing TempCNNs with biophysical variables such as surface dryness information can improve their performance. Finally, it was shown that using meta-learning the TempCNN can be adapted to be robust in shifting domains by learning the most optimal parameter initializations that allow for capturing the invariant embeddings that facilitate generalisation across domains possible.
Open Access
The importance of hydrologic refugia for the diversity of the Cape Flora
(2025) Nhlapo, Kagiso; Slingsby, Jasper; Verboom, George
Refugia are sites with more stable climate and hydrology than the surrounding area. There is growing recognition of the need to conserve refugia due to their role in harboring rare and endemic plants that might otherwise be prone to extinction due to global change. These sites are considered crucial for preserving species richness in the Cape Floristic Region (CFR) of South Africa, an area renowned for its exceptional plant species diversity and endemism, which is increasingly threatened by the impacts of global change. Despite the importance of refugia in the CFR, very little is known about their distribution. To fill this knowledge gap, I identify the distribution and character of refugia (mesic or xeric) by mapping total species richness (TSR), the distribution of widespread species (WS), narrow-range endemics (NREs), and wetland dependent narrow-range endemics (WD-NREs) and explore the environmental correlates of these distributions. I make use of Quarter degree square grid cells as a measuring unit, which are larger (~20 km across), than the scale at which microrefugia typically occur, however, the broad environmental heterogeneity within these units provides a meaningful basis for identifying potential refugial areas. From this study I find that the spatial distributions of TSR, WS, NREs and WD-NREs exhibited topographic and longitudinal gradients, with more species found in the southwestern CFR and in the mountains. Differences were observed in their correlates, where productivity, environmental stability explained TSR and WS, whereas environmental heterogeneity and environmental stability explained NREs and WD-NREs. Hydrological stability (such as groundwater-fed wetlands and seeps) and climatic stability (climatic stability index) have been documented to be indicators of refugia. Where hydrologic refugia provide broad-scale stable conditions for endemics and were found to play a significant role in maintaining persistence species in the CFR. These refugial sites face significant threats from groundwater abstraction, invasive species, and global change, highlighting the need for detailed fine-scale mapping and conservation of microrefugia within the broader refugial areas. Protecting these sites is essential for conserving biodiversity and ecosystem function in the CFR.
Open Access
Tracking socioeconomic vulnerability using network analysis: insights from an avian influenza outbreak in an ostrich production network
(Public Library of Science, 2014) Moore, Christine; Cumming, Graeme S; Slingsby, Jasper; Grewar, John
BACKGROUND: The focus of management in many complex systems is shifting towards facilitation, adaptation, building resilience, and reducing vulnerability. Resilience management requires the development and application of general heuristics and methods for tracking changes in both resilience and vulnerability. We explored the emergence of vulnerability in the South African domestic ostrich industry, an animal production system which typically involves 3-4 movements of each bird during its lifetime. This system has experienced several disease outbreaks, and the aim of this study was to investigate whether these movements have contributed to the vulnerability of this system to large disease outbreaks. METHODOLOGY/PRINCIPAL FINDINGS: The ostrich production system requires numerous movements of birds between different farm types associated with growth (i.e. Hatchery to juvenile rearing farm to adult rearing farm). We used 5 years of movement records between 2005 and 2011 prior to an outbreak of Highly Pathogenic Avian Influenza (H5N2). These data were analyzed using a network analysis in which the farms were represented as nodes and the movements of birds as links. We tested the hypothesis that increasing economic efficiency in the domestic ostrich industry in South Africa made the system more vulnerable to outbreak of Highly Pathogenic Avian Influenza (H5N2). Our results indicated that as time progressed, the network became increasingly vulnerable to pathogen outbreaks. The farms that became infected during the outbreak displayed network qualities, such as significantly higher connectivity and centrality, which predisposed them to be more vulnerable to disease outbreak. Conclusions/Significance Taken in the context of previous research, our results provide strong support for the application of network analysis to track vulnerability, while also providing useful practical implications for system monitoring and management.
Open Access
Understanding Highly Pathogenic Avian Influenza outbreaks in Western Cape Ostrich industry: did network dynamics enhance vulnerability? Christine Moore.
(2012) Moore, Christine; Cumming, Graeme S; Slingsby, Jasper
Disease outbreaks in both domestic and wild systems in recent years indicate the increasing potential for disease spill-over of generalist pathogens between domestic and wild species. Events of this nature are of considerable threat to rare or endangered species, while also being of significant economic concern for the farming industry. Understanding how disease moves within and between these contrasting systems is vital to prevent large-scale, multi-system epidemics in the future. This study utilized network analysis to understand how the movement of ostrich stock between farm locations in the Western Cape, South Africa may have contributed an epidemic outbreak of Highly Pathogenic Avian Influenza (HPAI) within the ostrich industry in 2011.

Browsing by Author "Slingsby, Jasper"

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