OpenUCT :: Browsing by Author "Hoffman, M T"

Browsing by Author "Hoffman, M T"

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Open Access
Collapse of an iconic conifer: long-term changes in the demography of Widdringtonia cedarbergensis using repeat photography
(2016) White, J D M; Jack, S L; Hoffman, M T; Puttick, J; Bonora, D; Visser, V; February, E C
BACKGROUND: Conifer populations appear disproportionately threatened by global change. Most examples are, however, drawn from the northern hemisphere and long-term rates of population decline are not well documented as historical data are often lacking. We use a large and long-term (1931-2013) repeat photography dataset together with environmental data and fire records to account for the decline of the critically endangered Widdringtonia cedarbergensis. Eighty-seven historical and repeat photo-pairs were analysed to establish 20th century changes in W. cedarbergensis demography. A generalized linear mixed-effects model was fitted to determine the relative importance of environmental factors and fire-return interval on mortality for the species. RESULTS: From an initial total of 1313 live trees in historical photographs, 74% had died and only 44 (3.4%) had recruited in the repeat photographs, leaving 387 live individuals. Juveniles (mature adults) had decreased (increased) from 27% (73%) to 8% (92%) over the intervening period. Our model demonstrates that mortality is related to greater fire frequency, higher temperatures, lower elevations, less rocky habitats and aspect (i.e. east-facing slopes had the least mortality). CONCLUSIONS: Our results show that W. cedarbergensis populations have declined significantly over the recorded period, with a pronounced decline in the last 30 years. Individuals that established in open habitats at lower, hotter elevations and experienced a greater fire frequency appear to be more vulnerable to mortality than individuals growing within protected, rocky environments at higher, cooler locations with less frequent fires. Climate models predict increasing temperatures for our study area (and likely increases in wildfires). If these predictions are realised, further declines in the species can be expected. Urgent management interventions, including seedling out-planting in fire-protected high elevation sites, reducing fire frequency in higher elevation populations, and assisted migration, should be considered.
Open Access
Distribution quantitative morphological variation and preliminary molecular analysis of different growth forms of wild rooibos (Aspalathus linearis) in the northern Cederberg and on the Bokkeveld Plateau
(2010) Malgas, R R; Potts, A J; Oettlé, N M; Todd, Simon Wallace; Verboom, George Anthony; Hoffman, M T
Aspalathus linearis (Fabaceae) is endemic to the Cape Floristic Region in the Western Cape and Northern Cape Provinces of South Africa. The reddish leaves and stems, primarily of one cultivar, are used to make a commercially important tea which is marketed locally and internationally as ‘rooibos’ or ‘redbush’ tea. In historical times rooibos was collected in the wild. In the twentieth century cultivation of a single cultivar increasingly replaced wild harvest to meet growing demand. Recently, tea from wild forms of the species, which vary significantly in growth form and reproductive strategy, has been marketed by small-scale farmers in Wupperthal and on the Suid Bokkeveld plateau in the northern part of the species' distribution. Little information on the wild forms of this species has been published, although a rich body of knowledge exists amongst local harvesters and other land-users. In this study, we focus on the northern part of the species’ distribution area where wild rooibos is harvested for commercial sale to niche organic and fair-trade markets. We adopt a transdisciplinary approach to (1) document the different growth forms, (2) develop a bioclimatic model of the potential distribution of the species, (3) quantify the morphological variation that exists between growth forms relative to the established cultivar and (4) use molecular techniques to provide a preliminary insight into the infraspecific diversity of different wild A. linearis growth forms. Our results show that local land users in the region identify four main growth forms of wild A. linearis. These are an ‘erect form’ and a ‘prostrate form’ in the Wuppertal area, a ‘shrub form’ in the Suid Bokkeveld, and a ‘tree form’ that has been observed at specific sites at Wupperthal, Biedouw and the Suid Bokkeveld. The PCA analysis of seven morphological traits identified three growth forms, which support the land user descriptions except in the case of the 'tree' and 'erect' forms which co-occurred in coordinate space. Both shrub and prostrate forms are wider than they are taller and possess more stems closer to the ground than erect forms. While the stems of both shrub and prostrate forms lie relatively flat on the ground, stem thickness is significantly greater in shrub forms. The tree type, the erect form and the cultivar studied possess the highest harvestable biomass. Prostrate forms and shrub forms resprout after fire while erect and tree forms regenerate from seed only. Haplotypic variation was assessed using DNA sequences from a single chloroplast region and revealed strong genetic differences between the different growth forms. Although preliminary, there is some evidence that sprouting and nonsprouting forms of the species are genetically isolated. This has important taxonomic implications for the species. Additional chloroplast regions and a nuclear region were also identified as variable and potentially useful markers for a multi-locus molecular approach to studying taxonomic and ecological questions within the species.
Open Access
Drought, Climate change and Vegetation response in the succulent karoo, South Africa
(2009) Hoffman, M T; Carrick, P J; Gillson, L; West, A G
For the winter-rainfall region of South Africa, the frequency of drought is predicted to increase over the next 100 years, with dire consequences for the vegetation of this biodiversity hotspot. We analysed historical 20th century rainfall records for six rainfall stations within the succulent karoo biome to determine if the signal of increasing drought frequency is already apparent, and whether mean annual rainfall is decreasing. We found no evidence for a decrease either in mean annual rainfall or in the incidence of drought, as measured by the Standardised Precipitation Index (SPI) over the 20th century. Evidence points to a drying trend from 1900-1950 while no significant trend in rainfall and drought was found at most stations from 1951-2000. In a second analysis we synthesised the information concerning the response of adult succulent karoo biome plants and seedlings to extended drought conditions. General findings are that responses to drought differ between species, and that longevity is an important life history trait related to drought survival. Growth form is a poor predictor of drought response across the biome. There was a range of responses to drought among adult plants of various growth forms, and among non-succulent seedlings. Leaf-succulent seedlings, however, exhibited phenomenal drought resistance, the majority surviving drought long after all the experimentally comparative non-succulent seedlings had died. Our synthesis showed that previous studies on the impact of drought on succulent karoo biome plants differ greatly in terms of their location, sampling design, measured values and plant responses. A suite of coordinated long-term field observations, experiments and models are therefore needed to assess the response of succulent karoo biome species to key drought events as they occur over time and to integrate this information into conservation planning.
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'Enough to be considered useful': John Acocks' contribution to South African botany
(2003) Hoffman, M T; Cowling, R M
John Acocks died in 1979 after working for nearly 50 years as a botanist in South Africa. His scientific contributions have had a profound influence on southern African ecology and this introductory article which includes a biographical sketch of his life, places his contribution in perspective. The 10 articles which appear in this issue have been prepared not only as a tribute to his contribution but also as an assessment of its current utility. Scientific thought is constantly changing and an attempt has been made to indicate where Acocks’ views are still supported by current evidence and where revision is necessary. His contribution lay in three main areas. Firstly, his description of the vegetation (or Veld Types) of South Africa remains a work of considerable importance and will probably never be repeated at the same level of scale, by a single individual, again. Three articles show that when assessed at a smaller scale his Veld Type concept is robust in some, but not all, cases. Differences between Acocks’ views and current treatments of the data are highlighted for the Bankenveld and for the vegetation of the Subtropical Thicket and Nama-karoo biomes. Acocks’ second important influence concerns his views on precolonial vegetation and the extent of human impact. He could not have presaged the explosion of palaeoenvironmental techniques which have arisen since his death and which have provided a somewhat different view of pre-colonial environments from the one he articulated, particularly with regard to the influence of fire on grasslands, savannas and forests. Although his views on the expansion of the eastern Karoo are not upheld in the light of recent findings there is good evidence for significant changes in the fauna and flora of the Karoo over the last 300 years. Acocks’ final contribution lay in the field of veld restoration and grazing management where he continues to have an influence on a small sector of the farming and range science community today. Not only has John Acocks provided a rich theoretical framework for scholars to test but he has also left a legacy of well-organised data for future generations. The final two papers in this special issue indicate the utility of his archives which will gain in value over the years. Challenges to Acocks’ views will continue apace and much of what he has written will undoubtedly be revised. This doesn’t detract from his contribution in any way but serves to highlight the importance of the foundation that he created during his lifetime and which we acknowledge in this special issue.
Open Access
Lack of polymorphism suggests a recent bottleneck of Aloidendron pillansii: Exploration into the population of Aloidendron pillansii from phylogeographical analysis of molecular data
(2014) Josephs, Randall Evan; Hedderson, Terry A; Hoffman, M T
The Karoo-Namib is a species rich region in which many iconic and keystone species are found, such as Aloe pillansii. The recent population history of A.pillansii is poorly understood. However the suggested climatic shifts that occurred throughout the Holocene era may have affected its distribution, demographics and gene flow. The glacial/interglacial refugia hypothesis predicts that the southernmost population served as a refuge population and that the subsequent expansion of the population was to the north in concert with the northward expansion of the winter rainfall regime. I evaluated this hypothesis by linking the molecular data (cpDNA and nDNA) of 84 individuals from three main populations with phylogeographical techniques. Based on the combination of percentage of mutations percentage per million years range and the chloroplast sequences, it has been estimated that the time of divergences was between 3.45 to 16.67 million years ago. The molecular analysis identified a significant lack of genetic diversity within and among the three dominant populations of A.pillansii. This suggests that the species experienced a severe bottleneck event prior to its recent expansion that has been suggested to have occurred within the time frame of 100 to 1000 years ago. This pattern is compared with its sister taxa Aloe dichtotma, which possesses variation within and among its populations. The lack of genetic variation evident within A. pillansii leaves it vulnerable to future climate shifts as low genetic variation within a species lowers the ability of that species to adapt to both environmental and climatic changes. This thesis has provided a brief insight into the population history of A.pillansii, but further research is needed.
Open Access
'Nature's method of grazing': Non-Selective Grazing (NSG) as means of veld reclamation in South Africa
(2003) Hoffman, M T
Acocks was concerned with the past, present and future state of South Africa’s vegetation and in the 1960’s, together with several farmers in the eastern Karoo, developed a grazing system which he thought would restore the vegetation to its former pristine condition. Acocks felt that the grazing systems advocated by the Department of Agriculture at the time were partly responsible for the degraded vegetation of the region as these systems encouraged livestock to graze selectively, thereby overgrazing the more palatable species in the vegetation. He felt that by forcing animals to graze all species non-selectively, the more palatable elements would be able to out-compete the less palatable species and dominate the vegetation as he believed they once did in pre-colonial times. Acocks found theoretical support for his argument which also relied on relatively long rest periods between grazing events and suggested that this non-selective grazing system simulated the way in which the pre-colonial ungulate herds utilised the vegetation. Although Acocks never conducted the key experiments needed to test his ideas, his approach was supported by several farmers in the eastern Karoo who conducted trials on their farms to test the principles of the method. The approach advocated by Acocks, however, was in direct contrast to that proposed by the Department of Agriculture who were concerned about the comparatively high stocking rates advocated under Acocks’ Non-Selective Grazing (NSG) system. Their own experiment on NSG found that it reduced plant cover and increased erosion and they believed that it would lead to further widespread degradation if implemented. Although Acocks was employed by the Department of Agriculture as a Botanical Survey Officer he was not a Pasture Research Officer and it was this latter group of employees who had the responsibility of researching and advocating appropriate grazing systems for South Africa’s rangelands. Acocks was, therefore, instructed not to promote NSG in his official capacity. Despite this, Acocks’ writing in the last ten years of his life is infused with the ideas of NSG which continue to influence the development of range management systems to the present.
Open Access
North-South cooperation through BIOTA: An interdisciplinary monitoring programme in arid and semi-arid southern Africa
(2006) Krug, C B; Esler, K J; Hoffman, M T; Henschel, J; Schmiedel, U; Jurgens, N
Connecting disciplines in a north– South collaboration has many advantages: mutualisms evolve, synergies are created and spin-offs emerge. The BIOTA South (Biodiversity Monitoring Transect Analysis in southern Africa) programme, with its long-term vision to generate knowledge of biodiversity along a north–south transect in southern Africa, is providing opportunities for research, technology transfer and capacity building while focusing on potential solutions for critical challenges that face this environmentally vulnerable part of the subcontinent
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The phenological pattern of vegetation in Namaqualand, South Africa and its climatic correlates using NOAA-AVHRR NDVI Data
(Taylor & Francis, 2005) Fox, S C; Hoffman, M T; Hoare, D
Remote sensing techniques were used to reveal the vegetation patterns in the greater Namaqualand area and to relate them to climatic variables. We related the normalised difference vegetation index (NDVI) to a range of different rainfall and temperature indices. Mean annual NDVI is significantly related to precipitation and potential evapotranspiration (PET). A multiple regression model explains 52% of the variance when Mean Annual NDVI is related to climatic variables. Mean NDVI in August (the month of maximum NDVI in most of Namaqualand) is significantly related to PET and the current plus two previous months of precipitation. A multiple regression model for mean NDVI in August and climatic variables explains 57% of the variance. We also assessed the annual and seasonal NDVI pattern exhibited within seven different vegetation types. The climatic variables influencing the summer rainfall and winter rainfall vegetation types differ significantly from each other. All vegetation types have a peak NDVI signal in August although Mountain Fynbos and North Western Mountain Renosterveld values are significantly higher than the rest. Upland, Lowland and Strandveld Succulent Karoo exhibit similar NDVI responses while the two summer rainfall vegetation types (Bushmanland and Orange River Nama Karoo) have a very low NDVI signal throughout the year. The results suggest that NDVI can be used successfully to detect growth and phenology patterns within different vegetation types in Namaqualand.
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The nature, extent and causes of land degradation in South Africa: legacy of the past, lessons for the future?
(2002) Meadows, M E; Hoffman, M T
The aim of this paper is to reassess the land degradation and desertification problem in South Africa. The country has a wide range of interrelated environmental and development challenges existing in a socio-political context recently subject to substantial change. The paper outlines the fundamental environmental constraints and opportunities that underlie degradation phenomena and illustrates the nature, extent and geographical distribution of the major forms of soil and vegetation degradation in the country. The principal causes of degradation are identified and some important land use and land tenure questions that need to be addressed in the near future are posited.

Browsing by Author "Hoffman, M T"

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