Browsing by Author "Hoffman, Michael Timm"

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    Open Access
    Above ground perennial plant biomass across an altitudinal and land-use gradient in Namaqualand South Africa
    (2010) Anderson, Phillipa Mary Levick; Hoffman, Michael Timm; O'Farrell, P J
    This study set out to generate estimates of the standing perennial biomass for six different vegetation types, and associated upland and lowland habitats, across the altitudinal gradient presented by the Kamiesberg mountain range in the Namaqualand region of the Northern Cape Province of South Africa. Volume-biomass regressions, established for 94 perennial species accounting for 70–80% of the plant cover, were used to generate these estimations. Comparisons to other studies give similar findings, corroborating the method adopted. Biomass was found to vary significantly in relation to the altitudinal, and associated rainfall, gradient, as well as by habitat type where the rocky uplands have considerably more biomass that the sandy lowland habitats. An examination of the impact of sustained heavy grazing associated with a communal rangeland on this standing perennial biomass, showed a significant decrease in on the lowland habitats. This loss in biomass is principally of palatable species, with no evidence of a response in terms of perennial biomass. These findings point to degradation on the lowland habitats of the communal rangeland, with negative consequences for livestock farmers in the region.
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    Open Access
    Conserving living landscapes: investigating the impacts of livestock grazing and assessing rangeland restoration potential in Overberg Renosterveld, South Africa
    (2021) Poulsen, Zoë Chapman; Chimphango, Samson; Hoffman, Michael Timm; Anderson, Pippin; Muasya, A Muthama
    Biodiversity is declining faster than at any other time in the Earth's history, driven mostly by land use change and degradation. Overberg Renosterveld, some of the most species diverse mediterranean type shrublands, are no exception with about 95% of their original extent lost to agriculture. Historically, large herds of indigenous grazing mammals roamed these landscapes. Today the Overberg's agricultural lands are fragmented by land cover change and divided by fences. In the contemporary landscape animals, largely domestic livestock, and plant resources are closely coupled, and overgrazing of remaining renosterveld fragments a significant threat, with potential to cause irreversible damage. The Conservation of Agricultural Resources Act (CARA) (Act 43 of 1983) states that farmers must not exceed the grazing capacity of the veld unless it is protected against deterioration and destruction, and that any land that is degraded or denuded must be effectively restored or reclaimed. Despite this legislation, there is little empirical research on the impacts of livestock grazing on renosterveld, as well as on restoration of overgrazed areas. It was the aim of this thesis to contribute to this gap in understanding. The thesis assessed the role of grazing by different livestock types, namely cattle and sheep, on biodiversity, the soil seed bank, and the restoration potential of renosterveld vegetation from resting the veld. The effect of livestock grazing by sheep and cattle on plant species richness and diversity and growth form diversity was assessed using Modified Whittaker plots and presented in Chapter 3. It was hypothesised that livestock grazing by cattle would have less effect on species richness and diversity and growth form diversity than sheep grazing and that both cattle and sheep grazing would lead to a reduction in species richness and diversity in comparison to renosterveld sites with a treatment of no grazing. Thirty sites where either no grazing has taken place or that have been grazed by cattle or sheep were selected with sites being evenly distributed between Eastern, Central and Western Rûens Shale Renosterveld. At each of the thirty sites, vegetation data were collected from a series of nested subplots of ten 1 m2 , two 10 m2 and one 100 m2 subplots nested within a 1 000 m2 plot. One soil sample was also collected from each 1 000 m2 plot to a depth of 10 cm for nutrient analysis. Findings revealed that sites grazed by sheep had significantly lower plant species richness (median richness = 29 species, mean Shannon-Weiner = 3.39) and diversity when compared to sites with a treatment of no grazing (median richness = 49 species, mean Shannon Weiner = 3.83). Sites with a treatment of no grazing had significantly higher richness of geophyte species (mean = 14.7) than sites grazed by cattle (mean = 7.0) and sheep (mean = 7.1) during the study. The results obtained were in line with the hypothesis that livestock grazing by sheep resulted in a reduction in species richness and diversity and vegetation cover in Overberg Renosterveld in comparison to sites where no grazing has taken place. Sites with a treatment of no grazing showed higher species richness and vegetation cover of non-succulent shrubs, annual forbs and perennial forbs than sites grazed by sheep. It was concluded that livestock grazing of Overberg Renosterveld by sheep needs to be done with care. This can be done by adopting a passive adaptive management approach. Here one set of management protocols can be developed and implemented and through science-based monitoring to inform management, these can be adapted as needed based on the key findings. Chapter 4 investigated ecosystem resilience and the restoration potential of Overberg Renosterveld through an exploration of its soil seed bank as a source for potential recovery. A glasshouse germination experiment investigated the effect of livestock grazing by cattle and by sheep in comparison with a grazing treatment of no grazing on the soil seed bank in Overberg Renosterveld, as well as the similarity between the standing vegetation and the soil seed bank. It was hypothesised that cattle and sheep grazing would reduce species richness, species diversity and growth form diversity in the soil seed bank in comparison with sites with a treatment of no grazing. Soil samples were collected from 30 sites that were also used in Chapter 3. The soil was then spread on top of a 6 cm layer of compost in seed trays, and smoke treated to enhance germination. Seedlings were assigned to growth form categories including forbs, geophytes, annuals, graminoids, succulent shrubs and nonsucculent shrubs and then identified to family, genus or species level. The results of the soil seed bank study were correlated with the vegetation results from Chapter 3 to examine the relationship between the standing vegetation and the soil seed bank. A total of 48% of taxa in the standing vegetation had seed present in the germinable seed bank. However, there were no differences in species richness, species diversity or number of individuals between grazing treatments. The results indicated that livestock grazing has a far less significant impact on the composition, species diversity and growth form diversity of the soil seed bank in Overberg Renosterveld than hypothesised. Instead, the results showed that there was a well-developed seed bank comprising mainly indigenous species with a variety of different growth forms including palatable grasses and shrubs. This indicates that Overberg Renosterveld vegetation has high restoration potential. Chapter 5 showed results on the effects of livestock grazing by cattle and sheep over time on plant species richness, diversity and growth form diversity in comparison with sites protected from grazing. Following collection of a baseline dataset, four years of follow up data were collected. A total of 22 fenced plots across Western, Central and Eastern Rûens Shale Renosterveld had a baseline dataset collected prior to being monitored on an annual basis over four years in grazed/ungrazed paired plots. Results on vegetation recovery from the fenced exclosures showed a significant increase in plant cover over time at sites that were not grazed. Mean species richness increased from 20.6 species to 25.4 species at sites with no grazing. Mean vegetation cover increased from 71% at T0 (the baseline time step) to 120% at T4 (the final time step) at the end of the study. Sites grazed by sheep had a decrease in vegetation cover over time each year from T0 to T4 from 75% to 50%. Results from a linear mixed model revealed that species richness between grazing treatments was significantly different at all time steps in the study. However, the significant differences were primarily due to comparisons between grazed sites and sites with a treatment of no grazing. Therefore, livestock grazing by sheep has a significant effect on renosterveld vegetation over time. Findings from this component of the study indicates that Overberg Renosterveld degraded by continuous heavy grazing has significant passive restoration potential by fencing renosterveld patches to facilitate more effective grazing management. Most of the renosterveld of the Overberg has been lost through habitat transformation for agriculture, and the future of that which remains is uncertain. This thesis affirms concerns around the impact of livestock grazing and shows the importance of improved ecological understanding around grazing management. Grazing by sheep was shown to cause greater impacts on renosterveld than other domestic livestock studied and is therefore a threat to renosterveld. These findings warrant closer attention to management practices around sheep grazing. However, the state of renosterveld soil seed banks offer considerable hope. Findings revealed a diverse indigenous seed bank, showing that renosterveld degraded by overgrazing has high restoration potential. Furthermore, fencing renosterveld to exclude livestock improves species richness and diversity over time. These findings highlight the need for caution when grazing renosterveld. However, where the damage has been done, the potential for recovery is high. Harnessing the soil seed bank in combination with excluding livestock grazing by fencing are effective tools in this critically endangered vegetation for achieving restoration and conservation goals.
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    Open Access
    Expanding grasslands? Structural biome shifts in the dryland rangelands of the eastern Karoo revealed through long-term observation of climate, vegetation and land use change
    (2021) Arena, Gina; Hoffman, Michael Timm; van der Merwe, Helga; O'Connor, Timothy
    Biome shifts caused by climate and land use change threaten global dryland ecosystems and the provision of services needed to sustain human livelihoods. The overarching drivers of early 20th century dryland degradation in South Africa have been attributed to high stocking rates and overgrazing by domestic livestock. Predictions of an eastward encroachment of shrublands into the semiarid grasslands, and resultant declines in productivity, emerged in the 1950s based on concerns that livestock overgrazing would persist. In response to these concerns, appropriate research and government interventions were implemented to address rangeland and livestock mismanagement. However, the role of long-term climate as a driver of semiarid vegetation change was poorly understood, particularly for changes occurring at the interface of the NamaKaroo and Grassland biomes where shrubland encroachment could be detected along a gradient of increasing annual rainfall. It is this gradient that controls the natural biome transition from shrub-dominance to grass-dominance. However, the prediction of shrubland encroachment was refuted when research in the early 1990s revealed that the vegetation of this region (known as the ‘eastern Karoo') had undergone an increase in grass cover; a trend that persisted into the early 2000s. Questions around the influence of changing rainfall patterns on driving this increase in grassiness have since been investigated at a single location in the eastern Karoo. Findings indicated that an increase in annual rainfall drove the increase in grass cover, but the generality of this relationship across the broader rainfall gradient of the biome transition has not been examined. In addition to rainfall, grazing by domestic livestock has had a significant influence on the relative proportion of grasses and shrubs in the eastern Karoo. While other studies have alluded to changes in land use, few data have been presented and the relative influence of land use change has usually been under-estimated. The availability of historical data from past vegetation surveys, climate station records and magisterial district livestock censuses makes it possible to develop a more comprehensive synthesis of how vegetation has changed across the rainfall gradient in order to address questions around biome shifts. This thesis, therefore, aimed to determine whether a biome shift is related to a broader shift in the climate regime through the assessment of long-term vegetation change along the rainfall gradient, in relation to climate and land use change since the early 20th century. Annual and seasonal trend analysis was applied to monthly rainfall, temperature, pan evaporation, evapotranspiration and wind speed data. Trends indicated that 32% of sites in the eastern Karoo showed an increase in annual rainfall between 1874 and 2019, which was also defined by a significant shift to increasing early summer rainfall. A concomitant trend of decreasing annual, and late summer rainfall, was detected at 32% of sites located on the mesic end of the rainfall gradient. Monthly maximum temperature between 1971 and 2019 also increased by 1.0 °C at a rate of 0.04 ± 0.06 °Cy-1 and monthly minimum temperature decreased by 0.3 °C at a rate of -0.03 ± 0.11 °Cy-1 . Wind speed, an important driver of evaporative processes, decreased at 44% of sites since 1971 and decreasing trends in A-Pan evaporation were found at all sites. No discernible trends in evapotranspiration were detected. Declines in wind speed and evaporation were suggested to promote an increase in vegetation cover which may restore ecosystem processes and improve rangeland productivity. An analysis of multidecadal wet-dry phases in the climate record was also undertaken using the Standardised Precipitation Index. Through this, it was observed that the rainfall regime over the eastern Karoo is defined by three significant phases over the last century: an early wet phase (1886- 1902), a 70-year long dry phase (1902-1970), followed by the recent wet phase (1970-2014) that commenced with heavy rains in the mid-1970s across the summer rainfall region of southern Africa. Long-term changes in the cover and composition of plant species and growth forms were compared between 1962 and 2018 at 27 sites using the point intercept method. Changes in species composition were determined from Bray-Curtis similarity distances. The dominant growth forms were organised along a rainfall-edaphic gradient and plant communities were further defined by altitude. Hierarchical clustering of species cover per site recognised this underlying gradient. Thus, sites were categorised into three vegetation units: ‘Karoo', ‘Escarpment' and ‘Grassland'. Results showed that vegetation cover increased, and bare ground cover decreased, significantly at all but one site. Perennial grasses increased by 20%, 24% and 35% at Karoo, Escarpment and Grassland sites, respectively. Palatable dwarf shrubs increased by 15% at Karoo sites. These growth form changes translated to a broader-scale shift in the relative dominance of dwarf shrubs to grasses at 59% of sites in the Nama-Karoo biome, coinciding with the spatial pattern of increasing early summer rainfall and temperature. However, a multivariate analysis provided minimal evidence of species shifts between vegetation units or biomes, suggesting that compositional stability was maintained across the rainfall gradient. Percent changes in the cover of grasses and dwarf shrubs were analysed in relation to the climatic and edaphic parameters using multiple regression analysis, but no significant linear relationships were found. However, summer rainfall and temperature combined explained 26% of the variation in the percent change in grass cover. The effect of these changes on rangeland grazing capacity (ha/Large Stock Unit) was also evaluated using the ecological index method of the assessment of veld condition. There was a fivefold improvement in grazing capacity at Karoo sites and a twofold improvement at Escarpment and Grassland sites. Changes in the cover of grasses, dwarf shrubs and trees were additionally determined through fixed-point repeat photography at 85 sites on two different landforms, plains and hillslopes. A shift towards grass dominance relative to dwarf shrubs was estimated at 90% of sites located on the plains and at 100% of sites located on the hillslopes in the NamaKaroo. Variable trends were found at Grassland sites in both landforms. Tree cover remained unchanged in both biomes. Agricultural livestock census data at the magisterial district-level showed a fivefold decrease in sheep numbers between 1911 and 2017 while the number of cattle and equine animals remained largely unchanged. Goat numbers fluctuated over time in response to the demands for Angora mohair. Supporting perceptions of significant reductions in livestock numbers since the 1960s were conveyed by 18 local landowners as derived from semi-structured interviews. The introduction of conservation-friendly grazing systems, seasonal rainfall shifts, and increased drought frequency were also perceived as important drivers of vegetation change. However, a number of socio-ecological challenges, for example, livestock losses to theft, vermin, or fire, emerged as having significantly influenced management decisions. Therefore, the persistent structural shift in growth forms across the rainfall gradient since the 1960s has largely coincided with significant climatic shifts in rainfall and temperature. A secondary influence of long-term reductions in livestock numbers, and a change to conservation-friendly management practices, is suggested to have supported the recovery of vegetation, especially that of perennial grasses, and rangeland productivity.
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    Open Access
    Mapping desertification: towards an approach for mapping regional land degradation in drylands
    (2020) Bell, Wesley; Hoffman, Michael Timm; Visser, Vernon
    Land degradation in drylands (desertification) is an issue that potentially impacts nearly half of the world's human population living on over a third of the Earth's land surface. Despite global concern of the impact of desertification on people and the environment, there is no universal method to assess and map desertification. Methods to assess desertification at the local to regional scale that can fit into a broader global desertification narrative are more appropriate. The overall objective of this thesis is to assess regional desertification using field and Earth observation data for the Namaqualand Hardeveld bioregion of South Africa. Field data on the condition of the land from 277 plots was analysed using Latent Class Analysis (LCA) and found to cluster into three separate states. The first state (S1) was comprised primarily of degraded plots. The third state (S3), on the other hand, was comprised primarily of non-degraded plots, while the plots in state two (S2) generally fell between those which were assigned to S1 and S3. Through the LCA, each plot was assigned a probability of belonging to each state, and the most important variables in distinguishing the three states (perennial plant cover and bare ground cover) were identified. A total of 16 remote sensing variables were determined for the project area. Five vegetation indices (NDVI, EVI, SAVI, OSAVI, MSAVI), as well as spectral mixture analysis (SMA) cover estimates for perennial vegetation, bare ground and bare rock were calculated using both Landsat 8 and Sentinel-2A data. These variables were used in a series of Partial Least Squares regression (PLSr) models to predict either the probability of a plot belonging to one of the three latent states, or the field estimated perennial plant and bare ground cover. The best performing PLSr model had ten remote sensing variables predicting the field estimates of cover (R2Ycum = 0.592; Q2cum = 0.554). Both Sentinel-2A and Landsat 8 SMA cover estimates were better at predicting field cover than any of the vegetation indices. Estimates of bare ground and perennial plant cover were projected over the project area using the PLSr model and ground truthed using data from 61 independent field test plots. There was a significant correlation between the PLSr estimates and the field estimates for both perennial plant cover and bare ground cover for the test plots with the best correlation found to be between the PLSr estimate of bare ground and field estimated bare ground cover (r = 0.827, p < 0.001, CI [0.727, 0.893]). The trendline slope and percentile range of a time series of the Landsat SMA bare ground estimate were used to create raster images. These images, along with images for the PLSr bare ground and perennial plant cover estimates, were converted into images representing membership values between zero and one for the habitat condition archetype. These three images were then combined to produce one raster representing the overall membership of the project area to the habitat condition archetype. The importance of five potential drivers of land degradation (elevation, slope aspect, slope steepness, rainfall trend, and land tenure) in predicting PLSrestimated perennial plant and bare ground cover were evaluated using a random forest model. All drivers were found to be important predictors of cover and were included in the construction of the final, multi-band archetype image. If habitat condition classes are designated according to the mean archetype membership value ± one / two standard deviations, then 17% of the project area could be considered moderately degraded, with just over 3% severely degraded. This novel method of assessing and mapping desertification leads to improved accuracy in predicting habitat condition in the context of potential drivers of change. The utility of SMA over traditional vegetation indices is supported for this particular environment. This methodology can be improved with better endmember designation as well as improved spatial data on the potential drivers of change in drylands. The archetype approach ensures less subjectivity in map production, and the retention of pertinent information in map products. The approach developed in this thesis will allow for more accurate desertification reporting for UNCCD member states and will ultimately improve efforts to combat desertification globally.
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    Open Access
    Modelling the complex dynamics of vegetation livestock and rainfall in a semiarid rangeland in South Africa
    (2010) Richardson, F D; Hoffman, Michael Timm; Gillson, Lindsey
    Predicting the effect of different management strategies on range condition is a challenge for farmers in highly variable environments. A model that explains how the relations between rainfall, livestock and vegetation composition vary over time and interact is needed. Rangeland ecosystems have a hierarchical structure that can be described in terms of vegetation composition, stocking rate and rainfall at the ecosystem level, and the performance of individual animals and plants at the lower level. In this paper, we present mathematical models that incorporate ideas from complex systems theory to integrate several strands of rangeland theory in a hierarchical framework. Compared with observed data from South Africa, the model successfully predicted the relationship between rainfall, vegetation composition and animal numbers over 30 years. Extending model runs over 100 years suggested that initial starting conditions can have a major effect on rangeland dynamics (divergence), and that hysteresis is more likely during a series of low rainfall years. Our model suggests that applying an upper threshold to animal numbers may help to conserve the biodiversity and resilience of grazing systems, whilst maintaining farmers’ ability to respond to changing environmental conditions, a management option here termed controlled disequilibrium.