Browsing by Author "Bond, William John"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Open Access
    Determining the potential drivers of invasive C4 grasses at De Hoop Nature Reserve, South Africa
    (2018) Munyai, Nthabeliseni Meryling; February, Edmund Carl; Bond, William John
    Grassland and shrubland distribution in SA is associated with rainfall seasonality. In grassland the vegetation is exclusively C4 while the shrub and tree component is generally C3. Shrublands, in contrast, are predominantly C3, for both woody and herbaceous species, though there can be a small C4 sedge and grass component. The C4 grassy biomes dominate the warm season rainfall region while C3 grasses predominate in the shrublands of the cool season rainfall regions. The C4 grasses are poorly competitive in cold climates. There are however anomalous patches of C4 grasses in cool season rainfall regions dominated by fynbos shrublands such as those at Potberg, De Hoop Nature Reserve, Overberg region, South Africa. Although the southern Overberg region receives rainfall year-round, more than half of the rain falls in the cooler months of the year. These C4 grassland patches appear to be invading the fynbos even on the nutrient poor podsols. The main objective of this study was to gain a better understanding of the ecology of these anomalous C4 grasslands in a fynbosdominated region. I first explored the role of roads as a conduit for grass invasion by sampling sites adjacent to and further away from the road verge. This was done by identifying all species in plots 5 m (Roadside pots) and 100 m (Adjacent plots) from the road verge every 500 m along a 14 km management road at Potberg. My results show that the most common C4 grasses were Cynodon dactylon and Eragrostis curvula whilst the most common C3 grass was Merxmuellera disticha. Both Roadside and Adjacent plots had more C4 grass species than C3 grasses. The frequency distribution of both C3 and C4 grass species was significantly higher on the Roadside plots than on the Adjacent plots. Although there was a higher incidence of C4 grass occurrence on the Roadside plots I conclude that, roadsides are not the conduit for C4 grass invasion into fynbos as several large patches of the C4 grass Eragrostis curvula and Imperata cylindrica can be found several kilometres away from the roadside and there are fewer C4 grasses away from the road verge. I then explored the impacts of C4 grasses on fynbos species diversity by identifying all the plants in 100 plots, half in fynbos-dominated vegetation and half in grassy patches. My results show that C4 grasses had a negative effect on fynbos species richness. The areas invaded by the C4 grass had fewer fynbos species (average of 7) compared to areas with C3 grasses (average 17). Some C4 grasses had more of an effect than others with some of the most aggressive the clonally spreading species Imperata cylindrica, Setaria sphacelata var. torta and Cynodon dactylon. C4 grasses such as Aristida diffusa and Themeda triandra occur naturally in the fynbos with little impact on fynbos species richness. Eragrostis curvula, a tall bunch grass, was also locally dominant in places at the expense of fynbos species diversity. To examine the drivers influencing colonisation of fynbos by C4 grass, I set up a transplant experiment designed to examine the extent to which topographic position along a slope (moisture availability), an increase in nutrient availability and a decrease in competition for resources influences the establishment and growth of C4 grass. To do this I transplanted tufts of Setaria sphacelata var. torta in three positions along a slope, with and without nutrient additions and with and without competition for soil resources. I removed competition for resources by trenching and inserting a plastic sheath around each grass tuft to a depth of 500 mm. My results show an increase in mortality in drier treatments at the top of the slope. Trenching and fertiliser addition had significant positive effects on grass biomass and negative effects on root shoot ratios. Slope position however only had significant effects on root shoot ratios. Plants at the top of the slope had significantly higher root/shoot ratios than those plants at the bottom (P<0.05). Trenching did not have a significant effect on root shoot ratios (P>0.05). Based on the results of this research, it can be concluded that the grass patches have a negative effect on fynbos species diversity. These grass patches are not a remnant from when the reserve was still a farm but are relatively new and are not invading out from the roads. I examined rainfall seasonality from 1910 to 2015 and found no change. My results from trenching show that physical disturbance that breaks up fynbos root systems will help promote C4 grass invasion. Thus, any activities that increases physical disturbance of the fynbos such as off-road vehicles and trampling by large mammals must be avoided to maintain intact fynbos. While this has not occurred at Potberg, I speculate that because of fire suppression, there has been a decline in species diversity and a reduction in competition for resources allowing the C4 grasses to establish and dominate. The impact of fire on C4 grass needs more research, to understand if too frequent or less frequent fire will disturb fynbos and allow invasion by C4 grasses.
  • Thumbnail Image
    Item
    Open Access
    Holocene vegetation history and environmental change in the forest-grassland mosaic of the Central Highlands of Madagascar
    (2022) Razafimanantsoa, Andriantsilavo Hery Isandratana; Gillson, Lindsey; Bond, William John
    The origin and classification of open and mosaic ecosystems, particularly in the tropics and subtropics, have led to controversy worldwide. This has affected biodiversity conservation and, in some cases, promoted the establishment of afforestation projects based on the assumption that open and mosaic ecosystems are degraded forests. Although this initiative can have benefits in terms of carbon storage and climate mitigation if carefully planned and managed, it can also cause biodiversity loss and degradation when afforestation takes place in areas that were previously open ecosystems, or where unsuitable species are used. Madagascar, a world biodiversity hotspot, is one of the countries targeted for the implementation of afforestation projects. The Central Highlands of Madagascar, dominated by grassland matrix with forest patches, is the main region targeted. The nature and origin of the landscape are hotly debated, however, and it is not clear whether these open ecosystems are ancient or anthropogenically derived. Understanding of landscape history is therefore required to identify and conserve ancient open ecosystems, and to distinguish them from areas that have been deforested by people. This research aims to reconstruct the vegetation history and environmental change in the Central Highlands of Madagascar during the Holocene using palaeoecological methods, in order to inform appropriate conservation and management plans. We provide new records of vegetation, hydrological change, fire and herbivory activities by using a multiproxy approach, which includes fossil pollen, stable carbon isotopes, diatoms, charcoal and coprophilous spores, that allows for a comprehensive investigation into the history and drivers of vegetation change. Sediment cores were collected from two sites, Tampoketsa-Ankazobe wetland and Lake Dangovavy, located in the eastern and western slopes of the highlands, respectively. Results indicated that the surrounding area of both sites was composed of mosaic ecosystems, comprising of forest patches of variable extent in a matrix of open grassland and ericoid shrubland vegetation, at least from the Early and Mid-Holocene to ca. 1000 cal years BP, driven mainly by climate variability and fire occurrence. In Tampoketsa-Ankazobe wetland (eastern slopes), the vegetation was characterised by a mosaic of ericoid shrubland and mid-elevation forest taxa, between ca. 11 200 and 8300 cal years BP, under warm/wet period and low fire occurrence. The vegetation in the area changed to a mosaic of ericoid shrubland with more dominance of high-elevation forest from ca. 8300 to 1000 cal years BP under a drier climate and consistent low fire occurrence. The abundance of shrubs and trees during those two periods were confirmed by the dominance of C3 plants as reflected by the stable carbon isotopes results, and coincided with low herbivory activities in the TampoketsaAnkazobe site from ca. 11 200 to 1000 cal years BP. In parallel, the pollen record from Lake Dangovavy (western slopes), between ca. 6200 and 5400 cal years BP, suggests a mosaic ecosystem, dominated by more C3 montane grass, ericoid shrubland and high elevation forest patches promoted by cool/dry climate with low fire occurrence and herbivory activities. Between ca. 5400 and 4200 cal years BP, vegetation in the area was dominated by a mosaic of ericoid shrubland and mid-elevation forest under a wetter period, moderate fire occurrence, and herbivory activities. This mosaic was controlled by climate, fire refugia and herbivory feedbacks. The vegetation changed into a forest-savanna mosaic with an abundance of grassland and pioneer/fire-resistant trees between ca. 4200 and 3000 cal years BP. The period was characterised by an initial increase of local fire followed by a regional drought event. This suggests that a threshold might have been reached, with a resulting shift in vegetation composition. Between ca. 3000 and 1000 cal years BP, reoccurrence of ericoid shrubland with woodland savanna taxa was recorded in the area. The vegetation was conditioned by variation of climate from wet (until ca. 2000 cal years BP) to dry period (ca. 2000–⁠1000 cal years BP) with moderate fire occurrence and herbivory activities. In addition, stable carbon isotope results show that between ca. 6200 to 1000 cal years BP, the site was characterised by C3 plants. During the last ca.1000 cal years BP, pollen records from both sites in the Central Highlands of Madagascar showed a shift to a more open landscape dominated by grassland. Trees and shrubland in the highlands experienced a massive decrease and this correlated with an abundance of C4 plants associated with reduced diversity. The shift of vegetation during this period was likely a result of a centennial severe drought period at ca. 950 cal years BP, as recorded in the literature and confirmed by the peak in aerophilous taxa in our diatom record. The drought was followed by a dramatic increase of fire occurrence and herbivory activities in the region, as recorded in the charcoal and spore records from both sites, indicating human activities at ca. 700 and 500 cal years BP for Lake Dangovavy and Tampoketsa-Ankazobe wetland, respectively. Though the vegetation at both sites in Central Highlands of Madagascar was very dynamic until ca. 1000 cal years BP, complex interactions between climate and fire allowed the forest and ericoid elements to persist, consistent with a heterogeneous mosaic landscape. This changed from 1000 years ago with the occurrence of a regional severe drought event followed by an increase in human activities leading to an increase of grass, a decline in forest and ericoid elements. Our findings suggest that although, the eastern and western slopes in Madagascar might have different vegetation histories over time as a response to the complex climatic-fire drivers at least until ca. 1000 cal years BP, they both: a) Contained ancient open ecosystems such as grasslands and/or ericoid shrubland, and a mosaic landscape which should be considered typical of the highland region. b) Experienced a loss of forest, woodland and mosaic elements, a trend that is consistent with the anthropogenic conversion of some forests to grasslands since ca. 1000 cal years BP. Such findings have implications in terms of conservation, fire management and afforestation projects in the Central Highlands, and provide additional knowledge that contributes to the understanding of its ecological processes and history prior to human arrival on the island. Indeed: 1) Ancient grasslands and ericoid shrubland need to be identified and conserved because of their antiquity and unique biodiversity. To date, there has been some focus on ancient grasslands, but the presence of ancient heathlands has not been discussed. 2) It is important to distinguish ancient from derived grasslands and to target the latter for reforestation, using species that are typical of the remaining forest patches. 3) Fire management should be conducted at a local scale and should incorporate the landscape fire history, considering, for example, the differences between two slopes in the Central Highlands.
  • Thumbnail Image
    Item
    Open Access
    Long-term ecosystem dynamics of contrasting grasslands in South Africa
    (2022) Dabengwa, Abraham Nqabutho; Gillson, Lindsey; Bond, William John
    Rainfall, fire, and grazing all control changes in vegetation and soil in grassland and savanna ecosystems. In these ecosystems, wetlands are key resource areas because they keep moisture and collect nutrients that support grass production. The grass production supports high grazer densities in landscapes, especially during dry climatic periods.