Browsing by Author "Hall, Alexander Stuart Warren"

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    Restoration potential of alien-invaded Lowland Fynbos
    (Stellenbosch : Stellenbosch University, 2018-03) Hall, Alexander Stuart Warren; Esler, Karen J.; Holmes, Patricia M.; Gaertner, Mirijam; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Widespread degradation of ecosystems on a global scale has led to the need for ecological restoration in order to more effectively conserve and manage threatened ecosystems. While this is an emerging field, substantial challenges remain in achieving optimal success in restoration interventions, which need to be addressed in order to improve recovery potential of degraded sites. This is especially important in highly biodiverse regions in urgent need of restoration intervention, of which the critically endangered vegetation type Cape Flats Sand Fynbos, found within the Cape Floristic Region (CFR) in South Africa is a prime example. Of particular concern in this case is the impact of invasive alien plants, especially where invasive plant removal alone does not facilitate native vegetation recovery. Use of alternative passive clearing methods, or the incorporation of active seed sowing interventions, may improve restoration success in such cases. This dissertation investigated the following aspects of restoration: (1) seed ecology and the determination of effective germination cues of structurally important species in this vegetation type (chapter 2), (2) different passive and active restoration treatments in order to determine best alien clearing and management protocols (chapter 3), (3) predicting long-term vegetation recovery trajectories under different restoration treatments and pinpointing ecological thresholds through the use of a dynamic model incorporating vegetation recovery rates under passive and active restoration treatments (chapter 4). A seed ecology study investigated the effectiveness of smoke and heat exposure as a pre-treatment of seeds in improving germination success of species within Cape Flats Sand Fynbos. Fourteen species were exposed to a range of durations of exposure at different temperatures, in combination with exposure to smoke residue, after which seeds were germinated on agar to determine treatment success. Most species fell into one of two main groups: Seed germination in the first group (e.g. Anthospermum aethiopicum, Metalasia densa and Watsonia meriana) was greatest following either a lower temperature heat pulse, an extended period of mild temperature exposure, or no pre-treatment with heat. Seed germination in the second group (e.g. Pelargonium elongatum, Phylica cephalantha and Trichocephalus stipularis) was promoted after brief exposure to higher (100°C) temperatures. Species within the latter group mainly possessed physical dormancy. Passive Acacia saligna clearing treatments in the field involving either burning or stacking biomass after initial alien clearing, as well as active intervention involving sowing seeds of native species either directly after burning or a year later and either with or without seed pre-treatment, resulted in very different recovery trajectories over a two year survey period. No treatment resembled a reference uninvaded vegetation condition after two years. Clearing alien biomass without burning was the cheapest method and facilitated recovery in less degraded areas, but otherwise there was very limited or no native vegetation recovery and this facilitated secondary invasion by herbaceous weeds. Burning after clearing controlled some guilds of secondary invasive species, but stimulated mass acacia reestablishment and did not result in native vegetation recovery. The native seed bank was found to have been depleted during the period of invasion, hence the lack of autogenic recovery under passive treatments. However, while clearing without burning resulted in low acacia reestablishment, two years later the acacia seed bank was similarly reduced both with and without burning. Active seed sowing was the most expensive treatment but resulted in the highest recovery of native shrub cover and diversity. These findings suggest that a biotic threshold has been crossed where passive treatment does not result in recovery of vegetation components. Active sowing was able to partially reverse this threshold through improved recovery of total shrub cover, while pre-treatment of seeds before sowing improved establishment of some species. However, non-sprouting shrub cover was over-represented while resprouting shrubs and species of Restionaceae, which germinated successfully under greenhouse conditions, were under-represented relative to the reference condition. This suggests that barriers still exist in preventing establishment of some species from seeds sown in the field. A dynamic model was designed to analyze the effectiveness of the different restoration methods, enabling the extrapolation of recovery trajectories over a long time span which could not be determined from short-term field surveys alone. Data for rates of vegetation recovery under different passive and active restoration treatments over the course of two years of field surveys were fed into the model. The modelled simulations showed that different treatments in similar degraded states at the time of initial clearing resulted in vastly different recovery trajectories when extrapolated over an extended period of time. Active seed sowing was initially the most expensive treatment but resulted in the highest indigenous shrub recovery, which in turn decreased the costs of longer-term follow-up alien plant clearing, as well as providing competition against establishment of secondary invasive species. Clearing without burning was the cheapest method, but resulted in limited recovery of indigenous or acacia cover over the long-term, leaving barren ground prone to secondary invasion by herbaceous weeds. The model therefore supported the finding of the field restoration experiments that ecological thresholds have been crossed which prevented recovery of certain vegetation components. Active sowing was able to partially reverse these thresholds and resulted in sustained shrub cover, but even this treatment did not resemble the reference condition in terms of restoring a balance of structural components after simulating over an extended period of time. By synthesising the knowledge gained from the three aspects of this dissertation, and in light of what was previously known to work effectively, management recommendations were formulated to provide practitioners with more effective guidelines for best practise in future restoration of lowland fynbos vegetation. Firstly, it was concluded that biotic thresholds to restoration have been crossed within the study site, due to the lack of autogenic recovery of native cover and diversity under passive treatments as a result of a depleted native seed bank. Active interventions can partially reverse this threshold through the reintroduction of native seeds, and increase resilience of the vegetation to secondary invasive species. Secondly, the additional knowledge gained through germination tests using heat and smoke pre-treatment of seeds is highly valuable for incorporating into further restoration work. The use of species-specific pre-treatments for stimulating optimal germination can improve restoration success following active seed sowing intervention in the field. However, some species which respond well to seed pre-treatment in greenhouse conditions fail to establish from seed in the field, and these species are better restored through nursery propagation for planting into the field. Thirdly, the best treatment to use in the field was found to depend on the level of degradation at the time of initial alien clearing. A less degraded site with representative native vegetation structure i.e. more than 10% cover of both non-sprouting and resprouting native shrubs will recover after clearing without burning, while a more degraded site lacking native vegetation structure or cover will require active intervention in order to facilitate recovery of native cover and improve resilience to invasion by primary or secondary invasive species. Since burning after clearing resulted in mass recovery of Acacia saligna, both in passive and active restoration treatments, two alternative treatments to those tested in the field are proposed. The most appropriate treatment would depend on the size of the secondary invasive seed bank. If this seed bank is small, clearing without burning and sowing pre-treated native seeds should be more effective than burning. If the secondary invasive seed bank is large, then clearing and waiting two years to facilitate acacia seed bank reduction, after which a management burn is conducted, should deplete the secondary invasive seed bank. Sowing native seeds after burning would result in native seedlings experiencing less competition from invasive species before establishing. It may not be feasible to restore the entire diversity found in a reference habitat, but focusing on restoring vegetation structure will provide resilience against invasive species establishment, facilitate an appropriate fire regime and provide suitable habitat for reintroduction of rare or threatened species in future once follow-up control of invasive species is more manageable.