Browsing by Author "Ngwenya, Duduzile K"

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    Simulating natural processes to scale-up ecological restoration in lowland sand fynbos
    (Stellenbosch : Stellenbosch University, 2023-03) Ngwenya, Duduzile K; Esler, Karen J.; Holmes, Patricia M.; Geerts, Sjirk; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The restoration of degraded ecosystems is set to gain momentum given that natural resources are dwindling and there is a need to mitigate the impacts of climate change. There is some reliance on ecological restoration to reverse environmental degradation and to facilitate sustainable economic development. Global initiatives such as the United Nation’s Decade on Ecosystem Restoration and the Global Biodiversity Framework have placed ecological restoration at the forefront of reversing environmental degradation while the Sustainable Development Goals embed biodiversity at the core of sustainability. Ecological restoration is set to direct degraded ecosystems on a restoration trajectory and recover the ecosystem’s composition, structure, and function. However, there is a mismatch between the extent of environmental degradation and that of restoration efforts. Consequently, the net contribution of current restoration efforts towards reversing environmental degradation is limited. What is lacking is an approach which converges the contributions made from different extents of restoration efforts into a global contribution. This is caused by the lack of a framing tool to guide the design and implementation of restoration interventions towards closing the gap between the extents of environmental degradation and restoration efforts. Environmental degradation is widespread and caused by various factors including biological invasions. Plant invasions are problematic in South Africa, particularly in the Greater Cape Floristic Region (GCFR), where they cause biodiversity loss. These invasions are usually dominated by the alien trees and shrubs native to the Mediterranean climate regions of the globe. Restoration efforts may be undermined because the invading species originate from similar, fire-prone environments with similar drivers of regeneration. This way, the invading species can easily adapt in fynbos environments and respond positively to restoration treatments meant to regenerate native vegetation. Therefore, the use of fire treatments to regenerate these fynbos types when they are heavily invaded by fire-adapted species is challenged by the invader’s equal responsiveness to fire treatments. A prime example is using fire treatments to regenerate Lowland Sand Fynbos found within the GCFR when it is heavily invaded by Australian Acacia saligna. In this case, fire treatments stimulate high- density, post-fire acacia recruitment instead of native species. This high-density acacia recruitment outcompetes the sparsely recruited native species which fail to establish thereafter. By coupling alien clearing interventions with restoration of native vegetation, the research sought ways to improve and scale-up ecological restoration efforts currently underway in Lowland Sand Fynbos. Alternative ways were sought to encourage native species regeneration while discouraging the invading species from resurging. Therefore, this dissertation investigated the efficacy of simulating and side-stepping the effects of fire to avoid high-density post-fire acacia recruitment. Guided by the findings from the reviewed literature, two restoration treatments were designed and trialled in Lowland Sand Fynbos ecosystems long invaded by A. saligna. These treatments were designed to improve the regeneration of native species by reducing the density of acacia emergence after clearing acacia stands. Regeneration of natives was encouraged by simulating the effects of fire required to stimulate the germination of native species and by sowing pre-treated native species to boost the depleted native seed bank. The recruitment of acacia was discouraged by avoiding and delaying the use of fire treatments when the acacia seed bank was relatively large and through follow-up clearing of recruited acacia. That is, the cleared areas were either sown without burning the acacia slash (“Fell, Stack & No Burn” treatment) or they were fallowed before burning the slash and sowing pre-treated seeds (“Fell, Fallow & Burn” treatment). By avoiding fire treatments, the recruitment of acacia was discouraged by depriving acacia of its fire-related heat pulse germination cue. This resulted in sparse recruitment of acacia which allowed the native species to germinate from pre-treated seeds and to establish. However, the recovered vegetation did not yet approach a structure comparable to the reference site because it lacked representation of a few plant guilds, e.g., proteoids and geophytes, and the species richness within individual plant guilds was poor. Delaying using fire treatments allowed the acacia seed bank to reduce in size, translating to reduced acacia seedling densities that allowed the recruited native seedlings to establish rapidly. However, this reduced post-fire acacia recruitment still yielded a dense acacia stand in competition with the recruited native species. But the cover from recruited annual species helped to suppress the establishment of dense acacia cover. When the annuals died back, follow-up clearing of acacia seedlings was necessary to facilitate the establishment of perennial species thereafter. Improving the management of alien plant invasions can be better executed by combining insights, efforts, and experiences from the fields of invasion and restoration ecology to inform restoration practice and the selection of suitable restoration treatments. Therefore, this research examined the effectiveness of treatments that side- stepped or simulated natural processes to scale-up native species recovery in a highly threatened Cape Floristic Region habitat. The technical and financial effectiveness of various treatments were compared and indicated context-specific outcomes. There was no ideal treatment which incurred the lowest cost but yielded the best outcomes in all categories of restoration outcomes. Instead, there were trade-offs between costs and outcomes in each treatment. For example, despite being able to recover good native cover and minimising acacia recruitment at lowest costs, the “Fell, Stack & No Burn” treatment retained the highest acacia seed bank after two years. The “Fell, Fallow & Burn” treatment resulted in the highest native cover after two years but it did not improve the species richness of the recruited native species and it was more expensive than the “Fell, Stack & No Burn” treatment. However, the resultant effects of fallowing and high-density post-fire acacia emergence yielded the lowest residual acacia seed bank compared to other treatments. Within the monitored period, none of the treatments allowed the restoration sites to approach an ecosystem structure that was comparable to reference conditions. The restored areas lacked the representation of a few plant guilds, e.g., the overstorey proteoid guild, geophytes and some ericoid resprouter shrubs. Furthermore, species richness within each recovered plant guild was low. The greatest barrier to stimulating the regeneration of a species-diverse Lowland Sand Fynbos ecosystem is likely to be the depleted native seed bank more than the effects of fire or the high-density post-fire acacia resurgence. A depleted native seed bank restoration intervention requires the addition of species-diverse seed mixes irrespective of the treatment, but seed supplies remain limited. The high-density acacia resurgence can be managed by manual removal of acacia saplings within the first year of sowing, though it is expensive and laborious to implement over large areas. An avenue for future research is to explore seed enhancement technologies such as polymer coating, since the combination of restoration treatments with follow-up clearing treatments can improve the establishment of native species in the field. Therefore, practitioners can use seed pre-treatments to enhance the germination of species that are easy to collect and re-establish in the field and plant nodes of rootstock for species and plant guilds that fail to establish from seeds. The choice of management approach depends on the budget constraints, priorities, context, and scope of the intervention. Despite the lack of an ideal treatment and an inconclusive evaluation, the synthesis of research findings indicated some best-practise management options that practitioners can adopt in future restoration efforts. Firstly, it was recommended that when restoring fynbos vegetation types that are heavily invaded with acacia, fires should be delayed until the risk of high-density post-fire acacia recruitment posed by the acacia seed bank is low. Secondly, the persistent portion of the acacia seed bank poses an ongoing risk of reinvasion and therefore requires long-term management planning and budget for follow-up clearing. Thirdly, given the dearth of available native seeds for active restoration in Cape Flats Sand Fynbos, scaling-up ecological restoration may require seed enhancement technologies to boost the depleted native seed bank and help to restore ecosystems that are species diverse, resilient, and resemble reference conditions. Lastly, restoration of functioning ecosystems takes time, over several fire cycles, and requires continuous improvement from the initial interventions.