Relating vegetation distribution to cycles of erosion and deposition in the Kromme River wetlands
- Authors: Jarvis, Samuel Cameron
- Date: 2023-10-13
- Subjects: Biogeomorphology South Africa Kromme Estuary (Eastern Cape) , Earth observation , Remote sensing , Niche construction , Wetland ecology , Geomorphology , Ecological succession , Optical radar , Prionium serratum
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424582 , vital:72166
- Description: The role of geomorphic disturbance has been increasingly recognized as fundamental in the creation and functioning of wetlands. This is true of the Kromme River wetland which has been formed through repeated cycles of erosion and deposition. However, the response – and influence – of wetland plants to these sorts of disturbance has not been investigated. This study sought to fill this knowledge gap by classifying vegetation communities over a range of hydrological and geomorphic disturbance regimes that have happened over the last few decades, and relating those vegetation communities to environmental factors. The study identified seven vegetation communities based on their species composition and abundance, which were related to geomorphic disturbance events. A conceptual model that accounts for vegetation distribution in the Kromme wetland was developed. Soil saturation was the most important factor explaining vegetation community distribution, which, in turn, is influenced by cycles of erosion and deposition. Following an erosional event on the valley floor, Prionium serratum dominated wetland is converted to a number of other vegetation communities. On the floodplain surface adjacent to the eroded gully, the Prionium serratum dominated wetland is transformed over time to Cynodon dactylon and Sporobolus fimbriatus communities. Prionium serratum clumps immediately adjacent to the recently incised gullies are able to persist, having sufficient access to water. Within the newly formed gullies, Juncus lomatophyllus colonizes the gully beds flooded to a shallow depth, Miscanthus capensis colonizes the gully bars and Setaria incrassata colonizes the exposed gully banks. Localised depositional features close to the thalweg in the gully are colonized by Prionium serratum seedlings and vegetative propagules. These plants represent the regenerating phase of Prionium serratum wetland, which also colonizes depositional floodouts downstream of the newly-formed gully. The Stenotaphrum secundatum community dominates drier, more elevated areas of the floodout. Over time, as the gully fills, Prionium serratum expands beyond the gully onto the valley floor, to replace the floodplain communities Cynodon dactylon and Sporobolus fimbriatus. Over time, Prionium serratum is thought to colonize the valley floor as the gully fills, stabilising it and promoting diffuse flow. Many restoration efforts in damaged palmiet wetlands have been focused on the preservation of intact palmiet communities upstream of erosional headcuts, with limited understanding of vegetation dynamics associated with the cut-and-fill cycles that naturally occur in these wetlands. Understanding the regeneration of Prionium serratum following erosional events is thus important for wetland restoration, as it should focus more attention on promoting palmiet restoration on depositional floodouts downstream of eroded gullies. A secondary aim of this study was to explore the possibility of mapping palmiet communities in Kromme River wetland using remote sensing techniques. Using a combination of ground-truthed data from this and previous studies in the Kromme River wetland, together with raster layers derived from a LiDAR survey, an overlay analysis was developed to effectively map the distribution of the Prionium serratum dominated community. The overlay was created using a machine learning library in RStudios known as Rpart. The results found that the model were 91% effective in classifying the distribution of the Prionium serratum community. A secondary finding was that the inclusion of a Relative Elevation Model in the overlay analysis allowed for the identification of Prionium serratum communities vulnerable to degradation following previous geomorphic disturbance events and those Prionium serratum communities that are likely to persist following a geomorphic disturbance event. , Thesis (MSc) -- Faculty of Science, Geography, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Jarvis, Samuel Cameron
- Date: 2023-10-13
- Subjects: Biogeomorphology South Africa Kromme Estuary (Eastern Cape) , Earth observation , Remote sensing , Niche construction , Wetland ecology , Geomorphology , Ecological succession , Optical radar , Prionium serratum
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424582 , vital:72166
- Description: The role of geomorphic disturbance has been increasingly recognized as fundamental in the creation and functioning of wetlands. This is true of the Kromme River wetland which has been formed through repeated cycles of erosion and deposition. However, the response – and influence – of wetland plants to these sorts of disturbance has not been investigated. This study sought to fill this knowledge gap by classifying vegetation communities over a range of hydrological and geomorphic disturbance regimes that have happened over the last few decades, and relating those vegetation communities to environmental factors. The study identified seven vegetation communities based on their species composition and abundance, which were related to geomorphic disturbance events. A conceptual model that accounts for vegetation distribution in the Kromme wetland was developed. Soil saturation was the most important factor explaining vegetation community distribution, which, in turn, is influenced by cycles of erosion and deposition. Following an erosional event on the valley floor, Prionium serratum dominated wetland is converted to a number of other vegetation communities. On the floodplain surface adjacent to the eroded gully, the Prionium serratum dominated wetland is transformed over time to Cynodon dactylon and Sporobolus fimbriatus communities. Prionium serratum clumps immediately adjacent to the recently incised gullies are able to persist, having sufficient access to water. Within the newly formed gullies, Juncus lomatophyllus colonizes the gully beds flooded to a shallow depth, Miscanthus capensis colonizes the gully bars and Setaria incrassata colonizes the exposed gully banks. Localised depositional features close to the thalweg in the gully are colonized by Prionium serratum seedlings and vegetative propagules. These plants represent the regenerating phase of Prionium serratum wetland, which also colonizes depositional floodouts downstream of the newly-formed gully. The Stenotaphrum secundatum community dominates drier, more elevated areas of the floodout. Over time, as the gully fills, Prionium serratum expands beyond the gully onto the valley floor, to replace the floodplain communities Cynodon dactylon and Sporobolus fimbriatus. Over time, Prionium serratum is thought to colonize the valley floor as the gully fills, stabilising it and promoting diffuse flow. Many restoration efforts in damaged palmiet wetlands have been focused on the preservation of intact palmiet communities upstream of erosional headcuts, with limited understanding of vegetation dynamics associated with the cut-and-fill cycles that naturally occur in these wetlands. Understanding the regeneration of Prionium serratum following erosional events is thus important for wetland restoration, as it should focus more attention on promoting palmiet restoration on depositional floodouts downstream of eroded gullies. A secondary aim of this study was to explore the possibility of mapping palmiet communities in Kromme River wetland using remote sensing techniques. Using a combination of ground-truthed data from this and previous studies in the Kromme River wetland, together with raster layers derived from a LiDAR survey, an overlay analysis was developed to effectively map the distribution of the Prionium serratum dominated community. The overlay was created using a machine learning library in RStudios known as Rpart. The results found that the model were 91% effective in classifying the distribution of the Prionium serratum community. A secondary finding was that the inclusion of a Relative Elevation Model in the overlay analysis allowed for the identification of Prionium serratum communities vulnerable to degradation following previous geomorphic disturbance events and those Prionium serratum communities that are likely to persist following a geomorphic disturbance event. , Thesis (MSc) -- Faculty of Science, Geography, 2023
- Full Text:
- Date Issued: 2023-10-13
The regeneration of palmiet (prionium serratum) following hydrogeomorphic disturbance: a case study of the Kromme River wetland
- Authors: Van Eck, Caydon Daniël
- Date: 2022-10-14
- Subjects: Biogeomorphology South Africa Kromme Estuary (Eastern Cape) , Wetland ecology , Prionium serratum Regeneration , Fluvial geomorphology , River channels
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/365555 , vital:65759
- Description: The Kromme River wetland in the Eastern Cape of South Africa occupies a broad valley (up to 350 m wide) with a gentle longitudinal slope of less than 2 % that has been formed by cut-and-fill cycles that are initiated by trunk-tributary interactions. These hydrogeomorphic disturbance events trigger gully erosion and generate sediments, the coarse fraction of which is deposited less than 2 km downstream, leading to depositional floodout features that fill gullies headwards. This process has been occurring at intervals for at least 10 000 years, and as such pre-dates the introduction of European farming practices in the area. Plants that regenerate by colonising these features are thought to have evolved adaptions necessary to colonise intermittently produced bare sandy sediments. This study aimed to examine the regeneration ecology of palmiet (Prionium serratum), the dominant vegetation community within these cut-and-fill wetlands, by relating its regeneration characteristics to environmental factors in a reach of the Kromme River wetland that has experienced a recent hydrogeomorphic disturbance event (approximately 10 years before the commencement of this study). Palmiet was found to be regenerating on sedimentary deposits on beds of gullies and on depositional bars on the margins of gully beds. The large depositional floodout downstream of a large gully also favoured palmiet regeneration, where it was found to be regenerating along not only the active channel, but also along old abandoned flow paths and sometimes areas well elevated above the channel. The geomorphic features that favoured palmiet regeneration were characterised by coarse-grained sediments (mean particle size approximately 310 μm) with low organic matter content (0.61 %), a low depth to the water table and low elevation above the thalweg (mean depth to water table is approximately 0.6 m), and a relatively close distance to the thalweg (< 10 m). This understanding of palmiet’s regeneration characteristics was viewed in relation to existing literature on undisturbed palmiet wetland plant communities and its reported contribution to conditions that favour wetland formation through gully filling, which allowed for the creation of a conceptual model of palmiet regeneration, colonisation and long-term persistence. This model was based on the Fluvial Biogeomorphic Succession concept. It suggests that palmiet’s interaction with the hydrogeomorphic environment throughout the different stages of its life cycle results in self-organising biogeomorphic landforms. Over hundreds of years, the reciprocal interactions between palmiet, sediments and water, fills gullies and restores valley bottoms, ultimately leading to the formation of a wetland landform. It is further proposed that through continued accretion, the geomorphic wetland landscape becomes more and more disconnected from the hydrogeomorphic dynamics of the fluvial system, such that the prevailing conditions begin to favour fynbos establishment, which may outcompete palmiet. , Thesis (MSc) -- Faculty of Science, Geography, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Van Eck, Caydon Daniël
- Date: 2022-10-14
- Subjects: Biogeomorphology South Africa Kromme Estuary (Eastern Cape) , Wetland ecology , Prionium serratum Regeneration , Fluvial geomorphology , River channels
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/365555 , vital:65759
- Description: The Kromme River wetland in the Eastern Cape of South Africa occupies a broad valley (up to 350 m wide) with a gentle longitudinal slope of less than 2 % that has been formed by cut-and-fill cycles that are initiated by trunk-tributary interactions. These hydrogeomorphic disturbance events trigger gully erosion and generate sediments, the coarse fraction of which is deposited less than 2 km downstream, leading to depositional floodout features that fill gullies headwards. This process has been occurring at intervals for at least 10 000 years, and as such pre-dates the introduction of European farming practices in the area. Plants that regenerate by colonising these features are thought to have evolved adaptions necessary to colonise intermittently produced bare sandy sediments. This study aimed to examine the regeneration ecology of palmiet (Prionium serratum), the dominant vegetation community within these cut-and-fill wetlands, by relating its regeneration characteristics to environmental factors in a reach of the Kromme River wetland that has experienced a recent hydrogeomorphic disturbance event (approximately 10 years before the commencement of this study). Palmiet was found to be regenerating on sedimentary deposits on beds of gullies and on depositional bars on the margins of gully beds. The large depositional floodout downstream of a large gully also favoured palmiet regeneration, where it was found to be regenerating along not only the active channel, but also along old abandoned flow paths and sometimes areas well elevated above the channel. The geomorphic features that favoured palmiet regeneration were characterised by coarse-grained sediments (mean particle size approximately 310 μm) with low organic matter content (0.61 %), a low depth to the water table and low elevation above the thalweg (mean depth to water table is approximately 0.6 m), and a relatively close distance to the thalweg (< 10 m). This understanding of palmiet’s regeneration characteristics was viewed in relation to existing literature on undisturbed palmiet wetland plant communities and its reported contribution to conditions that favour wetland formation through gully filling, which allowed for the creation of a conceptual model of palmiet regeneration, colonisation and long-term persistence. This model was based on the Fluvial Biogeomorphic Succession concept. It suggests that palmiet’s interaction with the hydrogeomorphic environment throughout the different stages of its life cycle results in self-organising biogeomorphic landforms. Over hundreds of years, the reciprocal interactions between palmiet, sediments and water, fills gullies and restores valley bottoms, ultimately leading to the formation of a wetland landform. It is further proposed that through continued accretion, the geomorphic wetland landscape becomes more and more disconnected from the hydrogeomorphic dynamics of the fluvial system, such that the prevailing conditions begin to favour fynbos establishment, which may outcompete palmiet. , Thesis (MSc) -- Faculty of Science, Geography, 2022
- Full Text:
- Date Issued: 2022-10-14
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