Steenhuisen, Sandy-LynnMartin, GrantPayne, StephanieMashamba, Tapiwanashe2025-06-042025-06-042024Dissertation (M.Sc.(Botany))--University of the Free State, 2024http://hdl.handle.net/11660/13089Invasive alien species present a critical threat to biodiversity, ecosystem services, and agricultural productivity worldwide. This study examines the distribution, demographic dynamics, and ecological impacts of 𝘚𝘢𝘭𝘪𝘹 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 L. (weeping willow) and 𝘚𝘢𝘭𝘪𝘹 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴 L. (crack willow) within the Grassland biome of the Eastern Free State, South Africa. These invasive species, introduced for ornamental and functional purposes, have shown extensive colonization, particularly in riparian zones, where they outcompete native vegetation and alter hydrological processes. The research combines historical and contemporary data sources to evaluate changes in the population structure and spatial distribution of these species over time. Using aerial photographs from 1984 and 2023, supplemented with roadside surveys and Google Street View (GSV) analysis, the study provides an assessment of the species’ demographics. Additionally, field-based verification of GSV-derived data validates its utility as a cost-effective tool for mapping and monitoring invasive alien plants. The study confirms that 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 and 𝘚. 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴 remain prevalent along watercourses, roadsides, and disturbed areas, with 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 exhibiting a broader distribution along roadsides with approximately 50% of the sites having a population size of 1-5 trees. Although 𝘚. 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴 is not as widely distributed as 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 it dominates the areas in which it is found, forming pure stands; 56% of the sites surveyed consisted of 6 or more trees. Gender analyses of 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 populations reveal a predominance of female trees which suggests that its propagation is mainly asexual. The demographic study done through road surveys and GSV reveals limited recruitment of 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 in recent decades, suggesting a potential decline in its invasive potential with almost 80% of the population consisting of adult or dying trees. In contrast, 𝘚. 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴 populations display a continuous expansion with healthy recruitment, with 44% of the trees recorded as seedlings, young trees, or mature trees. This expansion indicates that 𝘚. 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴 is now more invasive than 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢, which increases the need for a tailored management strategy to address this invasive species. The research also investigates interactions between invasive willows and invertebrate communities. Surveys at selected sites document the presence of 348 individual insects and one arachnid, representing 21 families, with 14 species associated with both willow species. Another finding was the presence of crown galls on 27.5% on the 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 population surveyed. Invasive willows provide habitat and resources for certain insect populations. Some of these insects, specifically 𝘗𝘭𝘢𝘨𝘪𝘰𝘴𝘵𝘦𝘳𝘯𝘢 𝘥𝘪𝘴𝘤𝘰𝘭𝘰𝘳, 𝘛𝘶𝘣𝘦𝘳𝘰𝘭𝘢𝘤𝘩𝘯𝘶𝘴 𝘴𝘢𝘭𝘪𝘨𝘯𝘶𝘴 𝘢𝘯𝘥 𝘏𝘦𝘭𝘪𝘰𝘯𝘪𝘥𝘪𝘢 𝘲𝘶𝘢𝘥𝘳𝘪𝘮𝘢𝘤𝘶𝘭𝘢𝘵𝘢, along with gall-inducing bacteria (potentially 𝘈𝘨𝘳𝘰𝘣𝘢𝘤𝘵𝘦𝘳𝘪𝘶𝘮 𝘵𝘶𝘮𝘦𝘧𝘢𝘤𝘪𝘦𝘯𝘴), recorded within the 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 populations could be resulting in the decline of this invasive species. This study explored the use of ArcGIS and GSV, to assess invasive tree distributions and potential of these tools to monitor the spread of invasive trees over time. The comparison of historical and current data reveals significant efficiency of GSV as a monitoring tool, offering a cheap and quick approach to invasive species management in resource-limited settings. Policy implications of the findings are discussed with reference to South African legislation, including the Conservation of Agricultural Resources Act (CARA) of 1983 and the National Environmental Management: Biodiversity Act (NEMBA). 𝘚𝘢𝘭𝘪𝘹 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 and 𝘚. 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴 have been classified as Category 2 invaders under CARA but are not found under the NEMBA Act. This may result in challenges in effectively controlling 𝘚. 𝘣𝘢𝘣𝘺𝘭𝘰𝘯𝘪𝘤𝘢 and 𝘚. 𝘧𝘳𝘢𝘨𝘪𝘭𝘪𝘴. This dissertation contributes to the field of biological invasions by providing an updated assessment of invasive willows in the Grassland biome, thereby offering insights for land managers, conservationists, and policymakers. The study concludes by emphasizing the importance of integrating historical data and modern geospatial tools, as an informative approach to develop effective management solutions for invasive alien plants. The methodological framework and findings presented herein offer valuable lessons for addressing invasive species challenges in similar ecological contexts globally.enDissertationUniversity of the Free State