Remote Sensing of Invasive Alien Plants in Strategic Water Source Areas of South Africa: Part I—A Systematic Review of Spatial and Taxonomic Biases and Their Consequences for Water Resources and Biodiversity

South Africa is a water-scarce country characterised by low and highly variable rainfall and strong spatial disparities in runoff generation, placing substantial pressure on already limited water resources. Although mountainous catchments occupy a small portion of the landscape, they contribute a disproportionate share of national surface water yield and are formally recognised as Strategic Water Source Areas (SWSAs). These areas are increasingly threatened by alien invasive plants, particularly woody species, which disrupt hydrological processes through increased evapotranspiration, canopy interception, and altered soil moisture dynamics, ultimately reducing streamflow and water availability. This study systematically reviews the spatial distribution and taxonomic focus of remote sensing-based mapping of woody invasive alien plants in South Africa using the PRISMA framework. The peer-reviewed literature was sourced from major scientific databases, screened using predefined eligibility criteria, and analysed through integrated spatial, bibliometric, and descriptive approaches. The findings indicate a marked increase in publications over the past decade, reflecting growing adoption of remote sensing in invasion ecology. However, research effort is spatially uneven, with strong clustering in eastern South Africa and limited representation in arid and interior regions. Only 46.3% of studies are located within SWSAs, with Southern Drakensberg dominating research activity. Taxonomically, woody species accounted for 79.2% of the unique invasive taxa identified across the reviewed literature, with Acacia, Eucalyptus, and Pinus representing the dominant genera, which are among the most hydrologically impactful invaders. The synthesis reveals a clear research focus on high-impact woody invasive plants but highlights critical spatial and taxonomic gaps that limit alignment between remote sensing research and hydrologically important landscapes, constraining effective prioritisation for water resource management. By identifying spatial and taxonomic biases in remote sensing-based invasion monitoring, this study provides an evidence base to strengthen monitoring and management frameworks that support Sustainable Development Goal (SDG) 6 (Clean Water and Sanitation), while contributing to SDG 13 (Climate Action) through improved catchment resilience and SDG 15 (Life on Land) by informing the conservation and restoration of invaded ecosystems.