This is a lay summary of the article published under the DOI: 10.1007/978-3-030-32394-3_15
Scientists say that uncontrolled invasive trees and other plants are responsible for the loss of massive amounts of South Africa’s water. They add that better strategies for measuring these losses, and controlling invasive plants, is essential for water security in the country.
Across South Africa, species like pine trees, black wattle and eucalyptus were originally introduced to provide wood. Scientists have known for a long time, however, that these trees use a lot of water and reduce the amount of runoff, or water on the ground’s surface, that reaches rivers and streams after rain.
In addition to trees, many other invasive plants have also been introduced and contribute to the problem.
In this study, the researchers reviewed the results of many past studies to get an overview of how invasive plants impact South Africa’s water systems. They were interested in understanding the extent of the problem, and how it can be studied and monitored in the years to come. They also provided some recommendations for how to study this issue in the future.
The researchers identified several key themes and issues from the scientific evidence. Among the biggest problems caused by invasive plants was that they increased water loss through evaporation and transpiration. Transpiration is a process whereby plants lose water from their leaves to the surrounding atmosphere.
The evidence also showed that a lot of water is lost when trees with deep roots invade areas near rivers, or areas where water is naturally stored underground. In cases where the trees were invading grassland, these effects were even stronger, because they were replacing plants that have shallow roots and reaching reserves of water that the indigenous plants couldn’t access.
The researchers added that the impact from trees in plantations can be managed, but their seeds often start new invasions in surrounding areas and can reduce the amount of water reaching rivers as a result.
The studies also showed that the Eastern and Western Cape, and KwaZulu-Natal provinces would be most affected by increasing water losses in the years to come. This means that these areas will have less water reserves during droughts, and less overall water security.
The researchers said that, considering these issues, it’s very important to be able to accurately predict the amounts of water that will be lost in the future. While many of the studies they looked at focused on specific types of invasive trees or plants, they pointed out that rates of water loss need to be understood on a much larger scale.
To do this, they recommended using technologies like remote sensing, where satellite photos are used to calculate the amount of water being lost across a larger area.
They also stated that the government departments that deal with water and water management in the country should include invasive plant control as part of their management strategies.
The study was conducted by scientists based in South Africa, and forms part of a book about biological invasions in the country.
Considerable advances have been made since the first estimates of the impacts of invasive alien plants on water resources in the early 1990s. A large body of evidence shows that invasive alien plants can increase transpiration and evaporation losses and thus reduce river flows and mean annual runoff. Riparian invasions, and those in areas where groundwater is accessible, have 1.2–2 times the impact of invasions in dryland areas. The magnitude of the impacts is directly related to differences between the invading species and the dominant native species in size, rooting depth and leaf phenology. Information on the impacts has been successfully used to compare the water use of invasive plants and different land cover classes, to quantify the water resource benefits of control measures, and to prioritise areas for control operations. Nationally, the impacts of invasive alien plants on surface water runoff are estimated at 1.44–2.44 billion m3 per year. The most affected primary catchments (>5% reduction in mean annual runoff) are located in the Western and Eastern Cape, and KwaZulu-Natal. If no remedial action is taken, reductions in surface water runoff could increase to 2.59–3.15 billion m3 per year, about 50% higher than current reductions. This review illustrates the importance of measuring water-use over as wide a range of species as possible, and combining this with information from remote sensing to extrapolate the results to landscapes and catchments. These methods will soon provide much more robust estimates of water use by alien plants at appropriate spatial and temporal scales. The results of these studies can be used in water supply system studies to estimate the impacts on the assured yields. This information can also be used by catchment water resource managers to guide decision-makers when prioritising areas for clearing and rehabilitation, and for targeting species for control measures.
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