Trees and forests provide a wide range of benefits and influence regional rainfall patterns. Forest cover in particular creates important microclimates that affect the weather and regulate the climate, often leading to wetter conditions. Furthermore, as climate change affects global weather patterns, it requires ongoing monitoring of the complex relationship between forests and rainfall patterns, and destructive flooding.
Forests: nature’s climate stabilisers
As far back as the mid-1800s, scientists were aware that forests create microclimates that affect rainfall. J.M. Anders noted in 1882 that forests increase rainfall uplifting wind currents high enough “for the moisture to be condensed into clouds and rain”.
Natural forests also have a stabilising effect on the climate, helping to regulate the Water Cycle. Natural forest biodiversity is one key aspect of this: their habitat complexity, diversity of vegetation size and shape, and dense understorey intercept, slow, and eliminate rainfall before it hits the ground. This canopy interception and evapotranspiration combine to return water to the atmosphere before it reaches the ground, improving water retention and reducing runoff.
Trees also rehabilitate soil and enhance soil health, elevating its ability to absorb and retain flood water. However, reducing the risk or severity of floods is not as simple as merely planting more trees. Compared to single-species plantation forests, biodiverse natural forests provide much more canopy interception and evapotranspiration, while the soils underneath them tend to be more porous with high infiltration rates. All of these factors result in lower rates of surface runoff and erosion.
Furthermore, the relationship between forests and rainfall patterns, particularly flooding is complex, and is also affected by a variety of other factors, including tree species, the position of trees in the landscape, soil type, the size of rainfall events and catchment areas, and climatic changes.
La Niña’s role in SA’s floods
La Niña is a climate phenomenon characterised by the cooling of central and eastern Pacific waters. It takes place every two to seven years, disrupting global weather patterns. These disruptions usually last between nine and 12 months, although the 2025 La Niña is anticipated to be relatively weak and short-lived.
Nevertheless, heavy rains linked to prevailing La Niña conditions this year have caused widespread flooding across South Africa in late February and early March. Flash floods have caused major infrastructure damage in Johannesburg, while a state of disaster was declared in KwaZulu-Natal after 22 flood-related deaths and R3.1 billion in damage.
This follows the devastating floods in April 2022 on the east coast of South Africa that resulted, in part, from a prolonged La Niña event. In response to the damage inflicted by these floods, a 2023 study on extreme rainfall and flood risk prediction was conducted by researchers from the University of Limpopo, in collaboration with the University of Johannesburg’s Global Change Institute and the South African Weather Service’s Climate Service.
The study found that rainfall had been significantly enhanced by interactions between unusually high winds (a strong low-level onshore airflow across the Agulhas Current) and the coastal escarpment, resulting in deep convection (heat transfer) and lifting. These effects were amplified by a cyclone that developed offshore, driving onshore southeasterly winds.
When combined with other environmental and human-related factors, these climatic variations are likely to lead to even more intense flooding events in the future.
The importance of land cover and land use changes
The current distribution of tree cover across South Africa is affected by changes in land use, which in turn can affect the severity and extent of flooding. The University of Limpopo study, for example, found that land cover/use was the highest predictor for flood risk, followed by elevation.
Because land use and associated land cover changes are closely tied to the needs of communities, they are crucial for mapping this risk. In other words, to decrease the impact of future floods, it makes sense to use mapping predictions to pinpoint areas most at risk from land use changes, such as the conversion of forest to farmland, the conversion of agricultural and forest land to urban areas, and the removal of natural slopes for infrastructure developments. This will allow informed decisions to be made regarding the use of land in specific areas, and the most effective areas to focus on for reforestation efforts with the potential to reduce flood damage.
While the entire coastal area was affected by extreme rainfall in 2022, the University of Limpopo study examined why the extensive damage to infrastructure and loss of lives was mainly concentrated in the eThekwini District. Bearing in mind the importance of land cover/use and elevation, it found that rapidly increasing urbanisation means most people are moving to low-lying urban areas where they “remove natural vegetation, flatten steep slopes for construction, and install paving on the ground, thus exacerbating flood risks.”
Urban flooding
Impermeable surfaces and poor drainage in these urban areas increase the risk and severity of flooding, as water that would otherwise seep into the soil instead accumulates as it flows over paved surfaces and roads.
Rapid urban development, an increasing number of impermeable surfaces, poor drainage systems, and changes in extreme precipitation are some of the most important factors contributing to increased urban flooding. Urban flood mapping and its use in making an urban development plan can therefore reduce flood damages and losses.
Urban trees and green spaces represent one of the most effective ways to mitigate the effects of flooding. Large green spaces and urban forests, if planned and positioned well, can provide a highly effective defence against flood damage by retaining vast amounts of water, while smaller green spaces strategically positioned throughout urban areas can also stem the build-up of flood water and reduce runoff.
Densely vegetated urban food forests, for example, are very effective in slowing water, allowing it to infiltrate the soil. Domestic gardens also help to manage surface water runoff and flooding in urban areas, particularly if they contain diverse trees and other plants.
The close and complex links between forests and rainfall patterns, particularly in forested areas, must clearly be kept in mind as climate change continues to affect the number and severity of La Niña events and associated flooding. A better understanding of the effects of land use changes, the removal of trees, and the potential to create green areas that can help to reduce urban flooding may well have a critical role to play in protecting communities in the most at-risk areas of South Africa.
