UCT study finds exotic plants fuel Cape Town wildfires

Certain exotic plants found in Cape Town’s urban areas may intensify the risks of wildfires, new research has found.

The study, conducted in the aftermath of the Table Mountain fire in April 2021, highlights how the flammability of vegetation is driven by specific plant leaf traits.

A team of scientists at the University of Cape Town’s department of biological sciences will publish their research in the September issue of the South African Journal of Botany.

Their goal: to examine how plant leaf characteristics influence flammability in an urban setting.

“While laboratory tests alone cannot fully predict how plants behave in real-world fires, they offer valuable insight into key characteristics that influence ignition, burn duration and intensity,” said Muthama Muasya, the study’s senior author and an internationally recognised plant taxonomist and evolutionary biologist.

“This information is essential for natural hazard planning, especially in fire-prone regions like the Western Cape,” he added.

The authors analysed 42 plant species — 22 native and 20 exotic — for leaf functional traits and key flammability components: ignitibility, sustainability and combustibility.

Their results showed that exotic species generally ignited faster and exhibited higher combustibility than native species. Conversely, native species tended to burn longer but at lower intensities.

“Leaf functional traits such as thickness, specific leaf area and leaf surface area were significantly correlated with flammability measures,” the study said. “Exotic plants were particularly flammable due to traits like large leaf surface area, higher specific leaf area and thin leaves.”

Further results showed that plants that burned in the 2021 fire were mainly associated with combustibility and moderate ignitibility, while those that partially burned tended to have thicker leaves. Those that didn’t burn were associated with longer ignition times.

The authors said the findings emphasise the role of some exotic ornamental plants in increasing fire risk in urban areas.

The study noted how the April 2021 Table Mountain fire was attributed to the abundance of exotic plants, which easily caught fire during the event. Two of the main exotic plants were at its epicentre: Pinus pinea (stone pine) and Phoenix canariensis (Canary Island date palm). 

The Canary Island date palm is a popular ornamental tree planted along Cape Town’s boulevards and near buildings, posing a significant fire hazard.

The April 2021 wildfire began on the slopes of Table Mountain National Park. It spread rapidly due to a build-up of debris from exotic trees and other indigenous plants.

Wind-carried embers caused spot fires that destroyed multiple buildings — including parts of the university. “The way that different localities surrounding the university were engulfed in flames, and the fire’s expansion into nearby areas in Cape Town, suggested that spot fires were ignited by wind-blown embers at distances up to 350m away from the main fire line,” the study noted.

The fire crossed barriers such as the M3 highway and rugby fields, demonstrating the limited effectiveness of fire breaks in such intense conditions. More than 650ha was burned, and the loss of infrastructure will take years to recover.

The study warned that wildfire risk is increasing globally, especially in Mediterranean-type ecosystems such as the Western Cape, because of climate change and flammable exotic species.

“Our region, like other Mediterranean regions, has hot, dry summers and cool, wet winters; however, with climate change, there is an overall trend showing that while atmospheric moisture levels have increased and rainfall events are becoming more intense globally, there is also an increase in the amount of evapotranspiration [moisture loss] occurring,” said lead author Dunja MacAlister.

This means that although there is rain during the winter months, a lot of evaporation occurs in the hotter months, making everything drier.

“In the last century of global warming, there has also been an increase in the frequency of occurrences of extreme events such as heatwaves and droughts,” she said.

“This all amplifies the risk of intense fires all over the world, as it leads to drying out of vegetation faster, including plants in gardens, green belts and in urban spaces, therefore increasing their flammability.”

At this point, exotic species, especially those with high oil content in their leaves and abundant biomass, “become very dangerous fire fuels”, MacAlister said.

Muasya said that exotic species that combine large, thin leaves with tall, woody growth and high retention of dry or dead foliage are of particular concern. “These characteristics dramatically increase combustibility and the ability to trap burning embers, especially under windy conditions.

“Species such as Cedrus atlantica [a species in the pine family], Cupressus sp [cypress], Hedera canariensis [Canary ivy] and, in particular, Phoenix canariensis and Piinus pinea [stone pine], are also a problem in other Mediterranean areas, such as California, with policies warning stakeholders to avoid planting them as they are flammable, invasive or both,” he said.

Fire-prone Mediterranean shrublands — including those in South Africa, California, Australia, and southern Europe — are increasingly facing extreme fire events known as megafires (fires greater than 10 000 hectares).

The study cites the 2017 Knysna wildfires, which killed seven people and burned 15 000ha, as an example of how exotic plants and extreme weather can drive fire severity.

The severity of the Knysna wildfires has been primarily attributed to the extensive fuel load caused by exotic plants in the region, an expansive wildland-urban interface, unprecedented droughts prior to the fire and very high fire danger weather conditions.

Become fire smart

“Urban greening projects need to be fire smart,” MacAlister said. “This means avoiding high-flammability species in zones such as homes, schools or infrastructure and prioritising low-flammability native species.”

She emphasised the need to integrate knowledge of fuel loads, leaf litter and fire behaviour into planning decisions — and not just aesthetics and biodiversity goals.

“Our research highlights the importance of assessing species flammability as part of plant selection. A great example is what happened in California recently and in the past. They have an abundance of exotic palm trees growing along boulevards and urban areas, and these trees are incredibly flammable, particularly if not regularly pruned.

“All that biomass sitting in the tree only adds more fuel to the fire, and as my paper shows, the Canary Island palm [trees] are very combustible, giving off a lot of heat during fires and therefore allowing the spread of fire to happen easier.”

She frequently thinks about how flammable the Cape Town suburb of Pinelands is. “There are palms and pines planted all over the suburb, and many houses have thatched roofs, making the idea of a fire occurring in the area even more terrifying.”

She advocates for replacing exotic vegetation with fire-adapted native species, which can restore biodiversity and offer aesthetic and ecological benefits without the same fire risk.

“Removing exotic species from native vegetation, especially those shown to increase fire risks, like certain invasive trees and shrubs, can significantly improve ecological resilience and reduce the likelihood of severe wildfires.”

The removal of exotic species must be done thoughtfully. “This is especially in places where they dominate, as their sudden removal could lead to habitat gaps and soil erosion if their removal is not followed up with active restoration of the area.”

Replacing these species with native vegetation, particularly those adapted to the local climate, can enhance natural biodiversity, restore ecological functioning and reduce long-term fire risks.

“Many native species, including trees and shrubs, can offer aesthetic value, shade, and most importantly, habitats for local wildlife, without the same flammability risks.”

Every year, Cape Town and surrounding areas experience hotter summers, and there is news of intense fires breaking out during the summer season, she said.

“Climate groups indicate worsening fire conditions in the coming decades, particularly in fire-prone regions like Cape Town and the Western Cape. We can’t wait for fires to occur before doing something.”

Planning and policy must embrace proactive planning instead of reactive responses. “We need to prioritise vegetation management as part of the fire mitigation strategies, which can be challenging in itself due to the fynbos biome being fire prone.

“However, removing high-risk species, restoring native vegetation in these areas, and maintaining fire breaks can significantly reduce the risk of high-intensity fires that reach urban areas in the future.”