Forest fires caused by invasive species: How foreign plants and animals increase the risk of fire

Invasive species – a term we encounter frequently. These animals, plants, fungi, and microorganisms, whether introduced accidentally or intentionally, can pose a threat to humans, animals, and nature and have become a global problem in recent decades.

In Germany, the large gland ant (Tapinoma magnum), which undermines and unstables houses and sidewalks, made headlines. The cute-sounding but poisonous lantana has not only poisoned the Indian population but even threatened the entire tiger population in a reserve there. Hawaii originally had no mosquitoes until they were introduced by whaling ships in the early nineteenth century—with them came diseases like avian malaria. And rotting carpets of water hyacinth, originally from the Amazon region, are now deteriorating water quality in over 50 countries on five continents.

Even native plants, such as blue-green algae, are problematic when they multiply uncontrollably. However, alien species inherently pose a greater threat than native species because natural enemies or defense mechanisms have not been able to develop over centuries and millennia.

We are increasingly confronted with fires that are increasing in frequency and intensity. Climate change is repeatedly cited as one of the main reasons for this. Heat and drought alone do not trigger the fires, but they do make forests more vulnerable to them. The same applies to invasive species. Pests such as the Asian longhorn beetle , the ash-infecting fungus False White Stem Cup , and the largely native bark beetle attack and weaken trees, causing them to lose their resistance to fire.

Invasive plant species cause more indirect damage. Many of them are designed for rapid growth, which displaces native plants. If they then replace plant species that retain a lot of water and are themselves rather dry, this creates the perfect conditions for fires.

In the western United States and Australia, this is no longer a theoretical problem. There are blacklists of invasive plant species and management plans to contain damage. One of the main problems is pasture grasses, which were deliberately introduced as fast-growing and robust forage plants and are now spreading almost uncontrollably.

In the US state of Arizona , the invasive red brome , originally native to Europe, and buffalo grass , originally native to Africa, Asia, and the Middle East, are currently causing concern. Brome grasses are considered high-risk fire plants because they are very dry and spread rapidly and extensively. Buffalo grass was originally intended to serve as a forage plant, primarily in the arid region of the Sonoran Desert, but it spread faster than expected and now dominates the landscape. It is highly flammable in the Arizona heat, burning quickly and intensely.

The Eurasian brome, native to the roof of the house, is causing a different problem in the Northwestern United States: It spreads so rapidly after a fire that other plants have little chance to regenerate. Thus, it not only contributes to the increased intensity of fires, but also to their frequency due to its flammability.

The Australian Department of the Environment has been warning for years that African grasses, in particular, can permanently alter the Australian savannah ecosystem. In the 1940s, gamba grass, among other crops, was planted on a massive scale as animal feed. It grows quickly, can reach several meters in height, and has now become a virtual monoculture in Australia's northern savannas. When a fire breaks out, it literally spreads through the grass carpet like wildfire, feeding it with its high biomass, which is significantly higher than that of native grasses. The flames destroy the trees that originally characterized the savannah, transforming it into grassy steppes.

Alien species don't just spread to new regions by chance. They are often introduced accidentally through trade and tourism, but also intentionally, for example, as animal feed or to stabilize soils. More resilient plants establish themselves more easily in challenging soils, compacting them and protecting them from erosion. For example, the risk of slopes slipping during heavy rains is reduced.

But it can also be the other way around: They degrade soil quality and promote erosion, for example, when their strong roots break up what should have been solid ground. In such cases, the risk of landslides on steep slopes increases, making it more difficult for firefighters to fight fires.

The spread of invasive species is also increasingly changing the daily lives of firefighters. Many aquatic alien species, such as snails, fish, crabs, and slugs, are considered highly invasive. Therefore, fire departments in North America are encouraged to prevent the spread of invasive species through firefighting operations. According to a joint guideline from environmental protection agencies , equipment should be decontaminated after each operation to prevent the transmission of invasive species from one area to another.

Every piece of equipment that has come into contact with water from natural sources should be thoroughly cleaned. This applies especially to hoses, valves, and firefighting water tanks on helicopters. Pumps, portable water containers, water tanks on emergency vehicles, the helicopters and vehicles themselves, tire treads, and shoe soles should also be cleaned with hot water and, if necessary, chlorine or bleach. Everything should be free of plants, small animals, and mud, and should be dry before the next deployment.

Various experts predict that forest and wildfires will become even more frequent in the future. The number of severe forest fires worldwide doubled between 2003 and 2023, with the six most extreme years occurring from 2017 onward.

A research group founded after the LA fire disaster estimates global warming at 1.3 degrees Celsius compared to pre-industrial times. This makes large fires 35 percent more likely and six percent more intense. These figures are expected to double by 2100.

Here in Europe, too, especially in the holiday regions of the South, the risk of forest and wildfires is increasing. How can we combat them, or better yet, prevent them? There are numerous approaches designed to answer this complex question.

A spatial concept is particularly needed in forests, said fire ecologist Alexander Held in an interview with RND in 2022. This includes, for example, paths with lateral protection zones for the fire service, the removal of dead wood, mowing (especially of fire-accelerating plants), mulching, an adapted selection of tree species, soil enrichment, and more.

Even if forests would never become completely fireproof, they could be made more natural, cooler, and more humid. However, this would involve a lot of effort, expense, and many hours of work.