Anna Wiederkehr | Sven Titz
800 degrees Celsius: that’s the temperature of a typical forest fire. At that point, aluminium has turned into liquid and the air temperature has been significantly cranked up.
But the heat emitted from wildfires is not the biggest danger to humans. As trees burn they release soot particles and toxic gases like carbon monoxide and nitrogen oxides. Wind transports these gases over kilometers. The destruction of protective forests can cause erosion and mudslides.
How do wildfires start?
Forests fires have existed as long as forest themselves. Generally, there needs to be three prerequisites in order for a fires to start: fuel, oxygen and heat.
Before humans made friends with fire, wildfires were ignited exclusively by natural occurrences, like lighting. Where volcanos are active, lava or hot rock debris can also start a fire.
In the more populated regions of the world however, wildfires are caused by humans. Arson, for whatever reason, happens more often than one might think. Carelessness is commonly the cause: campfires not properly extinguished can spread easily into a forest. Discarded cigarettes are also a frequent ignition source. Technical equipment, whether a chainsaw, car or power line, can produce sparks that quickly set small leaves and twigs ablaze.
How do fires spread?
All wildfires are the not the same. They can be broken down roughly into three types: crown fires, ground fires and peat fires. While ground fires are most frequent and crown fires are most dangerous, one could consider peat fires the more insidious. These smoulder subterraneously and can even travel below roads.
What role do wildfires play in nature?
How fires caused by humans affect untouched nature is not so easy to say. In Switzerland, for example, we know there are forests that no longer exist. What is clear is that natural wildfires have several different ecological purposes.
When burning starts, animals flee. For flora, the effects of a forest fire are never strictly negative. Iin fact, many natural forests depend on them. Fire destroys biomass, pathogens, and many chemical substances that have accumulated in the soil. Some of which have a toxic effect on the vegetation. Through fire the soil is sterilized and thus gives nature a chance to restart.
Cold regions take a long time to sufficiently decompose built up biomass. Fires accelerate this process: The ash contains important minerals and acts like a fertilizer.
Fires also rejuvenate forests. Clearings created in the aftermath make way for new plants to grow. In many cases this actually increases the plant diversity. Pine cones from coniferous trees are stimulated by heat, which allows them to open and germinate their seeds. These species are dependent on forest fires to survive.
How do we fight forest fires?
Along with the usual equipment of a fire brigade, helicopters and airplanes with the capacity for carrying water are also used to extinguish a wildfire. In addition, a method of creating firebreaks (or aisles) is used: a strip of forest is freed from all combustible material or moistened so strongly that the fire cannot spread. Infrared cameras help track down any remaining smouldering nests.
What is the situation in Switzerland?
32% of Switzerland is covered in forest. Formulated another way: for every one citizen, tower 66 trees. Two-thirds are coniferous trees and the other third is deciduous. The three most common species are spruce, fir and beech. The regions of Jura and the southern Alps have especially thick forests.
Which forests in Switzerland are threatened by wildfires and why?
Wildfires burn most frequently in the southern Alps. This finding emerged from the SwissFire database, founded by the research institute WSL (Wald, Schnee und Landschaft) and the Swiss Federal Office for the Environment (BAFU). Fires occur significantly less in the central Alps and Jura region and rarely occur in the Swiss midlands. Depending on the region, wildfires occur in different seasons.
Winter on the southern side of the Alps, specifically in Ticino, is extremely dry. As a result, it is a haven for forest fires. The Föhn, a hot southerly wind on the northern slopes of the Alps, plays a deciding factor by drying out the layer of branches and leaves at the bottom of chestnut forests. Virtually all of those fires are caused by humans — half of them from negligence.
A fire in the Misox Valley
It’s the end of December in 2006 and in the Misox Valley, in the southwest of canton Graubünden, there’s still no snow. Then, the forest starts to burn with strong winds that drive the fire up the valley slopes. In just a few days the fire expanded from 20 football fields in size to 100, or 119 hectares.
Waldbrände im Wechsel der Jahreszeiten
Unlike the dry, fire-prone winters in the southern Alps, the central Alps have less rain in the summer months. There, fires are more likely to burn in July and August, half of which are ignited by lightning strikes.
Are there negative consequences in Switzerland?
In Switzerland, flames are rarely the problem. The damage that affects people is more likely to occur in the days, weeks and months after the fire. This has to do with the fact that 40% of the forest in Switzerland are protective forests.
When trees burn their roots die. As a result, mountain slopes are destabilized which can lead to landslides. If it rains, the ash left over doesn’t allow the water to penetrate the soil. Grooves start to form, erosion picks up speed and eventually a lot of mud comes down the mountain. Avalanches are also much more likely to thunder down mountain slopes where forest fires have burned. All things considered, these dangers are still relatively rare in Switzerland.
What about forest fires in Europe?
Forest covers more than 215 million hectares in Europe — one-third of the entire continent. The majority grow in the lowlands of the north and east, as well as often further south in the mountains.
Which forests in Europe are threatened?
Whether it be the deciduous forest Macchia in the south of France or pine forests in the Alps — forests are at risk when a lot of combustible material is allowed to collect. Flames become particularly problem where the forest comes in contact with human settlements: in the so-called Wildland Urban Interfaces (WUI). Where, for example, trees give shade to houses or hiking paths leading through fire-proned areas. A WUI map, such as the following of the city of Bellinzona, shows where there is a higher risk of man-made forest fires:
A specific problem exists in exotic forests. In Portugal, eucalyptus trees were planted for many years, because it is very profitable as an export. However, Eucalyptus trees are indigenous to Australia and they burn «like torches» as Marco Conedera from the research institute WSL puts it.
A similar problem exists in northern Germany. In the past, large quantities of spruce and pine trees were planted. Spruce and pine typically grow in Scandinavia and northern Russia, where wildfires are a normal part of the forest rejuvenation process. The management of coniferous forests in Germany makes them even more susceptible to fires: The firebreaks in the forests which allow vehicles to drive through facilitate dehydration. These dry conditions impede the decomposition of pine needles. In times of drought, only a small spark is needed to start a major fire.
How do we stop forest fires?
People are the main cause of wildfires in most countries. This is why behavioral changes are a central component of fire prevention. A well-developed information and warning system is essential in regions threatened by wildfires. Often visitors are cautioned when lighting campfires, or a fire ban may be imposed.
In the longterm, natural silviculture is considered a useful preventive measure. If several generations of trees and many different species suitable for the region are allowed to grow in a forest, the risk of fire is often lower because a mixture provides greater moisture. Planting monocultures and only allowing for one generation forces forests to remain in a developmental phase for a long time. This in turn makes them more prone to fires.
How does fire management work?
In North America much experience has been gained overtime in terms of fire management. One preventative method is controlled burning: intentional fires in order to relieve accumulated fuel in order to reduce fire risk. It’s important to do control burning when the forest is dry enough to burn near the ground, but still wet enough for the crown and humus layer remain intact. Only then can the fires achieve the desired preventative and ecological effects.
Controlled fires were already set by the indigenous inhabitants of North America. Then the settlers came and tried to prevent forest fires completely for economic reasons — but also because they didn’t understand the role of fire in an ecosystem. Vast amounts of fire material accumulated as a consequence. At some point, huge fires started. This effect is called a “fire paradox”.
In the middle of the 20th century, there was a paradigm shift: We learned that longterm repression of forest fires increase the danger of forest fires. Prevention efforts were reduced and largely replaced with fire management and methods like control burning. In the meantime, other countries like Portugal and Italy have experimented with the practice of controlled burning.
How will the handling of forest fires in Switzerland change in a time of climate change?
Just by warming two to five degrees Celsius over the course of the 21st century, conditions will change so dramatically that some trees will be extremely uncomfortable where they currently burgeon. The rising temperatures is only one factor — the change in precipitation is the other. According to regional scenarios for Switzerland, the future forests will have to manage with less water during the growing season. The stress of severe droughts will increase.
Spruce and beech trees are already expected to be victims, which became apparent after the drought of 2018. They will no longer feel at home in the midlands and only in higher mountain regions will there be a place for the spruce to thrive. In contrast, Grape Oak, benefits from the change — it flourishes under warmer conditions.
Climate change is occurring faster than forests can adapt. During the transition period of rising temperatures and dryer environments, the risk of wildfires can increase. In order to cope, we are responsible to preventatively adapt the composition of the forests.
Methodology in Detail
In order to show the forested areas in Switzerland, we visualized data from the National Forest Inventory (NFI), a program from WSL and BAFU. This dataset uses a remote-sensing approach to map deciduous (leafy) and coniferous (needle) trees nationwide with a spatial resolution of ten by ten meters. A single data point thus provides an indication of the likelihood that an area will be leafy or needle-dominated.
Data showing the number of fires, amount of hectares burned and the distribution of wildfire causes is made available by the Eidgenössische Forschungsanstalt für Wald, Schnee und Landschaft (WSL) through their wildfire database (SwissFire).
The Wildland Urban Interface (WUI) map typically shows areas where building and other infrastructure come in contact with natural vegetation or forested areas. In alpine regions, the WUI is computed differently than in North America. Our map highlights areas where there is a large concentration of people and activity that causes fires. To define places with increased risk of ignition (i.e. the radius), we calculated a 100 meters buffer around roads and easily accessible buildings (buildings 100 meters from a drive-able road). How large the buffer is, is dependant on the characteristics and geographic location of the area in question.
In order to show active fires in Europe, we worked with NASA data. This dataset lists fires detected with an algorithm from the MODIS satellite that uses infrared radiation. The algorithm examines each pixel of the satellite image and sorts it into the following classes: missing data, clouds, water, no fire, fire, unknown. The algorithm assigns the following situations to class «fire»:
– intense fire activity on a part of the pixel surface
– fire activity that extends over a larger area.
We display only the «Hotspots» from the data which are categorized with a confidence interval of 80% or more.
Telephone interviews with Marco Conedera and Boris Pezzatti, WSL.
«Der Schweizer Wald im Klimawandel: Welche Entwicklungen kommen auf uns zu?», Merkblatt für die Praxis, WSL, 2017.
«Leben mit Waldbrand», Merkblatt für die Praxis, WSL, 2010.
Marco Conedera: «Die Schweiz in Flammen? Klimawandel und zukünftige Entwicklungen der Waldbrände» in Mitteilungen der Naturforschenden Gesellschaft in Bern, 2014.
Wird die Waldbrandgefahr in Zukunft zunehmen? Boris Pezzatti et al., Bündner Wald 70, April 2017.
Trocken und brandgefährlich. Naturwald Akademie, 6.6.2019.
Thanks to Marie-José Kolly, Manuel Roth and David Bauer.