Forest fires, the flickering disaster

Forest fires, the flickering disaster

NZZ explains:

Forest fires – the flickering disaster


Most recently, large areas in Australia burned. But at any time of the year, forests are on fire somewhere on earth, so humans and animals have to flee. Very often the danger is homemade. However, it can be contained. Surprising insights into a global phenomenon, with numerous data from Switzerland.

800 degrees Celsius: This is the temperature in a typical forest fire. Aluminium has long since liquefied under such embers. In large fires, the air can get even hotter.

However, the heat generated by forest fires is by no means the only danger for humans. When trees burn down, soot particles and toxic gases such as carbon monoxide and nitrogen oxides are also released. Wind can transport them over many kilometers into the distance. And then there are the long-term consequences: the removal of protective forest brings an increase in erosion and landslides.

How do forest fires develop?

Forests in flames have been around as long as the forests themselves. In order for a forest to start burning, three conditions must generally be met: There must be enough combustible material, sufficient oxygen – and enough ignition energy.

Before humans became acquainted with fire, forest fires were ignited mainly by thunderstorm lightning. Where volcanoes are active, lava flows or flying hot rock debris may also have started fires. These two natural causes still occur today.

Most forest fires are man-made

Proportion of forest fires in Switzerland caused by . . .
Most forest fires are caused by human resources - proportion of forest fires in Switzerland caused by . . .

Today, in most populated regions of the world, it is predominantly humans who cause the forest fires. Not infrequently, it is arson – for whatever motive. In addition, there is carelessness: campfires that have not been extinguished can spread into the forest. Discarded cigarettes are a possible source of fire. Technical equipment, whether cars, chainsaws or power lines, occasionally generate sparks, and then dry blades of grass or twigs may catch fire.

How do the fires spread?

Not all forest fires are the same. Roughly speaking, three types are distinguished: crown fire, ground fire and earth fire. While the ground fires occur most frequently and the crown fires pose the greatest danger, the earth fires can perhaps be described as the most insidious. These are underground smouldering fires, for example in peat soil.

Forest Fire Types

What role do fires play in the forest?

How nature, which is completely untouched by human hands, reacts to a forest fire is not so easy to say. In Switzerland, for example, such forests no longer exist. However, this much is clear: Natural forest fires have many different ecological effects.

Animals have to flee. However, they can often do this while the plants burn, unless they are protected in some way. However, the effect of a forest fire is by no means purely negative for the flora, rather natural forests are often dependent on it. The fire destroys biomass, including pathogens, and it destroys many chemical substances that have accumulated in the soil and are sometimes toxic to vegetation. The floor is virtually sterilized. This gives nature the chance for a fresh start.

Trees after a forest fire near the village of Cardigos, Portugal, on July 22, 2019. (Image: Rafael Marchante/Reuters)

Trees after a forest fire near the village of Cardigos, Portugal, on July 22, 2019. (Photo: Rafael Marchante / Reuters)

In cold regions, without fire, it would take a long time for the biomass to decompose sufficiently. A fire accelerates this process: the ash contains important minerals, it acts like a fertilizer.

In addition, the fire initiates the rejuvenation of the forest. New plants can settle in the clearings. In many cases, this increases biodiversity. The cones of many conifers can only open and the seeds germinate only after they have been exposed to the heat of a forest fire. These species are therefore dependent on forest fires for their survival.

In the human environment, fire-resistant species may hold up better than the more sensitive ones because of the more frequent fires. Among the species that do not tolerate fire well in Switzerland are, for example, the elms and the lime trees.

How are forest fires fought?

To extinguish fires, not only the usual equipment of fire brigades is often used, but also helicopters and airplanes that can refuel with water. In order to stop forest fires, aisles are also drawn: a strip of forest is freed from all combustible material or moistened so strongly that the fire is stopped there. Images with infrared cameras help to detect leftover ember nests.

The helicopter sets off the water. (Photo: Juan Medina / Reuters)
11 images

The helicopter sets off the water. (Photo: Juan Medina / Reuters)

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What is the situation in Switzerland?

32 percent of the country’s surface is forested. In other words, there are 66 forest trees per inhabitant. Of this, softwood accounts for two thirds, hardwood one third. The three most common species are spruce, fir and beech. The Jura and the southern side of the Alps are particularly densely forested.

Cover by deciduous trees (light green) or conifers (dark green)

Which forests in Switzerland are particularly threatened? And why?

Forest fires in Switzerland occur most frequently on the southern side of the Alps. This is the result of the SwissFire database, which was founded by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and the Federal Office for the Environment (FOEN). Much less often the forests blaze in the Central Alps and in the Jura and least often in the Swiss Plateau. The fires in these regions tend to occur in different seasons.

Most forest fires in Switzerland occur on the southern side of the Alps. . .

Annual number of forest fires between 1990 and 2014
Most forest fires in Switzerland occur on the southern side of the Alps . . . - Annual number of forest fires between 1990 and 2014

. . . where they also destroy the largest area of forest

Burnt area in hectares on the southern side of the Alps
. . . where they also destroy the largest area of forest - Burnt area in hectares on the southern side of the Alps

Winter on the southern side of the Alps, especially in Ticino, is often very dry. That’s why it often burns there. The foehn plays a decisive role: the warm north wind dries out the layer of branches and leaves on the bottom of chestnut forests. Then there are always ground fires. Almost all fires in winter are caused by humans, half by negligence.

Number of fires in the winter and summer months on the southern side of the Alps

A case study from the Misox Valley

At the end of December 2016, there was still no snow in a valley in the southwest of Graubünden – the Misox, which is located on the southern side of the Alps. Then the forest burned there. Strong winds drove the fire up. Over the course of a day, the fire grew from an area of 20 football pitches to one of around 100 football pitches: 119 hectares.

The Misox fire accounted for 26% of forest fires throughout Switzerland

Burnt area in 2016 in hectares
The Misox fire accounted for 26% of forest fires throughout Switzerland - Burnt area in 2016 in hectares

Forest fires in the change of seasons

On the southern side of the Alps, it rains less in the winter half of the year than in the summer half-year. That’s why the forest fires occur there, especially in the cold months. This is different in the other areas of the Alps. In the Central Alps, for example, lightning strikes are half the cause of forest fires in summer.

Monthly distribution of forest fires in the Alpine regions caused by . . .

What are the negative consequences of forest fires in Switzerland?

The flames are rarely the problem in this country. Rather, damage that affects people occurs in the days, weeks, and months after the fire. This has to do with the fact that four tenths of the forest in Switzerland is protective forest.

If the trees burn, their roots often die afterwards. As a result, mountain slopes are destabilized and landslides occur. If it rains, the water can penetrate poorly into the ground because of the ash. Instead, it flows off superficially. First gutters form, then the erosion picks up speed. Finally, whole mudslides come down the mountain. In winter, avalanches can thunder more easily into the valley on slopes where forest fires have raged. However, this problem occurs rather rarely.

What about forest fires in Europe?

Forests cover more than 215 million hectares in Europe, a third of the entire continent. Most of them grow in the lowlands of the north and east, further south often in the mountains.

Which forests in Europe are particularly threatened?

Whether the deciduous maquis in southern France or pine forests in the Alps – forests in which highly combustible material accumulates are particularly at risk. The flames become a problem especially where the forest and human settlements touch each other: in so-called Wildland-Urban Interfaces (WUI). Trees, for example, often serve as shade for houses. Roads and paths that lead through fire-prone areas can become a deadly trap. A WUI map like this, using the example of Bellinzona, shows where man-made forest fires can occur.

The Wildland-Urban Interface in Bellinzona

Areas with an increased risk of forest fires
The Wildland-Urban Interface in Bellinzona - Areas with increased forest fire risk

Exotics in the forests pose a special problem. In Portugal, eucalyptus trees were planted for many years because the wood quickly pays off as an export product. The trees, which come from Australia, however, burned “like torches,” says Marco Conedera of the WSL research institute.

A similar problem exists in northern Germany. In the past, large quantities of pines and spruces were planted there. However, these usually grow mainly in Scandinavia and northern Russia, where forest fires are a natural rejuvenation process. The management of the coniferous forests in Germany makes them even more susceptible to fires: The aisles in the forests for the access facilitate dehydration, the drought slowly decomposes the needles. In the event of drought, one spark is enough to trigger a major fire.

European countries with the most forest fires

Number of forest fires and total hectares burned between 2005 and 2016
Land Number of fires burned area (in hectares)
Portugal 238 192
1 265 666
Spain 166 860
1 258 359
Poland 99 932
44 617
Italy 73 877
858 927
Sweden 48 801
36 036
France 46 182
131 527
Turkey 28 021
101 007
Finland 15 851
7 076
Greece 14 413
26 540
Hungary 10 071
52 683

How do we keep the forest fires from our bodies?

In most countries, humans are the main cause of forest fires. That is why behavioural changes are a central component of prevention. In areas threatened by forest fires, visitors may be urged to exercise caution, for example when lighting campfires, or a fire ban may be issued. A well-developed information and warning system is essential for this purpose.

Near-natural silviculture is considered a useful preventive measure in the long term. 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 this mixture provides greater moisture. If, on the other hand, monocultures are planted and only one generation is allowed, entire forests remain stuck for a long time in a development phase in which they are susceptible to fires.

How does fire management work?

In North America, a lot of experience has been gained with “fire management”. For the prevention of fires, for example, there is a lot of work with controlled fires: These are fires set with the intention of reducing the accumulated fire material without risk. It is important to light such intentionally set fires when the forest is just dry enough to burn near the ground, but still moist enough that crowns and humus layer remain intact. Only then can the fire achieve the desired preventive and ecological effects and at the same time be easily kept under control.

Controlled fires were already set by the indigenous inhabitants of North America. Then the settlers came and tried to permanently stop forest fires because they wanted to use the trees economically. The result was that vast quantities of fire material accumulated. At some point, huge fires broke out. This effect is referred to as the “fire paradox”.

In the middle of the 20th century, a paradigm shift took place in North America: The inhabitants realized that with the permanent prevention of forest fires, the fire danger grew. So they reduced the fight against the fires and took up fire management. Controlled fires played an important role in this. In the meantime, this practice is also being experimented with in Portugal and Italy.

How is the handling of forest fires in Switzerland changing in the age of climate change?

Due to the warming of two to five degrees Celsius in the course of the 21st century alone, the conditions will change so much that some trees in today’s location will become uncomfortable. However, the rising temperatures are only one thing, the changes in precipitation the other. According to regional scenarios for Switzerland, forests will have to make do with less water during the growing season in the future. The stress of severe droughts will increase.

Bödmerenwald in the Muota Valley in the canton of Schwyz, with over 600 hectares the largest spruce forest in the Alps, 1350–1680 m above sea level.M. (Image: Gaetan Bally / Keystone)

Bödmerenwald in the Muota Valley in the canton of Schwyz, with over 600 hectares the largest spruce primeval forest in the Alps, 1350–1680 m above sea level. M. (Image: Gaetan Bally / Keystone)

Spruce and beech trees are expected to be among the victims, which is already becoming apparent after the drought of 2018. In the future, they will no longer be at home in the Swiss Plateau. In higher mountain areas, however, there will still be rooms for the spruce to thrive. The grape oak, on the other hand, benefits from the change – it spreads under the warmer conditions.

Climate change is currently happening faster than the forest can get used to. That is why the risk of forest fires can grow, especially in the transitional period. For this reason, humans must come to the rescue – and adapt the composition of the forests preventively.

People with umbrellas, taken on Thursday, July 8, 2021, in Rorschach. (KEYSTONE/Gian Ehrenzeller) (Gian Ehrenzeller / KEYSTONE)

50 degrees in Canada, flood of the century in Germany – this is how climate change affects us

Sven Titz (Text) / Alexandra Kohler and Balz Rittmeyer (Graphics)

The methodology in detail

In order to show the forested area of Switzerland, we visualized data from the National Forest Inventory (NFI), a program of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and the Federal Office for the Environment (FOEN). This dataset represents a remote sensing-based approach for a nationwide mapping of deciduous and coniferous trees with a spatial resolution of ten by ten meters. A data point thus makes a statement about the probability that such an area is deciduous or needle-dominated.

WSL provides data on the number of fires, burnt areas and causes in its forest fire database.

The Wildland Urban Interface Map (WUI map) with data from the SwissFire forest fire database shows areas where buildings and passable roads come into contact with natural vegetation, rural areas or forests. In the Alpine region, WUI is defined somewhat differently than in North America. This is about the areas where, because of the large concentration of people and their activities, most of the man-made fires arise. For the area depicted as threatened (i.e. the radius), we calculated a buffer of 100 meters around roads and easily accessible buildings (buildings 100 meters from a passable road). The size of the buffer depends on the characteristics and geographical location of the area concerned.

To show active fires in Europe, we worked with data from NASA. This dataset records fires detected by 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 the “Fire” class:
– violent fire activity on part of the pixel surface;
– Fire activity that extends over a larger area.
From the data, we only selected the “hotspots” with a confidence level of 80% and more.


Telephone conversations with Marco Conedera and Boris Pezzati, WSL.

“The Swiss forest in climate change: What developments are coming our way?”, Leaflet for Practice, WSL, 2017.

«Leben mit Waldbrand», Merkblatt für die Praxis, WSL, 2010.

Marco Conedera: “Switzerland in flames? Climate change and future developments of forest fires» in Mitteilungen der Naturforschende Gesellschaft in Bern, 2014.

Will the risk of forest fires increase in the future? Boris Pezzatti et al., Bündner Wald 70, April 2017.

Dry and flammable. Naturwald Academy, 6.6.2019.

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