Forest fires in Germany

Forest fires in Germany are only rarely the result of natural causes. Lightning strike, the sole natural cause, is a relatively insignificant cause of forest fires and as lighting is often accompanied by rain, it only damages individual trees or burns a very small area.

Additionally, the ecological importance of forest fires in central Europe is relatively low. Only a very few animal and plant species are dependent on forest fires for example the Black Fire Beetle (Melanophila acuminata) which has adapted its survival strategy very well to the seldom occurring natural forest fires.

Along with timber, which predominantly consists of cellulose and lignin, plant components such as leaves, needles, bark, grass and moss also burn in a forest fire. Fine fuels such as grass, small twigs or pine needles ignite at temperatures between 280 - 300 °C. These combustible materials are characterised by a low mass and a large surface area. The fuel’s flammability is determined by its density and moisture content. Dry grass and heather are more easily ignited than dense litter or forest slash. In dry and low humidity conditions, combustible materials dry out so much that even a little warmth is sufficient to evaporate the remaining moisture and ignite the potentially flammable material. The stronger the wind, which provides the fire with oxygen, and the drier the combustible material, the quicker the surface fire spreads as a narrow band.

Fires in spring mostly burn dried plant residues from the previous season. Winter precipitation protects the lower ground and plant layer. However, during a dry summer and on slightly permeable, poor soils the complete humus layer including stumps and upper roots can burn down to the mineral soil layer.

Depending on the fire’s origins, the weather conditions, vegetation ground layer and forest stand, fires can occur in various forms.

Surface fires are the most common and aerial (also known as crown) fires the most dangerous type of forest fire.

Along with these two forms there are also ground (peat) fires and torching fires. In ground fires, the subterranean peat layers of drained or dried out bogs smoulder away. Peat fires are characterized by their circular shape and slow rate of spread. The rarest form of forest fire in central Europe is torching whereby a single hollow or dry tree ignites mostly due to lightning strike.

A forest fire spreads out from its point of origin in all directions. A fire can climb up into the crown via a so called “fire ladder”. This occurs if the surface fire gets enough energy from the burning grass, litter, heather or brushwood that the needles and bark of densely branched trees dry out and ignite. Thus the surface fire ignites a crown fire and becomes an aerial fire. Crown fires are dependent on surface fires and can not jump from tree crown to tree crown. Although with strong winds or extreme droughts, they can precede the surface fire by a few meters, but they must be heated by an associated surface fire.

The smoke of a crown fire is dark, almost black, due to the accompanying charcoal. Thus the intensity of the fire is recognizable even from far away. A crown fire collapses into a surface fire again when the surface fire becomes weaker, there is a lack of densely branched trees or if it meets a patch of deciduous forest.

Firebrands are dangerous in crown fires. Rising hot air can carry sparks of pinecones, moss, charcoal or birch bark tar up to 400m ahead of the fire front to ignite spot fires. These flying sparks allow even wide patches of deciduous forest to be jumped over.

A fire is extinguished when no more combustible materials exist which can be ignited through radiation or firebrands. Extinguishing a fire aims for the same effect using retardants, rainwater or by suffocating with sand.

The shape of the burnt area is, depending on wind conditions, an elongated oval. It burns in an approximately 20–50 cm wide front on the fire perimeter. Only when there are large amounts of brushwood can this front expand to a width of one to two metres. Individual pieces of wood, trees or piles of litter can burn or smoulder behind the fire perimeter. Entering such an area is quite possible. Within one hour a fire can advance 200 to 1,200m (occasionally up to 2,000m).

Scenes of walls of flames over an entire area or fires which advance faster than people are very improbable in German and Austrian forests.

Wind dries the ground and pushes the flames downwards. This increases radiation and accelerates the fire’s spread. The same effect happens going uphill, so that the flames are driven upwards and the uppermost surface layer dries out quicker. Fires burn slower against the wind or when burning downhill.