Low rainfall periods or extreme droughts combined with extensive forest management are nowadays factors that increase the risk of forest fires. Modern extensive forest use causes combustible biomass and dead wood to accumulate on the forest’s soil. As a result, fires may spread with higher intensity during long, dry spells. Humans cause 90 percent of all forest fires in Switzerland through arson and carelessness.

Forest fires also increasing north of the Alps

In Switzerland, forest fires occur most frequently on the southern side of the Alps: in Tessin and in the south-facing valleys of the Grisons. Typically, the fires occur in the spring. During this time, the driest spells occur and deciduous trees do not yet bear a leafy canopy that protects the ground from the dehydrating sunrays.

There are much fewer forest fires in the central Alpine valleys. Nevertheless, 10 to 15 forest fires occur each year, especially in the Valais. Most of these fires are registered in the summer semester when the warm temperatures dry out pine forests in particular. Prolonged dry periods with high temperatures have caused the risk of forest fires in Switzerland to increase in the past 30 years. Forests north of the Alps are also becoming more and more affected.

Improving prediction

At the WSL, we research the factors that increase the risk of forest fires and develop tools for predicting them. We examine the questions of how to improve the reliability of forest fire prediction and whether there are any generally applicable prediction methods.

We have developed the Fireless2 and FireNiche early warning systems, which help public authorities assess the hazard situation. The nationwide forest fire database Swissfire, a collaboration project with the Swiss Federal Office for the Environment (FOEN), serves as a basis for medium-term prediction methods and forest fire risk maps.

Consequences for forests

We also observe how forest areas devastated by fire evolve after the fire. We look at how quickly life returns, how biodiversity changes, and which dangers are the consequence of forest fires – for example, reduced protection against rockfall and avalanches.