Wildfire, insect attack and windthrow in mountain forests are on the rise again

Numerous examinations of fossil wood and plant remains have demonstrated that natural disturbances, such as wildfire, avalanches, storms and bark beetle infestation, have been shaping our mountain forests for millennia. Many forests also bear the scars of human intervention, however, through logging, grazing or burning down to make land available for farming. Wood utilisation in the Alps peaked in the early 19th century, when both forest area and tree density reached an all-time low. Among the consequences was the creation of many new avalanche starting zones. Centuries of excessive exploitation and depleted wood resources reduced the forests' exposure to major disturbances – wildfire, bark beetle attacks and windthrow therefore played a lesser role in this period.

The situation changed radically in the mid-19th century, when rural dwellers moved to urban centers and the farming of a growing number of steep slopes was terminated. In many places coal and other fossil fuels replaced wood as an energy source. In addition, flood and other natural disasters gave rise to stricter forestry legislation, which fostered the conservation and regeneration of avalanche protection forests.

More forest area, more dead wood and more natural disturbances

An international research team headed by Peter Bebi have investigated changes in forest cover in the Alps in the period since the 19th century and produced a long-term picture by also looking at older sources of available information. Using forest inventory data for the last century they quantified the expansion of forest area across the entire chain of the Alps. They examined Switzerland especially closely by consulting not only inventories, but also the Siegfried Map (topographical atlas) of 1880 as well as modern maps and topographical data. They also used these resources to examine the relationship between changing forest cover and structure on the one hand, and the occurrence of natural disturbances on the other, and to establish differences in this respect between stands that have expanded since the 19th century and older forests.

Their analyses show that forest cover across the Alps has increased significantly during the past century – by 4 % a decade on average. Much the same applies to timber stock. While live tree volume has risen by 10 % a decade, dead tree volume has ballooned by 60 % as a consequence of natural disturbances, reduced harvesting and a greater acceptance of dead wood.

In the Swiss Alps, forests established after 1880 now constitute almost one-half of the forest cover. These are situated predominantly on slopes that are steeper than 30 degrees, which is where farming receded most substantially. Compared to older forests, these new stands are characterised by smaller tree diameters, less biomass and a greater prevalence of dense sections. In the last 50 years these forests have been much less intensively managed than older stands and, in many cases, show signs of grazing. Given their younger age and lower biomass, they were less severely damaged by the storms Vivian and Lothar, but have been more frequently affected by wildfires.

Larger cohesive forests and the increase in total biomass since the 19th century have prompted a rise in the number and scale of natural disturbances in the Alps. Warming and summer drought are elevating the dangers to which forests are exposed, in particular from wildfire and insects. In view of the long-term forest history and current developments, the researchers believe that natural disturbances caused by wildfire, insect attack and windthrow are currently a greater danger than at any time since man settled in the Alps, and will continue to gain in significance.