Learning the hard way: lessons from Vivian (1990) and Lothar (1999)

The windstorms Vivian (1990) and Lothar (1999) felled millions of trees in Switzerland. Trees are now growing at all windthrow sites. At higher elevations, however, the young forests often contain too many gaps to provide sufficient protection from natural hazards, as demonstrated by studies conducted by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, which have just been published in the Swiss Forestry Journal (SZF).

Fierce winter storms have laid waste to Switzerland’s forests time and time again over the past 150 years. 26 of these storms left at least 70,000 m³ of wood from broken or uprooted tree trunks in their wake. Three of them even resulted in over two million m³ of windthrown timber, namely the winter storm of 1967, Vivian/Wiebke (1990) and Lothar (1999). "Meteorologists have not yet established a growing trend of heavier storms in Switzerland", says forest scientist Tilo Usbeck, but "intense storms that cause significant damage to forests will also strike in the future".

Regeneration inventory at 90 sites affected by windthrow

20 years after Vivian and 10 years after Lothar Thomas Wohlgemuth and his team conducted a one-time regeneration inventory at 90 windthrow sites. Recent analysis of the data indicates that a new forest has grown at virtually all sites. On average, a sapling has grown on almost every square metre in areas at low elevations affected by Lothar in the Central Plateau, the Jura Mountains and the alpine foothills. In contrast, sapling density after two decades is only half as high in the storm in the areas at higher elevations affected by Vivian, which primarily hit forests in the Alps. Regeneration is less widespread here, particularly in large windthrow areas. Regeneration was somewhat more frequent in areas where the uprooted trees had been removed than in those where the timber had been left untouched. Wohlgemuth also noted that the height of the ten tallest trees per windthrow area was almost identical: 6.5 metres after a decade in lower-lying areas vs. 6.3 meters after two decades in the mountains. Areas affected by Vivian have mostly been repopulated with conifers, whilst broadleaf trees are more common in areas hit by Lothar. For Wohlgemuth, one conclusion from the inventory is that removing windthrown timber has a surprisingly positive effect on regeneration density. The bare soil left behind after clearing offers an ideal seedbed for the germination of most tree species .

Long-term research spotlights the potential of forest regeneration

Researching the slow natural development of forests after a storm requires patience. Peter Brang has analysed the data collected over the past 10 to 20 years from 19 sites affected by Vivian and Lothar, which have been monitored since they were hit by the windstorms (i.e. since 1990 and 2000). Brang found between 500 and 6,000 saplings per hectare at these sites. The lower values are particularly worth noting here, as 500 saplings only constitute a sparse forest and are not sufficient to protect a subjacent area from natural hazards. After 20 years, planted spruce trees were, on three Vivian sites, on average 1.0 to 2.4 metres taller than saplings grown from seeds fallen after the windthrow. Brang concluded that if there are not many saplings in major protection forests after a storm, it is advisable to extensively clear the sites, build preventive structures and plant saplings in groups. In other forests, several reasons speak for partial clearance with one-off planting at most.

Vivian sites still offer little protection

25 years on from Vivian, there is still no conclusive assessment of the protective effect of windthrow sites at elevations higher than 1,400 metres above sea level. Woody debris increased the surface roughness in the windthrow areas and thus provided effective protection from avalanches and rockfall in the initial years following the storm. Nevertheless, after 20 years the lying stems have settled at some 40% of their original height, as seen on the largest windthrow area at Cavorgia (Disentis, GR). Furthermore, as the density of the young trees is initially low and there are also places without trees, such areas offer poor protection from natural hazards. This was shown in studies conducted by Peter Bebi from the WSL Institute for Snow and Avalanche Research SLF in Davos . He thus advises planting saplings in such gaps early on.

Switzerland is intensively investigating the consequences of a storm

Immediately after the Vivian (1990) and Lothar (1999) windstorms, the economic damage caused by the natural phenomena was at the forefront of many a discussion. WSL, the Swiss Federal Office for the Environment (FOEN) and a range of other partner institutions have since extensively investigated the consequences of Vivian and Lothar and have published a number of decision-support tools. Forest managers and researchers have also learnt lessons from dealing with windthrow areas, especially in protection forests, and are now better equipped to act should similar situations arise in the future. Thanks to these results, there is now a relevant quantification that can be used as a reference for assessing forest regeneration after windthrow in Central Europe. We are learning the hard way, as the saying goes.  

Scientific articles in the Swiss Forestry Journal (in German, with abstracts in English)

• 25 Jahre Erfahrung mit Sturmschäden (Editorial T. Wohlgemuth, P. Brang)