idw – Informationsdienst Wissenschaft
Nachrichten, Termine, Experten
Nachrichten, Termine, Experten
idw - Informationsdienst
Wissenschaft
On the morning of December 26, 1999, the winter storm “Lothar” swept across Switzerland, knocking down around 14 million cubic meters of wood, three times the annual logging volume. WSL experts answer numerous questions about how the forest is doing 25 years later.
Citations:
“Lothar showed us the damage that extreme events can cause. In the Swiss Plateau, the extent of the damage was unprecedented. Today we would say, ‘inconceivable’.” Thomas Wohlgemuth, disturbance ecologist at WSL
“A windthrow event shakes up the otherwise rather ‘lazy’ forest vegetation. In places that were initially easily accessible, after two or three years it was sometimes impossible to get through because the undergrowth had grown so much. Some of the areas where windthrown timber had not been cleared could only be accessed by walking across the tree trunks lying on top of each other at a height of two to three meters – provided the bark was still holding.” Michael Nobis, botanist
“Thanks to the practical lessons learned from windstorm Vivian in 1990 and the research conducted by the WSL, Switzerland was much better prepared than it was for Vivian – in terms of organization, technology and ecology. In particular, there were also far fewer deaths during the cleanup than there were after Vivian, and more attention was paid to natural forest management afterwards.” Peter Bebi, forest ecologist and protection forest specialist at SLF
“It is impressive how, after 20 years, apocalyptic tree graveyards have become dynamic and lush eco-paradises with rare inhabitants that are rarely seen elsewhere.” Beat Wermelinger, retired entomologist WSL
How badly did the storm Lothar affect the Swiss forests?
It was by far the most devastating winter storm in Europe and Switzerland in terms of the damage to the forests. In Switzerland alone, it knocked over 14 million cubic meters of wood. That is almost three times the amount of wood that is felled in Switzerland each year, or around 3,500 truckloads
The storm hit the forestry and timber industry hard. Seventeen forest owners lost their lives during the dangerous clean-up operation, as did two people working for public forestry enterprises. Prices for roundwood fell by about a third in spring 2000 due to the oversupply.
Most of the damage occurred in the Swiss Plateau. The storm raged primarily in forests that are used mainly for timber production, but also in recreational forests near cities. Protective forests in central Switzerland suffered extensive damage, with up to 25% of these forests destroyed.
In the years that followed, and particularly after the hot summer of 2003, bark beetle outbreaks caused almost two-thirds as much damaged wood as the storm itself.
From their long-term observations, the WSL forest researchers have drawn the following conclusions:
How have the storm-damaged areas developed?
Trees have grown back on many of the large windthrow sites, and now reach heights of 10 to 20 meters on average. However, there are also areas on the Swiss Plateau – depending on soil properties and original vegetation – where blackberries or bracken have long held back the young trees, or where not the desired tree species, but hazelnuts, for example, thrive. Where the storm had blown down many spruce trees at lower altitudes, most of which had been planted, naturally occurring, climate-resilient, species-rich mixed deciduous forests have mostly emerged.
The long-term observation also revealed some surprising facts: for example, a pioneer phase with fast-growing tree species such as birch and willow was often absent. This means that in many areas, only the offspring of the tree species of the blown-down forest grew. Furthermore, despite clearing of damaged trees, a lot of deadwood remained on the windthrow areas, much more than is usual in Swiss forests. This is a plus for the important forest habitat of deadwood.
Lothar added structural diversity to many forests, creating new habitats for many animal and plant species. A 20-year WSL study showed that the insect diversity virtually exploded after the storm. Although this positive effect on species diversity gradually decreased as shrubs and trees overgrew the storm-damaged areas, it still persisted 20 years after the storms. The number of rare species was one-third higher in storm-damaged areas than in undamaged forests. “Storm-damaged areas that have not been cleared provide an excellent habitat for endangered species, especially in the later stages of decay,” says Beat Wermelinger, who has been studying insect diversity in storm-damaged areas for decades. A study by the Swiss Ornithological Institute showed that woodpeckers benefited from Lothar, as they feed on insects under the bark or in rotten wood.
What lessons have been learned?
When major storms knock down forests over large areas, there is almost always a sharp increase in the population of bark beetles in spruce-rich areas for several years. This initially affects the edge of the storm-damaged area, and then the neighboring weakened stand. If important forest functions need to be protected, damaged spruce trees need to be removed as quickly as possible, especially in low-lying areas. As soon as the weather warms up, bark beetles multiply rapidly on spruce trees. However, in the event of a major storm, the forestry service generally lacks the capacity to remove the infested spruce trees in time. That is why efforts are focused on preventing the beetles from infesting living trees.
In protective forest areas, it can be useful to debark the trees lying on the ground after storms and then leave them where they are, because in this way they can provide medium-term protection against rockfall and avalanches.
The spruce is not native to the Swiss Plateau; the forestry industry has planted it on a large scale for timber production. It is not only susceptible to winter storms and bark beetles, but also suffers from heat and drought. It is now becoming significantly rarer there, as shown by the National Forest Inventory (NFI) conducted by the WSL. Instead, many deciduous tree species that are considered climate-resistant, such as oak, cherry, sycamore maple and Norway maple, grew back naturally in the storm-damaged areas.
As several studies have shown, a storm is an opportunity for species diversity, for example of insects. In order to maintain and promote this at the landscape level, only part of the area affected by the storm should be cleared of storm-felled trees. A mosaic of cleared and uncleared storm-damaged areas, as well as intact forest, is best from a biodiversity perspective. Old deadwood in particular is a valuable habitat that has only become more common due to the increase in extreme events in recent years (storms, drought, bark beetles).
Which tree species are being planted more often today that are more resistant to storms and climate change?
Since deciduous trees lose their leaves in winter and thus offer less surface area for the wind to catch, they are generally less susceptible to winter storms than conifers – a correlation that becomes more pronounced with increasing tree height. Deciduous trees also tend to tolerate heat and drought better. In general, Swiss foresters plant far fewer trees than they did before 1990. Instead, they encourage naturally regenerating, desirable species by creating light for them and protecting them from game. Today, more and more planted trees are being protected from browsing in forest openings in order to save desirable tree species from rapidly emerging brambles and hungry game in the future.
Does the deadwood still provide sufficient protection against natural hazards?
Peter Bebi, an expert on protective forests at the SLF, has studied how protection against natural hazards in storm-damaged areas in the mountains has developed. His conclusion: fallen trunks and root plates lying around contributed to protection against avalanches and rockfall for an astonishingly long time. Although the trees lying on top of each other were compressed from several metres high to 80 centimetres within 20 years, the increased ground roughness mitigated the expected reduction in natural hazard protection after windthrow in many cases.
Before 1990, it would hardly have occurred to anyone not to clean up in protection forests after storms. Thanks to WSL research on windthrow areas, the range of options for action after windthrow has been expanded. Today, forestry services are better able to assess when it makes sense to leave storm-felled timber lying, to clear it only partially, or to combine it with plantings and other protective measures.
What is the situation at the end of 2024 and how are the forests doing today?
The year 2024 was wet and, for the most part, a time of recovery for the forests, after Switzerland had experienced a series of dry years since 2018. These had taken a heavy toll on the forest (see news ‘The Swiss forest is suffering from extreme weather’) and increased the mortality rate of trees.
Is Switzerland's forest better prepared for a ‘storm of the century’ like Lothar today?
Because there are significantly fewer spruces and more deciduous trees on the Swiss Plateau today than there were when Lothar hit: yes. In general, the bark beetles are finding slightly fewer trees. But the warmer weather and more severe droughts are likely to lead to even more widespread outbreaks. This was clearly demonstrated by the last major winter storm, Burglind, in 2018 and the severe bark beetle infestation from 2018 to 2020 north of the Alps.
Winter storms are a typical phenomenon in Western, Northern and Central Europe. After such storms, bark beetles multiply in abundance, first in broken, uprooted or weakened spruce trees. In the course of mass propagation, they often attack healthy spruce trees as well. When high temperatures allow more generations of beetles and drought further weakens the spruces, the number of beetles explodes and only decreases again after several years. This pattern was observed after Vivian in 1990, Lothar in 1999 and the storm Burglind in 2018.
It is reasonable to assume that major storms will occur again sooner or later; it is difficult to imagine stronger storms than Lothar in Europe, but they cannot be ruled out with climate change. However, forest experts are better prepared for them today, thanks to the experiences of Vivian and Lothar, but also to the long-term research projects of the WSL.
Dr. Thomas Wohlgemuth
Biodiversity
thomas.wohlgemuth@wsl.ch
+41 44 739 2317
Dr. Michael Nobis
Vegetation development
michael.nobis@wsl.ch
+41 44 739 25 35
Dr. Peter Bebi
Protection forest
bebi@slf.ch
+41 81 417 0273
Dr. Beat Wermelinger
Insects
beat.wermelinger@wsl.ch
https://www.wsl.ch/en/news/25-years-after-lothar-how-the-windstorm-rebuilt-the-f...
Lothar was by far the most devastating winter storm in Europe and Switzerland in terms of the damage ...
Reinhard Lässig
WSL
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