Reinvigorated forests 20 years after windstorm Lothar

On the morning of 26 December 1999, windstorm Lothar swept across Switzerland, flattening 12.7 million cubic metres of timber. The damage to the forest amounted to an estimated CHF 1 billion, including the consequences of subsequent beetle infestations. However, 20 years on, researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) also see some positive effects. For example, at lower elevations species-rich, mixed deciduous forests containing primarily climate-robust trees have emerged.

In some places barely a single tree remained standing

Where windstorm Lothar blew at speeds of 200 km/h and more on St Stephen's Day back in 1999, barely a single tree remained standing. The effect of the hurricane gusts was particularly great because it was relatively warm during the storm, the forest soils were not frozen and rain softened the soils. Therefore, trees and their root plates were more often knocked over than broken. Furthermore, in stormy conditions tall coniferous trees offer greater wind resistance than smaller deciduous trees, which do not have leaves in winter. Accordingly, forest ecologist Wohlgemuth is convinced that: "Shorter forest lifecycles could reduce the risk of windthrow, making many trees smaller and more stable".

Species-rich, climate-robust forests

Today, young trees 10 to 15 m tall are growing where the forest was flattened 20 years ago. WSL research shows that after the storm, pioneer trees such as willows, birches and rowanberries as well as tree species that dominated before the storm generally predominate. On the Swiss Plateau and in the Alpine foothills the predominant species growing back is beech, whereas spruce tends to preponderate at higher elevations. That said, our forests are now home to more species than before. "There are many indications that climate-robust forests are growing back here, containing additional species, like oak, cherry and maple", says senior forest scientist Peter Brang (WSL). For these tree species are more drought-tolerant than beech and spruce, surprisingly indicating that catastrophic disturbances can have a stabilising effect in the long term.

If regeneration is sparse, the question arises as to whether specific desired tree species ought to be planted. WSL's research on the windstorm Lothar has shown that diverse deciduous forests form in large, storm-hit sites 'troop'-planted with climate-adapted deciduous tree species, such as oak, because many other tree species establish themselves between the planted tree groups. Troop planting is inexpensive and achieves the intended objective faster than entirely naturally occurring young forest.

Avalanche barriers and planting in protected forests?

Sources of extensive disturbances, like windstorm Lothar, can immediately undermine the degree of protection offered by a mountain forest. Forest ecologist Peter Bebi (SLF) knew from experience and research into windstorm Vivian that toppled trees and uprooted root plates can help to provide protection against rockfalls or erosion, at least during the first few years after a storm. "The long-term impact was still largely unknown at the time", he says.

“Recent research work on windthrow areas has confirmed that the increased roughness of the mountain slopes due to lying dead trees and root plates in mountain forests often has a long-term effect against avalanches and rockfall,” says Bebi. Especially when the slopes are not extremely steep and reforestation takes place quickly thanks to young trees already existing before the storm. In the event that these are not yet available or are slow to emerge, technical safety measures and the plantings of target tree species have proved effective, especially in steep and high protection forests.