Rising temperatures and increasing dryness are placing trees under stress and changing the composition of our woodlands. The vegetation zones have moved up around 500 metres. This has been revealed by a research programme conducted by the WSL and the Federal Office for the Environment. It is also aimed at recommendations on how woodland owners and forest rangers can adapt woodland to reflect climate change.

Peter Brang, Leiter des Forschungsprogramms Wald und Klimawandel an der WSL. (Fotos: Kellenberger Kaminski Photographie)

The people leading the research programme briefed the media in November 2016 to report on the results. The people who were invited to attend met up on a forest path close to the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) in Birmensdorf. Peter Brang, Director of the Research Programme on Forests and Climate Change at the WSL, showed just how clearly this path separates the present and the future. He pointed at the trees on the left – predominantly spruce, between which there were a few isolated fir trees growing; it was a typical coniferous forest, the type you frequently encounter on the Swiss Plateau. "Woodland like this will be barely imaginable in 100 years," explained Peter Brang "because spruce is greatly endangered by climate change." Spruce struggles to withstand dry conditions and is susceptible to the "book printer", a type of bark beetle, which develops more quickly at higher temperatures.

Since the dawn of industrialisation, temperatures in Switzerland have risen by almost two degrees already, which is twice the global average. According to climate scenarios, they are set to rise by another two degrees or more. The WSL and the Federal Office for the Environment (FOEN) have been studying the implications of climate change for woodland since 2009 in a comprehensive research programme which incorporates over 40 projects.  The programme, which has also involved ETH Zurich and the Paul Scherrer Institute (PSI), has cost around 20 million francs.

"On the other side of the path," said Peter Brang pointing to the right, "you can see the type of woodland that will be more common in the future." There are durmast oaks standing alongside common hornbeams and cherry trees, with a few European black pines, which originally came from the Mediterranean and southeast Europe. It is warmer and drier here on the southern slope than it is in the spruce woodland on the northern side – a terrain in which deciduous trees dominate.

Practical recommendations

The varieties of trees that thrive in a particular location depend primarily on the water supply, on their alignment to the sun and on soil depth. The rangers in the forest in Birmensdorf have selected varieties of trees that are well adapted to local differences in conditions. However, they could soon be overwhelmed by the rapid pace of climate change, even though the expert says "don't expect us to talk about disasters".

Brang has used sticks in the spruce forest to mark trees which are barely twenty centimetres high and which will rejuvenate the woodland. There are many varieties of deciduous trees there, as well as silver fir which thrives better here than spruce – a good starting point for a mixed forest that will meet the challenges of the future.  However, that is not the case elsewhere. This is because seedlings are also eaten by deer and stags; they only have a dislike for spruce. "Therefore, rejuvenating nature is not a perfect solution," explained the scientist. In many places, efforts are required to protect against damage caused by wild animals, such as by erecting fencing. However, costly planting may also be required to supplement this.

Implementation in practice is one of the central concerns of the research programme. The principles for the adaptation strategies are set out in a book which summarises the results of the programme.  Specific recommendations about varieties of trees and woodland management can now be developed within the scope of forest tests, together with the cantonal agencies, as well as with woodland owner, timber industry and environmental organisations. This will also involve the implementation of a legal provision which parliament added to the existing Forest Act in April 2016 in order to guarantee that precautions are taken to counter climate change.

The multiple talents of woodland

"It is ultimately about preserving woodland activities," explained Peter Brang, giving way to a mountain biker for whom the forest is a recreation space just as it is for the walker who had overtaken the group shortly before. However, the forest is also used for timber production, provides habitat to flora and fauna, is an environment for hunting and offers protection. "There is a road a bit further down; this is a protected forest, with multiple talents," explained the scientist.

A third of Switzerland is covered by woodland, half of it protected. Severe storms or fires can undermine this natural protection against natural hazards. For example, following a forest fire above Leuk in the hot summer of 2003, it will take decades for the full protective effect of the forest to be restored, despite costly programmes of afforestation and avalanche barriers. Extreme events of this nature are likely to become more frequent in the future due to climate change.

It is not the risk of avalanches that is causing experts most concern. Climate change will cause the woodland to spread to and become denser in locations where it had previously been too cold.   This may even improve avalanche protection at higher levels. However, there is likely to be landslides and flooding in areas with shallow soil, and safeguards against these natural hazards will gain in significance in the long term. Therefore, the management of protected woodland is of particular importance.

Biodiversity lowers risk

"Our general advice is to increase the diversity of trees," explained Peter Brang summing up. "This will lower the risk of an area of woodland suddenly being stripped bare due to drought or pests." The experts are backing varieties of tree which already exist in Switzerland, such as beech, oak, cherry tree or silver fir. If the Swiss Plateau becomes warmer and drier in the coming decades, as is expected, the current woodland will shift to an altitude of 1000 metres in future.  "At lower levels, it could look like it currently does in an area to the south of Geneva," explained the forestry expert.

Should non-native varieties of trees also be grown increasingly in future which thrive better in a warmer, drier climate than the native spruce? This contentious issue was also explored by the WSL and the FOEN within the scope of the research programme. While conservation organisations feel that this poses a risk to biodiversity, Peter Brang and his colleagues come to a different conclusion. "All in all, non-native varieties of trees offer a certain potential to complement native varieties."

They have the Douglas fir in mind, in particular, which is a large conifer which originated in North America.  The spruce is still the "bread and butter" of the timber industry; deciduous trees are less lucrative. Coniferous wood will continue to be produced on the Swiss Plateau in the shape of the Douglas fir, even if the spruce only exists at higher levels, where timber is significantly more expensive to produce.  However, the experts recommend restraint in adding Douglas fir to the mixture.

The forester in the woodland in Birmensdorf has put red markings on some European black pines. Peter Brang explained that "they are to be felled because they are too close together." In future, forest owners will fell their trees for timber production earlier than is currently the norm; experts are advising that the "rotation cycle be reduced". While this may lessen economic risks due to climate change, this is likely to have negative economic consequences for the forestry enterprises. This is clearly evident from the results of the research programme. However, they have also come to the conclusion that the most costly approach would be to do nothing.