Forests are constantly changing. It is assumed that climate change increasingly contributes to these changes. WSL has examined together with the FOEN, how mountain forests have changed in the last few decades under the influence of climate change, what changes they will experience in the future and what kind of feedbacks with the climate we can expect from these changes.
The scientists worked with existing data from the Swiss National Forest Inventory LFI and the land use statistics. But they studied also processes in selected mountain forest ecosystems and worked with future scenarios.
Forests expand and become denser
The studies showed: Mountain forests have strongly expanded and became denser during the last 30 years, especially in steeper areas. In addition, a shift in species composition has been observed. Tree species such as spruce and larch reinforced their presence at higher altitudes and, in lower elevations, are competed stronger by other species such as fir and beech. The scientists believe that climate change has contributed to these changes: Higher temperatures accelerate the growth of trees. In addition, they also indirectly affect the trees by reducing the thickness and duration of snow cover. However, the ongoing forest changes are not only influenced by climate, but also by their history of cultivation and the disturbances they experienced. It is thus not possible to unanimously determine to what degree climate is responsible for the ongoing forest changes.
Increasing natural disturbances
On first sight, the expansion and growth enhancement of forests at high altitudes is advantageous, because the area covered by protective forest thereby increases. Furthermore, there are less avalanches in dense forests than in open forests. As a matter of fact, the protective effect of many forests has improved – although not always in those places where it would be preferable. However, a higher density of trees has risks as well: Denser forests are more vulnerable to forest fires, bark beetles, wind and snow damages. Furthermore, due to a lack of rejuvenation, they need longer to reestablish their protection effect after they have been disturbed.
If large parts of a forest are damaged by natural disturbances, its protection effect is seriously reduced. Therefore, the ongoing changes should be observed critically and require countermeasures. All the more because higher temperatures and longer periods of drought will probably promote forest fires and bark beetles in the future.
Less forest avalanches
Whether there are avalanches in forests or not depends heavily on the weather. According to an analysis of 189 forest avalanches, two typical weather situations are favorable for forest avalanches. Conditions that favor new snow avalanches are as follows: more than 50 cm of new snow within three days, a lot of wind and continuously cold temperatures. If the snow cover is thick enough, and if an increase in temperature weakens the snow cover, the conditions are favorable for old-snow avalanches.
Since 1971, i.e., since there exist reliable measuring data, these two typical situations of forest avalanches have decreased. However, since snow conditions strongly vary from year to year, there will still be weather situations in the future that are favorable for forest avalanches.
Forest expansion has feedback on climate
Forests reflect less sunlight than unforested areas, especially if they are covered by snow. Therefore, more energy is transferred to earth through forested areas than through unforested areas. Until now, it has not been clear, how great this “warming” effect is in relation to the “cooling” effect which results from the trees transforming carbon dioxid (CO2) into dioygen (O2). Recent studies for the Swiss Alps showed: In mountain areas above 1200 m, the “warming” effect is of great importance in the long term and reduces the “cooling” effect by more than 60%. The positive effect of forest expansion on climate is therefore considerably smaller than expected before.
Challenges for forest managers and researchers
The research project showed that the relationships between climate change and avalanche protection forests are highly complex. In the short term, the protective effect against avalanches and rock fall will increase, if the forests further expand and become denser. However, without active management measures to reduce tree density, the forests become more vulnerable to fire, bark beetles and wind. Changes in weather conditions will increase the risk of forest fires and bark beetle outbreaks on the one hand, but will reduce the frequency of situations favoring forest avalanches on the other hand. To better understand these complex feedbacks, further research is necessary.