Mixed forests can reduce the risk of forest damage in a warmer climate

Forests with few tree species pose a considerably higher risk of being damaged, and the introduced lodgepole pine is especially vulnerable. This is the finding of a new study published in Ecosphere by researchers from Umeå University and the Swedish University of Agricultural Science in Uppsala. The results can be useful for preventing forest damage and financial losses related to the forest industry.

Fungi, insects, fires and cervids, such as moose, are examples of natural factors behind tree damage in Swedish forests. Sometimes, the damage becomes so extensive that it impacts the function of forest ecosystems, not least the ability of forests to provide wood and other tree products.

“In a warmer climate with more extreme weather and new pest organisms, and with more intense forestry, forest damages are expected to become more common and more severe. It is therefore important to understand causes of forest damages and whether it can be prevented,” says researcher Micael Jonsson at Umeå University, who led the study.

The Swedish national forest inventory has collected extensive data from Swedish forests. Since 2003, data on forest damage have also been collected.

In the current study, the research group analyzed 15 years of data from all over Sweden, to investigate which damages are most common and which factors determine the risk of a tree becoming damaged. The study is more extensive both in time and geographically than previous studies.

Wind and snow cause the most damage

The results show that wind and snow are the most common causes of tree damage, followed by forestry and then fungi. Damages from cervids—mostly moose—are in fifth place. Overall, 94% of all trees showed some kind of damage. Coniferous trees and young stands showed the highest risk of damage, and in warmer parts of Sweden, stands with few tree species showed a considerably higher risk of being damaged compared to stands with a higher number of tree species.

“Our results show that there is a potential to reduce the risk of forest damages via changed forest management. Especially, a higher proportion of broadleaf trees in the otherwise coniferous-dominated production forest would result in fewer damages. We can, for example, see that the lodgepole pine, introduced by the forestry industry, has the highest risk of damage. Its introduction therefore counteracts a profitable forestry,” says Jonsson.

Weaknesses in data material

The results also indicate that a higher number of tree species in a stand act as an insurance against extensive forest damage in a warmer climate.

“We must adapt Swedish forests and forest management methods to a future warmer climate. Including more tree species in production forests seems to be an adaptation that could work,” says co-author Jan Bengtsson at the Swedish University of Agricultural Science.

However, the study also shows that the data material has some weaknesses. For example, it has not been possible to establish the cause behind a large proportion of the damage.

“The national forest inventory collects important data for our understanding of the forest, but when it comes to the damage inventory, the data quality needs to improve to be fully usable in forestry practices,” says Jon Moen, co-author at Umeå University.