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

Fungi, insects, fires and cervids, such as moose, are examples of natural factors behind tree damages in Swedish forests. Sometimes, the damages become so extensive that they impact 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 a 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 damages have also been collected.

In the current study, the research group has analyzed 15 years of these 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.

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 on fifth place. 94 percent 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 a changed forest management. Especially, a higher proportion of broadleaf trees in the otherwise so 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 Micael Jonsson.

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

"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.