Forest damage caused by global warming can be predicted

Lately, visitors to the Koli National Park have been met with a sight of dead trees in the nature reserve. Global warming is contributing to increasing forest damage in Finland and elsewhere in the world.

"Koli is a special case in many respects. It is a visible destination, a national landscape and a national park visited by many people," says University Lecturer Olli-Pekka Tikkanen of the School of Forest Sciences at the University of Eastern Finland.

"Dead trees cause concern among visitors. However, they are not removed because it is a nature reserve. Koli thus serves as a showcase for climate change to the public."

Tikkanen points out that insect damage is a natural part of life in forests—for example, wind-damaged areas are a natural habitat for pest insects. Traditionally, the reproductive efficiency of insects has been weak in Northern Europe, but the situation is changing due to intensive forestry and climate change.

"Even healthy trees are now at risk because drought causes them stress, leaving them defenseless. Normally, spruce trees are able to produce resin, which is usually enough to repel pest insects. Now, however, drought is hampering their resin production," Tikkanen explains.

The forest growing in Koli is relatively old and the soil is very thin, which makes it more susceptible to drought. As spruce trees age, their ability to protect themselves weakens, while younger forests are better at fending off pest insects. Dry, warm summers have increased damage caused by the European spruce bark beetle, especially in areas already otherwise prone to forest damage.

The European spruce bark beetle is efficient at reproducing

The insect-damaged trees in Koli, and also in the Puijo Nature Reserve in Kuopio, are only a small part of the problem. The European spruce bark beetle is causing increasing damage across the southern part of the country.

Researchers in the School of Forest Sciences at the University of Eastern Finland are currently investigating how European spruce bark beetle damage can be better predicted, identified and prevented. Their latest initiative is to examine whether damaged trees affect the nature experience of people spending time outdoors in nature.

"Forest damage can be addressed early. Currently, the best approach would be for a forest manager or owner to inspect their forest every three weeks, which is labor-intensive and expensive," Tikkanen notes.

"We are now studying damages at the level of individual trees and stands, using drone-operated remote sensing as well as modeling large, open-source forest datasets. Stand-level data on forests is available from the Finnish Forest Center, so we know which tree species grow in specific areas, how old the trees are and what their volume is, and whether they are growing in mineral or peat soil."

According to Finnish legislation, felling is subject to a forest use notification. If felling is done because of insect damage, that, too, must be reported. In spruce forests, the culprit is almost always the European spruce bark beetle, so its prevalence can be traced and assessed throughout Finland. The findings are then fed into a forest simulator, which can predict how different forest management practices affect the risk of forest damage.

Drones help to map insect damage

A joint project by the University of Eastern Finland, the Finnish Geospatial Research Institute, the Swedish University of Agricultural Sciences, and the Slovak Academy of Sciences is exploring remote sensing methods to identify insect damage sites. By intervening early, when the larvae are still under the tree bark, it is possible to stop the damage from spreading.

"With a multispectral camera attached to a drone, trees infested by European spruce bark beetles can be identified. Looking through the lens of a regular camera, a spruce will look green. However, when the spectrum of light captured by the camera is processed, it is possible to find the correct wavelength ranges that show insect damage in the tree even before the needles visibly start turning yellow," Tikkanen says.

UEF researchers have measurement plots in Koli, which are visited every couple of weeks. Kelluu Oy, a company specializing in autonomous airships, and the National Land Survey of Finland, are conducting aerial photography of the area, after which all the data will be collated.

Scent detection dogs trained to find insects can also be used to pinpoint sites infested by the European spruce bark beetle.

"The scent detection ability of dogs is unparalleled, and scent detection dogs are already used to some extent in countries like Sweden. Doctoral Researcher Reetta Kangaslampi has trained three dogs to search for European spruce bark beetles. It will be interesting to see how well the dogs find the pest insects and whether the tree marked by the dog eventually dies. These experimental sites are located in Pirkanmaa," says Tikkanen.

If all the methods currently being studied are put into practice, different damage management strategies can be considered, and information and options can be offered to Finnish forest owners.

"There is no blanket solution for all. When economic constraints are included in the models, we can better evaluate different methods for the future."

More research on tree mortality is needed

The European spruce bark beetle isn't the only thing that can cause mass mortality of trees.

Associate Professor Samuli Junttila at the School of Forest Sciences has studied changes in tree mortality in Southeast Finland using aerial images and computer vision models. His findings show that the number and volume of dead standing trees have increased tenfold over six years.

Now, his focus is on tree mortality worldwide.

"We are using aerial and satellite images to study tree mortality in different parts of the world. Currently, we are in the process of developing remote sensing methods to automatically detect the deaths of individual trees in various forest ecosystems," Junttila explains.

"We are creating a large database of individual tree deaths to better understand tree mortality in a changing climate. Research sites have been established, e.g., in Australia, Switzerland, Estonia, Canada, the United States, Brazil, Panama and Romania."

Drought and heat are major causes of tree mortality across the world. In many places, bark beetles deliver the final deadly blow, but drought makes trees more susceptible to bark beetle attacks. Trees can also die directly from a lack of water or excessive heat, causing their crowns to turn entirely brown in only a few days.

"Our observations have shown that the situation can deteriorate rapidly in Finland as well. Unlike in Central Europe, Finland's cold temperatures hinder the reproduction of bark beetles, but, for example, populations of the European spruce bark beetle have grown significantly in recent years," he says.

"Currently, no one knows just how quickly and severely the situation can deteriorate as our climate changes. Due to a lack of reliable models of tree mortality, we don't know whether we'll lose 10% or 50% of the world's trees, and that's why more research on tree mortality is urgently needed. One significant bottleneck is a lack of tree mortality observations, which we are now trying to address."

According to Junttila, climate change causes tree mortality, underscoring just how crucial it is to mitigate it.

"We also need forests that are more diverse in terms of their species and structure to better withstand the stress caused by climate change. Even-aged monocultures are the most vulnerable to disturbances. Forests need to be diversified, and we need to be open to southern tree species. Our climate is changing faster than tree species can adapt."

Provided by University of Eastern Finland