Photo credit Babtunde Dosumu
Enhancing the resilience of Swedish forests A forest dominated by just a couple of species can be regarded as more vulnerable to pests and diseases than one with a wide variety of different trees. The team behind a Formasbacked project are investigating whether planting silver birch could enhance resilience and protect the future of sustainable forest production in Sweden, as Michelle Cleary explains. The Swedish forestry
sector is heavily dependent on spruce and pine, which together cover about 80 percent of the country’s productive forest land. These two species face serious problems however, which threaten the long-term health of Swedish forests. “We’re currently experiencing the biggest spruce bark beetle epidemic that we’ve ever seen in Sweden, millions of trees have died, and many places across Europe are facing similar challenges. We’re seeing more severe and frequent insect outbreaks as a result of climate change,” explains Michelle Cleary, an Associate Professor at the Southern Swedish Forest Research Centre at the Swedish University of Agricultural Sciences. There are also a variety of threats to the health of pine trees. “Several fungal diseases cause problems in pine, such as fungi causing root rot and foliar and stem diseases” continues Professor Cleary. “An over-reliance on pine and spruce trees, with the different health challenges that forests are facing, threatens the security of Swedish forests.”
Diversifying forests This issue is central to Professor Cleary’s work in a research project backed by Formas, the Swedish government research council for sustainable development. The project team are looking into planting silver birch - which is already quite common in Sweden - as a way to diversify forests and enhance their resilience. “A less diverse forest is more vulnerable, given the current and future challenges that forests will face, particularly in the context of climate change and global trade which is leading to the introduction of sometimes damaging organisms. Diversifying forests is a means of spreading the risk and adding security to the forestry sector,” says Professor Cleary. While silver birch is fairly common in Sweden it hasn’t traditionally been managed intensively, whereas a lot of attention has been centered on pine and spruce. “Damaging agents such as root rot caused by Heterobasidion spp. have been researched extensively in Sweden and, in practice, attempts are made to control the disease to reduce economic losses,”
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Photo credit Michelle Cleary
continues Professor Cleary. A variety of thinnings may occur over the course of a rotation, and each time trees are harvested, freshly cut stumps can then be colonised by the Heterobasidion fungus. When Heterobasidion grows in stumps it can then grow down through the roots and infect neighbouring trees, which Professor Cleary says can be a serious problem. “It can cause significant decay in the timber, in particular spruce trees. So, in Sweden we treat the stumps at every thinning operation, as a protective measure. A biological control agent is sprayed on the stump, as soon as the tree is cut,” she explains. If more silver birch is to be planted in future, it will be important to assess its vulnerability to the Heterobasidion fungus, a topic that researchers are investigating in the project. “Do we anticipate that Heterobasidion will be a serious problem in silver birch, to a point where we would also need to also treat birch stumps with this biological control agent, at thinning?” outlines Professor Cleary. The project team have conducted some field experiments where they have thinned out the silver birch, and looked at its susceptibility to infection by Heterobasidion.
This work has not yet concluded, but results so far suggest that Heterobasidian is not a major threat to silver birch, in large part because of the presence of another fungus inside the tree which seems to have inhibitory effects. “This seems to be very prevalent in silver birch trees. It doesn’t cause any disease, but it clearly comes into the tree at some point - maybe via an insect vector - and colonises the wood,” says Professor Cleary. Photo credit Kinga Stolarek
EU Research
The importance of this fungus was not anticipated at the outset of the project, and the discovery of it has raised new questions, which Professor Cleary and her colleagues are also addressing in their work. “We’ve started some additional studies, looking at antagonistic effects. We’ve been growing the two fungi together to characterize their interactions,” she outlines. This focus on silver birch has also brought other issues to prominence, one of which is the impact of pine weevil populations, which have recently been shown to attack newlyplanted birch. Seedlings are typically planted on clear-cuts, open areas within a forest created when trees are harvested, and pine weevils are attracted to the volatiles that are then emitted. “They can smell the stumps, and they essentially over-winter in the stumps of trees that were previously cut. When new trees are planted, weevils can then feed on those trees,” explains Professor Cleary. Pine and spruce seedlings are commonly treated with a protective coating to prevent seedling mortality, now Professor Cleary and her colleagues are looking at whether birch seedlings also require this protection. “Our early observations on young plantations showed a high percentage of the seedlings had been damaged by weevils, but lower than that which is typically seen in pine,” she outlines. “This could be an exceptional result though, so we need to study this further.”
weren’t, from which researchers could then assess its effectiveness. Researchers have also probed other issues which may influence the vulnerability of trees to pine weevil populations, including their age. “We looked at a site which had been clear-cut a year prior, while a second site was a very fresh clear-cut. We wanted to see if there was an age difference, in terms of the level of weevil damage,” says Professor Cleary. This work has so far focused on a single site, but it’s unclear whether this is representative of weevil damage more broadly, so Professor Cleary plans to scale up this research in the future. “We’ve got financial backing to
establish a nationwide experimental series, in which we will be able to track weevil damage over one or two growing seasons, and see how variable it is,” she continues. This will also allow researchers to monitor the long-term impact of weevils on the quality of its growth rate, which is an important consideration in forestry. A tree that has been girdled by weevils may not necessarily be dead, so Professor Cleary says it’s important to continue with this research and make further observations. “It may well be that the stalks around the roots are still alive and the tree can resprout,” she explains. Browsing by moose, deer or other
“A less diverse forest is more vulnerable, given the current and future challenges that forests will face, particularly in the context of climate change. Diversifying forests is a means of spreading the risk and adding security to the forestry sector.”
Field experiments A number of field experiments have been established, with researchers laying out traps, catching and identifying insects and assessing damage across the growing season, from around the end of April until September. This work involves getting down on the ground, and digging around the base of seedlings to assess the extent of any damage. “We did this pretty intensively over the course of a full growing season - on a weekly basis for the first few months, then maybe bi-weekly for the rest of the period. We were very concerned about catching the insects at the right time, so that we could identify them,” says Professor Cleary. “We have been looking to identify which weevil species are involved in causing damage to trees. There are four different weevil species that are damaging trees – two of them are actually causing mortal damage, by girdling the stems, while the two others are mostly chewing on leaves.” The project team have conducted an experimental trial in which half of a group of silver birch trees were treated with mechanical protection and the other half
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Photo credit Michelle Cleary
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