According to Rein Drenkhan, a professor of forest pathology at the Estonian University of Life Sciences and the lead author of the study, milder and snow-free winters are paving the way for diseases arriving from the south. Since these diseases have not previously spread in the local climate, the trees have not adapted to cope with them.

Pathogens are also increasingly arriving in Estonia with biological materials, including tree saplings. "You might bring a beautiful tree into your garden, but you'll notice its entourage later. This entourage may include pathogens that harm both the newly introduced tree and the neighboring trees already growing in your garden," said the professor.

Planting trees of unknown origin in Estonia's natural environment is highly risky, according to Drenkhan, and carries a significant risk of damage. These trees may simply not be suitable for the local environmental conditions. Unsuitable conditions lead to stress in the trees, making them more vulnerable to various forms of damage.

"If you were to send seeds of Estonian origin that have been tested here to another country, grow plants from them, and then bring them back, the risk would be low. The characteristics of the tree's origin don't disappear. A separate issue is what 'entourage' might accompany the plant, but this risk can always be mitigated with pathogen testing of imported plants based on random sampling," the scientist noted.

Currently, two tree diseases are receiving particular attention in Estonia: ash dieback caused by Hymenoscyphus fraxineus and Dutch elm disease caused by Ophiostoma novo-ulmi. "Ash dieback has been present in Estonia for approximately 20 years, leading to a significant reduction in the proportion of ash trees. It is caused by an invasive pathogen from the Far East, which we have never encountered before," said Professor Drenkhan.

Dutch elm disease arrived in Estonia as early as the 1920s and has previously damaged elm trees in waves. According to Drenkhan, the last major period of damage occurred in the 1990s.

"However, a new outbreak has now begun, which is more widespread than ever before. As a result of global trade, subspecies of the Dutch elm disease pathogen from America and Europe have met, producing hybrids that are even more aggressive in damaging trees," he noted.

Forest owners in Estonia are also concerned about root rot, which affects coniferous stands. "This is a typical root decay pathogen that makes trees more vulnerable to storm damage and harms the trunk wood. In spruce forests alone, this disease causes landowners in Estonia to lose €8.5 million annually," the professor explained. He added that this figure accounts only for spruce forests, not other affected tree species.

Outlook grim for spruce stands

Rein Drenkhan's study predicts a significant decline in the proportion of spruce-dominated forests in Estonia. However, there is no need to fear the complete disappearance of the Norway spruce. It is a native species in Estonia and one of the few shade-tolerant trees capable of growing and regenerating under the canopy of other trees.

"You can still plant spruce, but it's wise to shape young stands into mixed forests, which foresters are already doing. After regeneration cutting, natural regeneration usually occurs in the planted area, with deciduous trees leading the way. This allows the creation of, for example, a mixed forest of spruce and birch," Drenkhan said.

However, he emphasized that it is no longer advisable to plant single-species spruce stands. These are already in poor condition and will only worsen in the future. Pure spruce stands, particularly those with trees of the same age, are much more susceptible to pathogens and pests.

Spruce stands are increasingly threatened by climate change on a broader scale. "For instance, we can expect storms to occur more frequently in the future, and these storms typically knock down parts of the forest. In the aftermath of storm damage, spruce bark beetles thrive, potentially leading to outbreaks that could harm entire surrounding forest areas," noted the forest pathologist.

As a result, the study recommends avoiding the establishment of pure spruce stands and seeking climate- and disease-resistant offspring of Norway spruce. In Estonia, with the support of the State Forest Management Center (RMK), areas are being established to test the offspring of pine, spruce, birch and black alder. "Additionally, in collaboration between Estonia and Sweden, geographic trial plantations of spruce and pine have been established. These provide information on where, besides Estonia, suitable tree progeny grow that could be appropriate for future forests," Drenkhan explained.

The downsides of nesting peace

The spruce bark beetle (Ips typographus) is a native species in Estonia and has been present in the region for a long time, explained Rein Drenkhan. As a result, bark beetle outbreaks have historically occurred in Estonia, usually following storm damage. However, the study highlights that climate change is making life increasingly favorable for the beetle, leading to higher population numbers.

"Due to climate change, storms are becoming more frequent, and we are also clearly seeing a warming trend. As a result, winters are becoming milder and summers drier. Our native Norway spruce, however, does not tolerate drought well," the professor explained.

Drought conditions, in turn, cause stress to trees, especially spruces, making them more susceptible to various pathogens and pests. "In this context, the spruce bark beetle is a secondary pest, meaning it acts as a consequence of extreme weather conditions," the professor clarified.

The forest pathologist also pointed out that warmer weather conditions favor the spruce bark beetle. The warmer it gets, the more beetle generations can emerge each year, increasing the potential for damage. "Currently, in favorable years, one to two generations of spruce bark beetles develop. In the future, up to three generations could emerge in a single season, posing a greater risk to the health of spruce forests," Drenkhan noted.

In the past, damage caused by the spruce bark beetle has been kept under control with knowledge-based management techniques. However, during the recent outbreak, this approach was not followed, according to Drenkhan. "Beetle control must be done in the spring at the right time by cutting down and removing infested trees from the forest. Instead, the principles of the spring bird-nesting peace were applied. These two things are incompatible and cannot be implemented simultaneously," he emphasized.

"Taking commercial forests under protection and leaving them untouched and unmanaged, even for the purpose of pest control, is foolish. This can lead to damage not only within protected areas but also in private forests along the boundaries of these areas," Drenkhan added. He stressed that this approach puts both commercial and private forests inside and around protected areas at risk.

According to Drenkhan, former commercial forests cannot simply be placed under protection with the expectation that nature will take care of itself. "These forests need to be managed in a way that helps them become more natural," the forest scientist added.

While strict protection is necessary to some extent, it should not be the main principle of nature conservation in a forest-rich country like Estonia, Drenkhan argued. "The principle for protected forests should allow for moderate management, which would alleviate unnecessary pressures. We need to be honest and face reality – the state does not, and will not in the near future, have sufficient resources to compensate landowners for the losses incurred due to conservation restrictions," he concluded.

Ash stands being replaced by mixed broadleaf forests

In addition to spruce, the study also examines the condition of other tree species, noting that the status of the common ash remains consistently poor. The situation is particularly dire in denser, older forests with a higher proportion of ash trees in western Estonia. Damage caused by ash dieback is increasing in ash forests across Estonia, progressing from east to west.

"Former ash groves are being replaced by mixed broadleaf forests, which is a direct consequence of ash dieback," Rein Drenkhan noted. Only time will tell if the general health of individual ash trees improves as their overall presence in forests decreases. Given the current situation, Drenkhan said it is not particularly advisable to plant ash trees either in forests or in green spaces at this time.

"It is necessary to select more disease-resistant ash progeny from trials to use in the future," said Drenkhan. "Naturally regenerated plants should be preserved during maintenance cuttings." He also emphasized the importance of monitoring ash trees growing along the eastern border of Estonia to detect and prepare for the potential spread of the emerald ash borer (Agrilus planipennis), a dangerous pest that has already been found in northwest Russia and is not far from Estonia's borders.

The common elm is also in poor condition, but its relative, the wych elm, is in much better health. For this reason, the study recommends planting wych elm in both forests and urban green spaces.

In addition to Dutch elm disease, elms are also affected by other pathogens, such as the fungus Sphaeropsis ulmicola. "Therefore, continuous monitoring of elm health is essential and will be key in selecting trees for future progeny trials," Drenkhan added.

Among alders, the Phytophthora root rot is spreading, primarily affecting grey alder. In a survey of 360 alder trees, 5 percent showed signs of Phytophthora infection. Notably, the pathogen was found on only one black alder, with the rest detected on grey alder. "The damage caused by Phytophthora on alders is relatively modest in the short term, but the pathogen is present, and we need to monitor its spread and impact on various host plants," assessed Professor Drenkhan.

"These kinds of changes in forests are becoming apparent, and we must take them into account. Monitoring and analyzing tree health consistently will help us adapt to climate change and guide the development of future forests," Drenkhan emphasized.

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