Wind damages expected to increase in a warmer climate

Forestry annually suffer significant losses due to natural disturbances in production. These include both biotic pests such as fungi and insects, and abiotic such as fire and storm. The amount of timber yearly affected by such disturbances in Europe increased ten-fold between 1950 and 2020, from 10 million cubic meter to over 100. The most devastating of these factors is wind, which is responsible for on average 46 % of the affected volume.

Several studies estimate that wind damage will continue to increase throughout the 2000’s. One reason is climate change, but it is not given that the wind itself will increase. There are conflicting theories as to how a warmer climate will affect the formation of storms. Warmer seas will have an impact, as hurricanes do grow more powerful the more water and warmth they contain. Most scientists also agree that the hurricanes of the 2000’s will be stronger than in the past, but that is not to say they will be more common. How extreme weather in Scandinavia will develop is uncertain. Regardless, climate change also brings other side effects which in and of themselves will affect forests’ susceptibility to wind.

Problems in a warmer climate

One of the factors affecting the outcome of a storm is the characteristics of the soil. The more water it can potentially hold, the more susceptible it is, and thus forests on sandy, permeable soils are less exposed to wind damage than are forests on clay soils. However, no soils are as hard as the frozen ones. Winter ground frost is an important storm protection, especially since the winters of northern Europe are generally more windy than the summers. In Finnish studies from 2017, scientists estimated that if winter temperatures were to rise by four degrees, the period of deep ground frost would shorten by two months, at least in southern Finland. This would mean that 80 % of all storms with winds stronger than 11 m/s will occur on unfrozen ground, compared to 55 % today. Also, precipitation is expected to increase by 18 % until 2100, which also leads to less sturdy soils and more wind-felled trees.

There are other reasons the scientists expect wind damage to increase. According to the Finnish Forest Research Institute, 70 % of all trees planted in Finland in 2009-2013 were spruce trees; three times as many as in the early 1990’s. The increase was caused by escalating problems in pine plantations, mainly due to wild life grazing but also fungal disease such as blister rust, snow blight and pine twisting rust. The shallow root system of the spruce makes it less resistant to wind than the pine, and if a greater part of the standing forest consists of spruce, the cost of wind damage will increase. On top of that, the scientists estimate that higher levels of carbon dioxide and a warmer climate will lead to faster growing trees, and forests containing larger timber volumes will lead to greater losses in storms. But the shallow roots also make spruce less resistant to drought. Since recurring droughts are expected to increase in the 2000’s, the conditions for spruce will grow worse. It is thus expected that more birch and pine will be planted in the future, and since these species are less storm susceptible, the problems should then decrease.

Storms and rot

These observations were complemented by another study, investigating how the interaction between wind and spruce root rot may develop in the future. Root rot is caused by a pathogenic fungus that degrades the root system of the spruce trees and hollows the stem, which increases the risk of both stem break and windthrow. The study calculates that at least half the diameter of the stem needs to be rotted for the risk of stem break to increase significantly. However, rot inside the roots is a much greater issue. It has been calculated that spruce trees with root rot may fall at wind speeds 20-30 % lower than what healthy trees withstand, and in thrown spruces, the prevalence of rot has been estimated being six times higher than in the trees that did not fall. The amount of root rot in the forests of Finland and Sweden increases continually thanks to forestry practices, and is also expected to increase further due to a warmer climate. This is why root rot is expected to contribute to the increasing problems with wind damage in the future, especially in combination with decreasing ground frost.

So, rot can lead to wind damage, but wind damage may also lead to rot. The resilience of the tree is determined by, aside from stem strength and soil sturdiness, the depth and strength of the roots. The root strength on the lee side is especially important. There, the roots are exposed to bending and compression, and more often suffer mechanical damage than the roots on the wind side. This may serve as an entry point for root rot, and lead to both stunted future growth and increased susceptibility during storms to come.

Damage in surviving trees

The future fate of trees surviving storms was investigated in a study from 2012 by Rupert Seidl and Kristina Blennow. They studied survivors in the years following the storm Gudrun in Småland, 2005. They found that growth was decreased by up to 10 %, and that the decrease was largest in the areas where the storm had been hardest. The total growth loss after Gudrun was estimated to 3 million cubic meter over three years, which was more than the losses due to spruce beetles, hatched in the millions of wind-felled trees. The scientists believe that the reduction is caused by several factors. The survivors have suffered damage to root systems, greenery and branches, impairing conditions for growth and potentially leading to pathogen infections. Also, the trees relocate their resources after one storm to better handle the next, and strengthens their root system and the thickness of the stem. Thus, less resources remain for height growth, which means less tree volume.

Adapting to wind

Being exposed to wind is necessary to be able to withstand wind. Both stem and root system adapt to prevalent wind speed and direction, and adjust growth to cope with storms in the future, or even change the form of their own crown to catch less wind. A tree that never experiences a storm, or is physically prohibited to sway, will grow slender and weak.

This happens in dense stands, where the trees shelter each other. The wind force that the average tree is exposed to can be one fourth of that of solitary trees. Also, clustered trees can support each other, and interlocked root systems further increase resilience. After a harvest, spared trees are exposed to more wind than previously, and has less physical assistance from neighbors.

Risks after harvest

That is why the risk of wind damage always increase after thinning. The support and shelter from surrounding trees are removed, and the tree thus suffers wind forces it is not used to. It can take up to 5 years before the canopy closes and provides some shelter again, and 15 years for the trees to adapt to the increased wind. The more intense thinning, the greater the challenge for the remaining trees. This is especially true in older, never thinned stands, with many high and slender trees. Since the highest trees probably have been able to grow rather unrestricted and thus have thicker stem in proportion to height, it may be advised to thin “from below” and remove the smaller, slender trees.

Similarly, there are risks associated with final harvest, when spared trees and trees along the edges of neighboring stands become exposed to new wind conditions. The impact of the wind lessens if the edges are cut straight, and even more if they run along the dominant wind direction. Edges perpendicular to the wind suffer most damage.

Thus, harvest along corridors constitute an even greater risk, since any given area would contain considerably more meters of edge zones than at a larger clear cut. The same goes for harvesting in smaller patches, or any other method that creates more edge zones and thus more wind exposed trees. The form of harvest that confer least risk for significant economic losses due to wind is probably some sort of selection harvest, where a multi-layered forest always remains and provides shelter and support.

Different types of forestry means different risks

This was studied by Maria Potterf in 2022 and 2023. She compared the risk for wind damage in traditional rotation-based forestry with even-aged stands, to that of multi layered stands and selection harvests. Potterf discovered that the economic risk is higher in even aged stands since more trees are exposed to the wind, the stand contains more trees, and the entire stand is vulnerable at the same time since every tree is the same size. However, the risk for wind damage to occur in any way, shape or form is larger in multi layered forests, since those always have at least some trees susceptible to wind, always large trees, and since trees are felled more often, exposing more trees.

In another study from 2022, based on observations in hurricane-damaged pine forest in Alabama, similar conclusions were drawn. The scientists stated that shortening generation times in even-aged stands, or removing the highest trees in multi layered stands, would decrease wind damage. However, they also point out that this would lead to less timber being produced, higher risk of fire, and a bigger ecological impact.

These studies, and others, indicate that multi layered forests, with trees in many different ages, is more resistant and has a greater potential for recovery after a storm. In such forests, a spruce of any given diameter would be shorter than in an even-aged forest, and thus more robust in relation to how much wind it catches. A mixture of tree sizes makes the whole stand more resistant since each storm is the most devastating on trees of a certain size. If every tree would be of that size, they all suffer equally, whereas risk is more evenly distributed in a multi layered forest. Following this logic, at least some trees are always susceptible to a certain storm in such forests, while a young even-aged forest can survive storms unscathed – most problems begin once the stand exceeds 10 meter height.

The better performance of the multi-layered forest has also been recorded from Schwartzwald in Germany, where several even-aged and multi-layered stands stood next to each other. The amount of wind-thrown timber that was removed over the years from the multi-layered stands were significantly smaller than from the even-aged. The same author also estimated that the multi-layered stands are more resistant to wind the closer they come to being fully layered.