Damage from the storms this August in the territory managed by JSC “Latvijas valsts meži” (LVM) reached more than 100 thousand cubic metres of felled or broken timber, while in the LVM Tērvete Nature Park, a popular recreation area in the Zemgale region, thousands of trees were broken and uprooted, altering the familiar landscape of the park.
LVM is actively working to adapt its management operations to the challenges brought by climate change: methods for identifying nature-induced disturbances and digital tools for the prompt detection and assessment of potential damage risks are being developed, management measures for the prompt remediation of damage are being implemented, and management methods and techniques are being improved. Additionally, employees and partners are receiving training to improve their understanding of the risks and opportunities posed by climate change, and various research projects are being carried out in cooperation with Latvian scientific institutions.
Wind – the most significant risk
As confirmed by scientists, wind is the most common natural disturbance in European forests, and the proportion of stock damaged by it has shown a tendency to increase in recent decades, causing significant socio-economic and ecological losses.
It is expected that the financial losses caused by storms to foresters will increase not only due to their frequency but also because of changes in the type of damage: higher wind speeds increase the probability of tree breakage, reducing the proportion of valuable assortments that could otherwise be produced from windblown trees by carrying out sanitary fellings. Although birch stands are considered more resistant to wind than, for example, spruce or aspen stands, an increase in the volume of damage has been observed in them as well, which is partly linked to storm damage occurring while the trees are in leaf.
Primary and secondary tree breakage
As wind acts on a tree trunk, significant structural damage to the wood occurs – compression of the vascular tissue or tracheids (primary breakage), which forms even under lower wind loads than are necessary for it to be uprooted or snapped (secondary breakage).
Under the influence of primary breakage damage, the tree remains viable, but its stability and vitality decrease. In the long term, primary breakage contributes to the death of the tree under the influence of various adverse conditions, such as wind load, drought, the spread of pests, or diseases. Primary breakage damage can also be reversible, as studies have observed the restoration of the mechanical stability of trees; however, this process is slow, and during it, the weakened tree is exposed for a long time to regular wind impact and other damage risk factors. This, in turn, increases the recurrence of structural wood damage and consequently increases the risk of snapping or falling under the influence of even lower wind speeds.
Wind resistance of forest stands
The parameters of individual trees, as well as the taxation indicators of an entire forest stand, such as density, average height and diameter, species composition, and maintenance regime, characterise the stand's resistance to wind-induced damage.
The tallest trees in a stand are more exposed to wind; the thickest trees are more resistant. However, during a rapid growth phase, trees in denser stands may develop a larger height-to-diameter ratio, which reduces their resistance. “The largest trees in a stand, as well as trees on the edges of openings, can absorb 7 to 15 times more wind load than trees of average height and those in the second canopy layer. This mechanism is particularly important in heterogeneous stands where tree swaying in the wind is uneven. The death of wind-blocking or 'catcher' trees can lead to a decrease in the stability of the entire stand, as the structure of a forest stand significantly influences its overall resistance to the destructive effects of wind,” explains LVM development project manager Elmārs Pēterhofs, describing the findings of scientists.
Different tree species have varying wind resistance. The load required to achieve primary and secondary breakage in individual trees increases with the volume of the tree trunk. For example, aspens often reach the largest dimensions, yet as they grow, they frequently develop high and asymmetrical crowns, exposing the stand to greater wind impact and reducing their resistance. Conversely, birch, although flexible, will be better able to resist secondary breakage because the successful adaptive capabilities of its root system provide significant stability even in unstable soils. In Latvian conditions, the spruce forms a shallow root system, which is particularly vulnerable to being uprooted as a form of secondary breakage. However, when a spruce is located in the second canopy layer of a stand, the risk of wind-induced damage is lower.
What do scientists recommend for foresters?
To reduce wind-induced forest damage, scientists recommend creating forest stands that are more homogeneous in terms of both species composition and the dimensions of individual trees. This is particularly important as stand height increases, and with it, the expected wind load. Therefore, thinning of young stands and final felling – once the target diameter is reached – must be carried out in a timely manner. In this way, the time period during which mature forest stands are exposed to the risk of wind-induced damage can be reduced.
Over the last 20 years in Europe, the impact of major cyclone activity has caused forest damage exceeding 340 million cubic metres. In total, approximately 89 thousand cases of forest stand damage have been registered during this period, with a total area of about one million hectares. The volume of wind-damaged timber increases along with the total growing stock in European forests, yet a statistically significant trend of an increase in the proportion of damaged stock has also been identified.
https://www.lvm.lv/jaunumi/7243-klimata-parmainas-un-veja-ietekme-uz-mezsaimniecibu?idU=1