Snow damage can occur anywhere in Finland, if the snow-load on trees grows heavy enough. The risk to trees is especially high when a snow-load accumulates from wet snow or supercooled droplets within a short period of time.
Spruce is able to withstand the weight of snow better than pine. The snow-load on thick pines tends to concentrate on the crown, and its weight can cause the trunk to snap. Pine stems are especially liable to snapping in very cold temperatures. Recently thinned pine stands that were previously overcrowded for a long time are the most susceptible to snow damage.
Snow-loads on spruce are spread across the entire height of the tree. A typical consequence is the top breaking off, in which case the tree can still continue to grow. Some broad-leaved trees bend so much under the weight of snow that their trunks become permanently twisted.
Ice, rime, and snow
Snow-loads accumulated over a long period of time usually consist of a combination of hard ice, rime, and snow. These kinds of snow-loads are especially common in northern Finland and in the hilly regions of North Karelia, but they can also occur elsewhere in Finland.
The accumulation of snow-loads depends on the frequency of warm and windy spells during winter. Snow-loads accumulated in early winter melt quickly at lower altitudes. At altitudes of more than 300 metres above sea level, the air warms slowly, which allows more snow to accumulate, causing snow-loads that sometimes last all the way through midwinter.
In the north, moisture released by the Bay of Bothnia cools down above the mainland and freezes to the windward side of trees. Moisture is carried to the hills of Lapland and the north-eastern parts of Northern Ostrobothnia for as long as the Bay of Bothnia remains free of ice.
The heaviest snow-load recorded on a spruce weighed 3,197 kilograms. The average snow-load was 100–150 kilograms per one metre of tree trunk.
Snow breakages governed by the Forest Damages Prevention Act
If snow breakages amount to more than 10 cubic metres per hectare in a spruce stand or to more than 20 cubic metres per hectare in a pine stand, damaged trees beyond these figures need to be removed from the forest before bark beetles mature in July/August.
In areas prone to snow damage, pine stands in particular need to be thinned often. The risk of snow damage is especially high in pine stands where the crowns have already shrunk to small tufts. Pine should ideally not be grown in areas that are prone to snow damage. If winters when the ground does not freeze become more common, more trees are likely to be lost due to being torn up by their roots.
Picture on top of the page: Erkki Oksanen / Luke