Current issue: 56(4)
Under compilation: 57(1)
Within the European Community snow damage affects an estimated 4 million m3 of timber every year, causing significant economic losses to forest owners. In Northern Europe, for example, the occurrence of snow damage has increased over the last few decades mainly due to the increase in total growing stock. The most common form of damage is stem breakage, but trees can also be bent or uprooted. Trees suffering snow damage are also more prone to consequential damage through insect or fungal attacks.
Snow accumulation on trees is strongly dependent upon weather and climatological conditions. Temperature influences the moisture content of snow and therefore the degree to which it can accumulate on branches. Wind can cause snow to be shed, but can also lead to large accumulations of wet snow, rime or freezing rain. Wet snow is most likely in late autumn or early spring. Geographic location and topography influence the occurrence of damaging forms of snow, and coastal locations and moderate to high elevations experience large accumulations. Slope plays a less important role and the evidence on the role of aspect is contradictory. The occurrence of damaging events can vary from every winter to once every 10 years or so depending upon regional climatology. In the future, assuming global warming in northern latitudes, the risk of snow damage could increase, because the relative occurrence of snowfall near temperatures of zero could increase.
The severity of snow damage is related to tree characteristics. Stem taper and crown characteristics are the most important factors controlling the stability of trees. Slightly tapering stems, asymmetric crowns, and rigid horizontal branching are all associated with high risk. However, the evidence on species differences is less clear due to the interaction with location. Management of forests can alter risk through choice of regeneration, tending, thinning and rotation. However, quantification and comparison of the absolute effect of these measures is not yet possible. An integrated risk model is required to allow the various locational and silvicultural factors to be assessed. Plans are presented to construct such a model, and gaps in knowledge are highlighted.
Snow damage to forests in Southern Finland in November 1991 was examined in relation to meteorological conditions. The combined effect of different factors proved to be necessary for severe damage. First, the snow load, in terms of precipitation, should exceed a certain limit. The limit can be set for weak or moderate damage at about 40 mm and for very severe damage at about 60 mm. Second, temperature at the time of precipitation should be above 0°C, which enables the slightly wet snow to attach to twigs during the subsequent period with temperature below 0°C. On the other hand, temperatures exceeding 0.6°C prohibit damage by permitting the snow load to fall from the branches. Wind speed exceeding 9 ms-1, as observed 15 m above ground, were strong enough to dislodge the snow which is not attached, and thus reduce the damage. There are few statistics either of snow damage or of the relation between the snow damage and precipitation. However, there is causal connection between snow damage and heavy snowfalls. Therefore, the regions with a high frequency of heavy snowfalls, as indicated by orographical features and occurrence of thick snow cover, were investigated.
Abundant snowfalls and thick snow cover influence forest ecology mainly in two ways. Snow loading increases the number of damaged stems, which increases the amount of decay in stems, in its turn important for many animals. Second, the ground remains unfrozen under the snow cover, which is of crucial importance for many perennial species of ground vegetation. These winter phenomena also have influenced the early Finnish culture as man in his everyday life in the wilderness was in close contact with nature. In this paper, ecological interactions between snow conditions, forest flora, fauna and early culture are discussed mainly with reference to the province of Uusimaa in Southern Finland.
Snow and rime, attached to branches of conifers, seriously damaged forests in a region of 11,000 km2 in Southern Finland during a passage of two nearly occluded cyclones in 1959. The roles of different weather elements were studied by considering the variations occurring in them over this region and its surroundings. Damage occurred only inside an accentuated pattern of copious orographic precipitation. Precipitation only became attached to and retained on branches in such parts of the area where temperature varied on both sides of freezing point but did not exceed 0.6°C. Furthermore, damage only occurred in forests where rime formed (above a certain level and on sloping towards the prevailing wind).
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Exceptionally widespread snow damages occurred in January 1959 in the southern coastal region of Finland. An inquiry showed that significant devastation had occurred over an area of 42,620 ha. The purpose of the present investigation was to study the susceptibility to snow damages of different stands in different locations. Only the stem breakage was recorded. 924 stands along 92 one-kilometre lines were studied in the western continuation of Salpausselkä ridge in the summer 1960. A supplementary study was carried out in 1961 in separate stands.
Most heavily damaged stands were found in a damage zone closest (31–40 km) to the coast of Gulf of Finland. The damages were 39% fewer in the zone 61–70 km from the coast. No stands over 140 m above sea level escaped damage. Stands on the edge of an open area such as a field, lake etc. fared better than areas within the forest. Eastern slopes were more susceptible for snow damages in these weather conditions. Also, conifers were more frequently damaged than deciduous trees. Dense stands, and stands aged 61–100 years had most damages.
The PDF includes a summary in English.
Finnish tree species have adapted differently to heavy snow loads that occur especially in fell areas in Kuusamo and Salla as well as Maanselkä area in Sotkamo and Rautavaara in Northern Finland. Norway spruce (Picea abies Karst. L) is adapted better than Scots pine (Pinus sylvestris L.). The aim of this study was to investigate how crown and stem form of Norway spruce in the snow damage area of Maanselkä area differ from other areas in the same region.
Relatively broad crown at the base of the stem, quickly tapering crown and narrow and even upper crown were typical for trees growing in the snow damaged areas. The higher the altitude is, the stronger tapering the crown is. The tapering begins usually in a height of 4-5 meters. Even the stem diameter begins to taper strongly at this height. In the areas where heavy snow does not cause snow damage, top of crown is broader. Also, in the snow damage areas the damaged trees seem to have broader crown shape than the trees with little damages.
Height of the trees decreases in the snow damage areas compared to forests in lower altitudes, which can be caused both by wind and snow load.
The article includes a German summary.