Current issue: 53(4)
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.
Monitoring of heavy metal accumulation in plants has been used to reflect the deposition of heavy metals in terrestrial ecosystems. In some cases, the accumulation rates in plants are linearly correlated to deposition measured as bulk precipitation collected in funnel samplers. It is uncertain, however, how large the contribution due to adsorption/impaction of small particles is to this relationship. The need for design of enlightening experiments on deposition rates in different vegetation types and their relation to immission and bulk precipitation data is discussed.
The first three-year effects of PK(MgB) and NPK(MgB) fertilization on the dry mass accumulation and nutrient cycling were studied in a Scots pine (Pinus sylvestris L.) stand growing on a drained low-shrub pine bog in Eastern Finland. The total dry mass of the tree stand before fertilization was 78 tn/ha, of which the above-ground compartments accounted for 69%. The annual above-ground dry mass production was 6.3 tn/ha, 51% of it accumulating in the tree stand.
The study period was too short for detecting any fertilization response in the stems. The total dry mass accumulation was not affected, because the increase in foliar and cone dry masses after both fertilization treatments, and that of the living branches after NPK fertilization, were compensated by the decrease in the dry mass of dead branches.
The nutrients studied accounted for 392 kg/ha (0.49%) of the total dry mass of the tree stand before fertilization. The amounts were as follows; N 173 kg/ha (44%), Ca 90 kg (23%), K 58 kg/ha (15%). The rest (18%) consisted of P, Mg, S and micronutrients combined. The unfertilized trees took up the following amounts of nutrients of the soil: N 15.6, Ca 12.8, K 4.1, P 1.3, MG 1.7, and S and Mn 1.5 kg/ha. The uptake of Fe and Zn was 510 and 130 g/ha and that of B and Cu less than 100 g/ha. More than 50% of the nutrient uptake, except for that of K and Fe, was released in litterfall. The results indicated very efficient cycling of K, Mn and B between the soil and trees.
The fertilized stands accumulated more N, P, K and B than the unfertilized ones during the tree-year study period. The increased accumulation corresponded to 35% (52 kg/ha) of the N applied on the NPK fertilized plots, 10% of the P, 25% of the K and 10% of the B on the PK and NPK fertilized plots. The increased amount of B released in litterfall after fertilization was equivalent to 4% of the applied B. Fertilization inhibited the uptake of Mn and Ca.
The PDF includes a summary in Finnish.