Destructive storms in European forests: past and forthcoming impacts
Résumé
Storm Damage and Classification • More than 130 separate wind storms have been identified as causing noticeable damage to European forests in the last 60 years (~2/year). • Storms are responsible for more than 50% of all primary abiotic and biotic damage by volume to European forests from catastrophic events. • Storm damage is categorized in this report into 3 components: o Primary damage: Initial mechanical damage to the trees caused by the storm o Secondary damage: Subsequent damage following the initial wind storm. This is mostly from bark beetles, but can be from other biotic factors, fire, sun, snow/ice and even additional wind damage. o Tertiary damage: Loss of production in shortened forest rotations and other long-term constraints on forest operations • A proposed storm classification has been developed based on the percentage of growing stock (%GS) initially damaged (primary damage) by the storm. • 11 Storms (January 1953, September 1967, September 1969, November 1972, October 1987, January-March 1990, December 1999, November 2004, January 2005, January 2007, January 2009) were selected for study in greater detail. • An extensive database of storms; and a more detailed database for the 11 specific storms have been created. These are available online at: http://www.efiatlantic.efi.int/portal/databases/European_storms_catalogue
Contributing Factors • The amount and location of storm damage and the vulnerability of forest stands is a complex interaction between the meteorological conditions and stand location, soil type, stand composition, and past forest management. • The evidence for the impacts of certain forest operations, species choices or site conditions is often weak, missing or contradictory. Therefore, we have only made statements when there is clear scientific evidence to support these statements. • Gust peak wind speed is strongly correlated to the maximum potential levels of damage: o No appreciable damage for gust peak wind speeds below 30 ms-1 o Moderate levels of damage (maximum damage up to 2% of national growing stock) for gust peak wind speeds between 30 ms-1 and 40 ms-1 o High levels of damage (maximum potential damage between 2 - 4% of national growing stock) for gust peak wind speeds between 40 ms-1 and 45 ms-1 o Severe levels of damage (maximum potential damage > 4% of national growing stock) for gust wind speeds above 45 ms-1. • Tree height has an important impact on vulnerability. • Statistical analysis of storm damage suggests that spruces and poplar are among the most vulnerable and silver fir and oak among the least vulnerable respectively of the conifers and broadleaves. However, such differences, and 5 the generally higher susceptibility of conifers to damage, are confounded by differences in species management and the choice of sites on which they are planted. • Soil condition is very important. Root anchorage strength is increased by soil freezing, and reduced by water-logging and heavy rain and by poor drainage that allows soil saturation during storms. • Recent thinning, particularly in older stands, is often associated with increased damage. • The vertical structure of stands (e.g. irregular versus regular) appears to have little influence on stability. • Taking any site or stand factor in isolation as a way of assessing vulnerability and/or risk of a forest can be completely misleading.
Current and Future Trends • The increase of growing stock and average forest age across Europe in the last 60 years has contributed to the increase of observed damage. • If the total growing stock and average age of European forests continues to increase there will be a proportional increase in the volume of storm damaged trees. • There is some evidence that storm intensity is increasing and that storm tracks are penetrating further into mainland Europe and along a wider swathe, increasing the risk to forests in Eastern Europe. • With climate change, higher temperatures will lead to longer periods of unfrozen soils during European winters, potentially leading to an increase in damage particularly in Fennoscandia. • Storms will tend to be accompanied by heavier rainfall leading to more saturated soils and increased risk of wind damage. • If the current build-up of growing stock continues together with predicted changes to the climate, damage levels are expected to at least double, and possibly quadruple, by the end of the century. • Best current estimates suggest that storm damage to European forests results in an annual reduction of 2% in the carbon sequestration by forests. This figure could exceed 5% by the end of the century if the current build-up of growing stock continues.
Existing Responses to Storms • There is no consistent recording and reporting system for wind damage across Europe or for reporting damage from different hazards (abiotic and biotic). This leads to uncertainties in relative levels of wind damage within different parts of Europe and in assessing the importance of specific hazards. • There is a large amount of information and knowledge within Europe on the causes of forest storm damage and the best methods for dealing with their aftermath. However, this information is widely dispersed, is often out-of-date and may only be available in certain languages. • Most countries in Europe affected by storm damage to forests respond in a similar manner. This includes providing subsidies for harvesting, transport and 6 forest restoration, the short-term derogation of controls, and the production of guidelines on the best methods to re-establish or regenerate the storm affected forests. The similarity in approaches makes the production of a European wide set of generic advice, best practice guidelines and policy instruments a possibility.
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