Réf. Usbeck & al. 2010

Référence bibliographique complète

USBECK, T., WOHLGEMUTH, T., DOBBERTIN, M., PFISTER, C., BÜRGI, A., REBETEZ, M. 2010. Increasing storm damage to forests in Switzerland from 1858 to 2007, Agricultural and Forest Meteorology, 150, 47-55.

Abstract: The most severe damage to forests in Central Europe occurs during winter storms caused by Northern Hemispheric mid-latitude cyclones. Storm events in the winter semesters of the past 150 years were investigated to quantify changes and evaluate whether damage rates, forest properties and climate had changed. Records of damage extent (wind throw/snap/breakage), forest area and growing stock in Switzerland were comparatively analysed. Storm damage (m3) was 17 times greater during the period 1958–2007 than during the period 1908–1957 and 22 times greater than in the period 1858–1907. Forest area in Switzerland has increased by 63% and growing stock by 292% over the past 150 years. The significant recent increase in storm damage could only partially be explained by increased growing stock. Weather reports prior to storms indicated that severe storm damage occurred almost always when soils were unfrozen (96%) and wet (96%). During the observation period mean winter temperature has increased by nearly 2 °C and winter precipitation has increased by nearly 50% in the study region. In the Zurich region, daily maximum gust wind speed and storm damage were compared. Maximum gust wind speed above 35 m s−1 was associated with extensive storm damage. Catastrophic storm damage and maximum gust wind speed measured during storms have increased during recent decades. In conclusion, increasing growing stock, warm winter temperature and high precipitation, and even more markedly, increasing maximum gust wind speed have all contributed to the recent increase in windstorm damage to forests.

Mots-clés
Gusts, Maximum wind speed, Extreme wind speeds, Wind damage, Wind throw

Organismes / Contact

• WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Zurcherstrasse 111, 8903 Birmensdorf, Switzerland
• WSL Swiss Federal Institute for Forest, Snow and Landscape Research, CP 96, 1015 Lausanne, Switzerland
• University of Berne, Institute of History, Erlachstrasse 9a, 3012 Bern, Switzerland
• Corresponding author: matthias.dobbertin@wsl.ch


(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature, Precipitation, Daily maximum gust wind speed Forest Tempest Winter storms

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland         1858-2007

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

Winter climate and prestorm weather:
In the regions affected by severe winter storms, winters have become warmer, with a 2 °C rise during the last 140 years. They have become wetter, with an increase in precipitation of nearly 50%. Mean winter temperature above 0 °C were rare in the 19th century, but have become frequent since the mid-1970s.

‘Frozen soil’ prior to winter storm damage occurred in one out of 25 events. In 96% of all events considered, soils were unfrozen. Severe winter storm damage occurred in all but one event (96%) during ‘wet soil’ conditions. The ratios of both ‘wet/not-wet’ and ‘unfrozen/frozen’ soil conditions preceding a winter storm event have slightly increased with respect to the comparison of 50-year periods while the minimum damage recorded also increased. Before severe winter storm damage soil conditions were mostly wet and unfrozen in contrast to non-severe events where the respective ratios were smaller.

Daily maximum gust wind speed and forest damage in Zurich:
An average of 0.21 days per year with a UDMG of at least 30 m s−1 for the first period January 1891–1929, 0.85 days per year for the second period 1930–1968, and 2.51 days per year for the third period 1969–2007. The extremes of daily maximum gust wind speed increased by roughly 12 m s−1 between the first and last period. [...]

Discussion:
[...] The present results show that winter temperature and precipitation have increased during the observed period (in agreement with, e.g., Begert et al., 2005; Schmidli and Frei, 2005; Scherrer et al., 2006; Rebetez and Reinhard, 2007). Winter storm damage usually occurred when the soil was ‘unfrozen’ and ‘wet’. The authors found that this weather type is characteristic for winter storms that are always caused by cyclonic weather situations (e.g., Kraus and Ebel, 2003; Allaby et al., 2006). [...]

Maximum gust wind speed has also increased during the time period for which data are available. It exceeded 30 m s−1 for the first time at the beginning of the 20th century, reached nearly 35 m s−1 in the mid-1930s, nearly 40 m s−1 at the end of the 1960s and nearly 45 m s−1 in 1990. [...]

The present results of increasing storminess since the 1960s correspond well with the four Central European pressure records Kremsmünster, Vienna, Karlov, and Prague (Matulla et al., 2008). The observed changes in maximum gust wind speed over the investigated period evoke the questions of comparative observations in Central Europe and of possible meteorological causes. Extreme mid-latitude cyclones during the extended winter season (October to March) are the main origin of severe windstorm events affecting Central and Northern Europe (Leckebusch and Ulbrich, 2004). Their occurrence may be linked to the North Atlantic Oscillation (NAO) (Seierstad et al., 2007; Trigo et al., 2008; Pinto et al., 2009) that is the dominant pattern of atmospheric circulation variability in this region during winter (Matti et al., 2009). Strong positive NAO phases are well known to cause more extreme cyclones (e.g., Pinto et al., 2009; Trouet et al., 2009). The present findings of maximum wind speed observations correspond to the increasing severity of storms during the period 1960–2000 (Leckebusch et al., 2008a). Also the temporal pattern of storminess for Austria and the Czech Republic (Matulla et al., 2008) covering the period from the end of the 19th century to the 1990s resembles well the pattern of maximum gust wind speed in Zurich. In Austria, several extreme winter storms in the years 2007 and 2008 produced the most severe forest damage in more than 60 years (Steyrer et al., 2008).

Modélisations
 
Hypothèses

Discussion:
[...] The parameter maximum daily wind speed is claimed to be best for determining changes in wind speed and number of storm events (Rockel and Woth, 2007). For the future climate in Central Europe, extreme wind speed is predicted to increase (e.g., Rockel and Woth, 2007; Leckebusch et al., 2008a,b; Pinto et al., 2009), both in maximum wind speed of extreme events (Leckebusch et al., 2006) and in spatial extent (Leckebusch et al., 2008a).


Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Meteorological data were used, mainly from federal and cantonal publications and databases of the past 150 years covering an area of roughly 35,000 km² with highly structured landscape. A meteorological network of measuring stations was installed in 1863. Reliable maximum gust wind speed has been recorded since 1891. Daily average temperature and daily total precipitation served as explanatory variables to explain soil status at the time of the damaging events. [see details in the study]


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

Damage-causing severe winter storms:
The storm damage during the period 1958–2007 was estimated to be 17 times greater than during the period 1908–1957 and 22 times greater than during the period 1858–1907, and storm frequency was about twice as high compared to the period 1858–1907 or the period 1908–1957 to the period 1958–2007.

For the 25 severe winter storms we found data at a cantonal level (in total for 90.4% of all cases). Little or no data were found for Canton Ticino, which usually is not affected by winter storm damage, and for Canton Geneva, where damage is not reported because of its small forest area. Most severe, i.e., catastrophic winter storms happened in 1879, 1935, 1967, 1983, 1990 and 1999.

Winter climate and prestorm weather:
‘Frozen soil’ prior to winter storm damage occurred in one out of 25 events. In 96% of all events considered, soils were unfrozen. Severe winter storm damage occurred in all but one event (96%) during ‘wet soil’ conditions. The ratios of both ‘wet/not-wet’ and ‘unfrozen/frozen’ soil conditions preceding a winter storm event have slightly increased with respect to the comparison of 50-year periods while the minimum damage recorded also increased. Before severe winter storm damage soil conditions were mostly wet and unfrozen in contrast to non-severe events where the respective ratios were smaller.

Daily maximum gust wind speed and forest damage in Zurich:
[...] In total, 22 data pairs were analysed to compare forest damage caused by winter storms in Canton Zurich and UDMG at the Zurich MeteoSwiss station. Pearson’s and Spearman’s rank correlation coefficients between UDMG and damage were rp = 0.61 and rs = 0.68, respectively. While for the first period almost no correlation was found, for the second and third periods, both Pearson’s and Spearman’s rank correlation coefficients increased. They reached statistical significance in the third period.

Storm damage was found to be significantly associated with maximum gust wind speed (p = 0.0015). In addition, the authors found an increase in the maximum damaged part of the growing stock and a temporal increase in extremes of UDMG in winter. Highest UDMG are usually also indicated by reports on forest damage or damage to other goods. All UDMG events of at least 35 m s−1 were caused by winter storms.

Discussion:
This study showed an increase in severe winter storm damage in both amount and frequency (confirming Bütikofer, 1987; Münchner Rück, 2001; WSL and BUWAL, 2001; Schelhaas et al., 2003). The damage potential, i.e., forest area, growing stock as well as growing stock per hectare has also increased (confirming, e.g., Brändli, 2000; Mather and Fairbairn, 2000). However, the damage adjustment by forest area and growing stock cannot fully explain the damage increase during the time period 1858–2007. Various authors have suggested that other major influencing variables have also considerably changed (e.g., Mayer and Schindler, 2002; Schmoeckel and Kottmeier, 2008). The present results show that winter temperature and precipitation have increased during the observed period (in agreement with, e.g., Begert et al., 2005; Schmidli and Frei, 2005; Scherrer et al., 2006; Rebetez and Reinhard, 2007). Winter storm damage usually occurred when the soil was ‘unfrozen’ and ‘wet’. The authors found that this weather type is characteristic for winter storms that are always caused by cyclonic weather situations (e.g., Kraus and Ebel, 2003; Allaby et al., 2006). With one exception, severe winter storm damage in Switzerland occurred when soils were wet.

Modélisations
 
Hypothèses

Discussion:
[...] The present results show that winter temperature and precipitation have increased during the observed period. [...] With one exception, severe winter storm damage in Switzerland occurred when soils were wet. This suggests that the increasing winter precipitation – as regionally predicted in climate change models (Frei et al., 2006; Schmidli et al., 2007) – predisposes forests to higher winter storm damage.


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

The severity and extent of storm damage in forests are a function of three types of factors (Hubrig, 2004; Schmoeckel and Kottmeier, 2008):
• the damage potential, i.e., the amount of growing stock exposed to strong winds;
• the susceptibility to wind, depending, for example, on tree species or stand height;
• the extent and severity of the causing event, i.e., storm extent, wind speed, and gusts.

Susceptibility of forests to storms has been intensively studied. Earlier studies of winter storm damage to Swiss forests focused almost exclusively on their susceptibility to particular cyclonal storm event series (e.g., 21, 23 and 28 February 1967; Vivian and Wiebke 1990; Kurt, Lothar and Martin 1999) or to single storm events. Forest damage has been separately analysed for the storms Vivian in 1990 and Lothar in 1999. In the two studies, various factors such as stand height, conifer proportion, altitude, slope and soil pH were found to correlate with the extent of damage. The damage were compared in both events, Vivian (5 million m3) and Lothar (14 million m3), and similarities were found in damage probabilities, e.g., for stand height, development stage, percentage of conifers, soil-water logging and soil depth. However, cross-validation using data from the two storms to predict damage failed. In a few studies a relation between weather conditions and soil stability was detected. High soil moisture content caused by high precipitation and temperatures above 0 °C can weaken soil stability, and in contrast, freezing can increase soil stability. Finally, the extent of storm damage to forests depends on the wind in space and time, in particular on the dynamic wind field and its interaction with dimensions of trees or stand structures, in particular gap size, topography, crown shape, dumping effects, wind gust frequency, and their dynamic interactions. [see references in the study]

The observed increase of maximum gust wind speed is decisive due to the fact that wind force, i.e., the destructive energy, increases quadratically when wind speed increases linearly (e.g., Otto, 2000; Hubrig, 2004). The results from the case study region clearly indicate that winter storm damage to forests corresponds to the velocity of the maximum gust wind speed. As a rule of thumb, severe winter storm damage exceeding 2 m3 ha−1 occurred mainly when maximum gust wind speed exceeded 35 m s−1.

Discussion [See references in the study]:
[...] Due to inconsistent or missing data the following forest and site factors often used to describe the susceptibility of forests to storm damage were not considered: proportion of conifers, stand structure, management system, and stand height. Various reports suggest that proportion of conifers and stand structure in Swiss forests have not changed substantially since 1860. An increase in mean stand height was assumed to be positively correlated with growing stock. Stand height was found to be one of the most important factors explaining forest damage following the storm Lothar in 1999. In contrast, one author concluded that stand height was less important than species composition and stand structure. Concerning site conditions, the question whether changes in soil chemistry reduce forests stability cannot be answered due to a lack of relevant data. However, a few studies have examined the effects of changes in soil chemistry on stand stability. After the Lothar storm, more uprooted trees on soils with higher base saturation were found, but a direct effect of root damage and uprooting could not be shown. Some authors found that Lothar caused more damage to stands on soils with lower pH values. However, they found no relationship between storm damage and modelled atmospheric deposition, which may have induced changes in soil chemistry. In summary, beyond several factors not treated in this study, the present results show that storm caused forest damage in Switzerland from the middle of the 19th century until today is a function of wind force, growing stock and weather (temperature and precipitation) conditions preceding winter storm events.

[The authors] analyse severe winter storm damage since the late 1850s in Swiss forests on the basis of few but available variables concerning damage potential, susceptibility to wind and wind severity. For damage potential data on forest area and growing stock per hectare were used. Further the meteorological conditions prior to the storm were used, i.e., precipitation and below or above zero temperature that affect soil stability and indirectly the susceptibility of trees against uprooting. For wind variables the daily maximum hourly wind gust were selected from the Swiss meteorological station Zurich, which had been shown as a good proxy for storm severity in a prior study (Usbeck et al., 2009).

Severe winter storm damage is defined here as forest damage in timber volumes in three or more cantons exceeding 70,000 m3. This limit was chosen to obtain storms with large-scale impact and sufficiently reliable damage estimates. Reliable data on forest growing stock, damage and meteorological data with sufficiently fine-resolution are available for the last 150 years. In order to detect general causes of winter storm damage, available winter storm damage data of severe winter storm damage events were correlated with the explanatory variables on forest area, growing stock, temperature, precipitation and wind speed.

Both historical and recent forest and meteorological data were used, mainly from federal and cantonal publications and databases of the past 150 years covering an area of roughly 35,000 km² with highly structured landscape. The first large inventory of forested areas in Switzerland started in 1858. Detailed information on winter storm damage to forests in subregions (cantons) has also been available since the late 1850s. A meteorological network of measuring stations was installed in 1863. Reliable maximum gust wind speed has been recorded since 1891. The amount of damage (m3) from severe winter storms since 1858 was compared with various explanatory variables. Daily maximum gust wind speed was correlated to nearby recorded forest damage. Daily average temperature and daily total precipitation served as explanatory variables to explain soil status at the time of the damaging events. [see details in the study]


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations

Daily maximum gust wind speed and forest damage in Zurich:
The authors found an average of 0.21 days per year with a daily maximum gust wind speed (UDMG) of at least 30 m s−1 for the first period January 1891–1929, 0.85 days per year for the second period 1930–1968, and 2.51 days per year for the third period 1969–2007. The extremes of UDMG increased by roughly 12 m s−1 between the first and last period. [...]

Storm damage was found to be significantly associated with maximum gust wind speed (p = 0.0015). In addition, the authors found an increase in the maximum damaged part of the growing stock and a temporal increase in extremes of UDMG in winter. Highest UDMG are usually also indicated by reports on forest damage or damage to other goods. All UDMG events of at least 35 m s−1 were caused by winter storms.

Discussion:
This study showed an increase in severe winter storm damage in both amount and frequency (confirming Bütikofer, 1987; Münchner Rück, 2001; WSL and BUWAL, 2001; Schelhaas et al., 2003). The damage potential, i.e., forest area, growing stock as well as growing stock per hectare has also increased (confirming, e.g., Brändli, 2000; Mather and Fairbairn, 2000). However, the damage adjustment by forest area and growing stock cannot fully explain the damage increase during the time period 1858–2007. Various authors have suggested that other major influencing variables have also considerably changed (e.g., Mayer and Schindler, 2002; Schmoeckel and Kottmeier, 2008). The present results show that winter temperature and precipitation have increased during the observed period (in agreement with, e.g., Begert et al., 2005; Schmidli and Frei, 2005; Scherrer et al., 2006; Rebetez and Reinhard, 2007). Winter storm damage usually occurred when the soil was ‘unfrozen’ and ‘wet’. The authors found that this weather type is characteristic for winter storms that are always caused by cyclonic weather situations (e.g., Kraus and Ebel, 2003; Allaby et al., 2006). With one exception, severe winter storm damage in Switzerland occurred when soils were wet. [...]

Maximum gust wind speed has also increased during the time period for which data are available. It exceeded 30 m s−1 for the first time at the beginning of the 20th century, reached nearly 35 m s−1 in the mid-1930s, nearly 40 m s−1 at the end of the 1960s and nearly 45 m s−1 in 1990. [...]

The present results of increasing storminess since the 1960s correspond well with the four Central European pressure records Kremsmünster, Vienna, Karlov, and Prague (Matulla et al., 2008). The observed changes in maximum gust wind speed over the investigated period evoke the questions of comparative observations in Central Europe and of possible meteorological causes. [see above]. The present findings of maximum wind speed observations correspond to the increasing severity of storms during the period 1960–2000 (Leckebusch et al., 2008a). Also the temporal pattern of storminess for Austria and the Czech Republic (Matulla et al., 2008) covering the period from the end of the 19th century to the 1990s resembles well the pattern of maximum gust wind speed in Zurich. In Austria, several extreme winter storms in the years 2007 and 2008 produced the most severe forest damage in more than 60 years (Steyrer et al., 2008).

Modélisations
 
Hypothèses

Discussion:
[...] The present results show that winter temperature and precipitation have increased during the observed period. [...] With one exception, severe winter storm damage in Switzerland occurred when soils were wet. This suggests that the increasing winter precipitation – as regionally predicted in climate change models (Frei et al., 2006; Schmidli et al., 2007) – predisposes forests to higher winter storm damage. [...]

The parameter maximum daily wind speed is claimed to be best for determining changes in wind speed and number of storm events (Rockel and Woth, 2007). For the future climate in Central Europe, extreme wind speed is predicted to increase (e.g., Rockel and Woth, 2007; Leckebusch et al., 2008a,b; Pinto et al., 2009), both in maximum wind speed of extreme events (Leckebusch et al., 2006) and in spatial extent (Leckebusch et al., 2008a). However, regardless of the significance of the correlation between UDMG and winter storm damage to forests, the dynamic wind field in forests may be the main cause of the majority of storm damage in forests. [...]


Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Daily maximum gust wind speed

[See above]


(4) - Remarques générales
 

(5) - Syntèses et préconisations

Conclusion:
The present results show that since the 19th century and particularly over the last four decades, wind velocity and strengths of wind fields have increased in Switzerland while forest area and growing stock were also increasing. In the meantime, winter temperature and precipitation have also increased, potentially driving forest stands to be more sensitive to wind storms through wetter and more often unfrozen soils. The combination of all these factors may explain why forests stands nowadays are generally more vulnerable to winter storms than they were in earlier times. Changing growing stock alone cannot explain the observed increase in forest damages.

To combine forestry and damage prevention, more detailed information on wind effects during winter storms is needed. This knowledge would help to better quantify storm vulnerability of forests with respect to topography. It would require the analysis of more long-term wind data series and their relation to nearby winter storm damage records. More localised information on maximum gust wind speed would be a basis for highly resolved wind gust risk maps that could help forest managers to take appropriate preventive measures.

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