Réf. Zumbrunnen & al 2009 - A

Référence bibliographique complète

ZUMBRUNNEN, T., BUGMANN, H., CONEDERA, M., BÜRGI, M. 2009. Linking Forest Fire Regimes and Climate - A Historical Analysis in a Dry Inner Alpine Valley. Ecosystems, 12, 73-86.

Abstract: Forest fire regimes are likely to experience considerable changes in the European Alps due to climatic changes. However, little is known about the recent regional fire history and the impact of local climate on the fire regime during the 20th century. [The authors] therefore reconstructed the fire history in a dry continental valley of the Swiss Alps (Valais) over the past 100 years based on documentary evidence, and investigated the relationship between the reconstructed fire regime and the local climatic variability. [They] compared the impact of temperature, precipitation, drought and dry foehn winds on fire frequency, extent of burnt area, and fire seasonality on various spatial and temporal scales. In the subalpine zone, the fire regime appears to have been mainly driven by temperature and precipitation, whereas these variables seem to have played only a secondary role in the colline–montane zones. Here, foehn winds and, probably, non-climatic factors seem to have been more important. Temperature and precipitation played a major role in shaping fire frequency and burnt area in the first half of the 20th century, but lost their importance during the second half. [This] case study illustrates the occurrence of different fire regime patterns and their driving forces on small spatial scales (a few hundred square kilometers). [The authors] conclude that the strong rise in temperature over the past century has not profoundly changed the fire regime in Valais, but in the second half of the 20th century temperature was no longer a strong determinant for forest fires as compared to human activities or biomass availability in forests.

Mots-clés
Fire history; Climate; Continentality; Documentary evidence; Central Alps; Switzerland.

Organismes / Contact

• Research Unit Land Use Dynamics, WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland
• Department of Environmental Sciences, Institute of Terrestrial Ecosystems, Swiss Federal Institute of Technology Zurich (ETH), Universita¨ tstrasse 16, 8092 Zurich, Switzerland
• Research Unit Ecosystem Boundaries, Swiss Federal Institute for Forest, Snow and Landscape Research, Belsoggiorno 22, 6500 Bellinzona-Ravecchia, Switzerland
Corresponding author; e-mail: thomas.zumbrunnen@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, Drought and Dry foehn winds   Forest fire  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Canton of Valais       Past 100 years (1904–2006)

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

During the 20th century, an increase in annual mean temperature of 1.3°C was observed in Valais (Bader and Bantle 2004).

Modélisations
Climatic projections suggest further warming and increased summer drought for this area (for example, Schär and others 2004).
Hypothèses
 

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

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

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

 


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

The Fire Regime in Valais (1904–2006)

Fire Frequency: The mean fire frequency amounted to nine fires per year (SD = 8). Two periods of change are conspicuous: a slight increase during the 1940–1950s and a stronger peak during the 1990s. Fire frequency evolved differently in the two elevation zones, that is, the colline–montane versus the subalpine zone. Until the end of the 1940s, they had approximately the same fire frequency per unit of forest area. Subsequently, fire frequency increased at lower elevations, whereas it remained almost constant and increased only slightly toward the end of the 20th century at the subalpine level.

Annual Burnt Area: During the study period, forest fires burned about 2700 ha in Valais, corresponding to an annual mean value of 26 ha, with a high interannual variability (SD = 65). For instance, more than half of the total area burned in only 6 years (1906, 1921, 1979, 1981, 1996, and 2003), and this was mainly due to very few large events. These 6 years were all within the first or last third of the study period. In other words, the first (1904–1940) and last (1971–2006) decades were characterized by a few years with a large area being burnt, whereas in most years the area burnt was very small or none was burnt at all. The intermediate period (1941–1970) was characterized by a relatively small but regularly distributed annual burnt area.

Fire Sizes: [...] The fire size distribution fluctuated over the study period. If we exclude the fires of unknown size, the Wilcoxon rank-sum test provides strong evidence for the null hypothesis of identical medians for the 1904–1940 and 1941–1970 periods (P = 0.112), but it suggests that the median of the 1941–1970 period was greater than that of the 1971–2006 period (P < 0.0001). Yet, if we assume that the fire events of unknown size were rather small and add them to the smaller than 0.1 ha class, the test provides strong evidence for the alternative hypothesis, that is, that the median was greater in the 1941–1970 period than in the 1904–1940 (P < 0.0001) and 1971–2006 (P = 0.004) periods. However, in both cases, that is, regardless of whether fires of unknown size are included or not, the last two periods saw a decrease of the median fire size. In both cases, maximum size decreased during the second period, and then increased again during the last period.

[...]

Climatic Controls on the Fire Regime

Relationship Between Temperature/Precipitation and Fire Frequency: The cross-correlation analysis between the time lines of temperature/precipitation and the fire frequency revealed different patterns according to area (entire study area, low elevation, high elevation) and to whether the period before or after 1950 is considered. In most cases, fire frequency was positively correlated with temperature and negatively with precipitation of the same year, but there was no correlation with either temperature or precipitation for the period 1951–2006 and in the colline–montane zones. In addition, a positive correlation between fire frequency and precipitation was evident with a negative time lag of 3 years for almost all regions and periods considered.

Fire Activity and Drought: With one exception (1914), there was a water deficit (Thornthwaite index <0 mm) every year during the fire season, that is, March–October. The 1940s and 1950s were the driest decades, whereas the driest single years occurred during the first part of the 20th century (1906, 1911, 1921, and 1943). Despite a strong rise in temperature since the end of the 1970s in Valais (Bader and Bantle 2004), the fire season has not become markedly drier. The fire seasons with more than 12 fires and a burnt area greater than 19 ha were clearly drier during the first half of the 20th century than during the second. Although most of the seasons with a high number of fires were relatively dry, the season with the highest number of fires, 1990, was one of the wetter seasons in the study period. The seasons with a high burnt area corresponded mostly, but not always, to dry periods. For instance, the fire season 1979 with 160 ha burnt was wetter than average. Since the 1950s, large fires (>10 ha, >50 ha) have tended to occur in months that were wetter than the seasonal average, whereas prior to 1950 they tended to occur in months that were drier than average. Furthermore, the frequency of these ‘‘large’’ events increased noticeably in the period 1950–1980 even though climatic conditions were less favorable for fires than they had been during the preceding decades.

[...]

Discussion

Fire Frequency: Most years with a large number of fires were dry to very dry, such as 1911, 1962, or 1996. Nevertheless, there are some exceptions and 1990, the year with the highest number of fire events, was actually quite wet. The reason for this exceptional number of fires remains unclear. One non-climatic cause could be the occurrence of a devastating foehn storm that blew down about 600,000 m3 of wood in Valais during the winter preceding the 1990 fire season (Etat du Valais 2000). This led to an increase in fine fuels and coarse woody debris in the forests, coupled with more insolation of the forest floor due to the absence of a canopy in the affected areas. Frelich (2002) and Kulakowski and Veblen (2007) have pointed out possible interactions between blowdowns and fire activity.

The overall impact of temperature and precipitation on fire frequency was confirmed by the cross-correlation analysis. However, there was a distinct significant correlation between fire frequency and temperature (r = 0.59)/precipitation (r = -0.48) in the same year during the first half of the study period (1904–1950), whereas no such correlation (T, r = 0.01/P, r = -0.09) was found during the second half (1951–2006). This change could be due to new factors that interfered with the climatic signal. The economy of the study area changed greatly from being mainly agriculture oriented (during the 19th and the first half of the 20th century) to becoming more industry and service oriented. As a consequence, many traditional forms of forest use (for example, pasture in forests, collecting of litter and dead wood) have been abandoned or reduced in intensity and extent (Kempf 1985; Kuonen 1993; Gimmi and Bürgi 2007). Among others, these changes in forest use have contributed to an increase in coarse woody debris in the forests. Also, a significant fraction of felled logs is often left in stands to provide protection against rockfall, and so is wood that has become unmarketable because of economic rationalization (Bugmann 2005). Living and dead biomass have thus increased (Gimmi and others 2008), which may have influenced fire frequency and fire intensity, causing a relative decline in the importance of climatic factors such as temperature and precipitation. The fact that there have been several years since the middle of the 20th century with a high number of fires despite moister conditions supports this hypothesis. Moreover, fuel load clearly also plays an important role in the fire regime because fire frequency was found to be significantly and positively correlated with precipitation with a time lag of 3 years. This indicates that rainfall tends to boost the production of fine fuels. [...]

Fire frequency varies with altitude in Valais and was considerably higher at the colline–montane than at the subalpine level. This reflects the more fire-prone climatic conditions and the higher ignition potential at lower elevations due to denser human settlements (Bundesamt für Statistik 2005). Although at the beginning of the 20th century fire frequency at these two altitudinal levels was fairly similar, the frequencies started to differ strongly after the end of the 1940s.

These dichotomous trends are probably caused by human populations shifting toward the lowlands (abandonment of high-elevation agricultural land, urbanization at low elevations) and by a concomitant relative decrease in ignition sources at higher altitudes (for example, less forestry and agriculture). Fire activity at the subalpine level has not noticeably increased, although the forested area and fuel load have increased greatly since the 1950s, mainly at high elevations (Kempf 1985; Julen 1988; Gimmi and others 2008). This suggests either that fuel load is not very relevant as a controlling factor of the fire regime at higher elevations, or that the fuel build-up has been counterbalanced by a decrease in human population. Fire frequency at the subalpine level was found to correlate well with temperature and precipitation in the same year, although there was no correlation at the colline–montane level. This implies that temperature and precipitation have lost their relevance as factors influencing the fire regime over time at low elevations. This is probably because the progressively much more favorable fire weather has allowed other fire drivers to gain importance, such as the presence of anthropogenic ignition sources. Thus, climate as a key driver of fire frequency still seems to predominate at the subalpine level, whereas several other drivers operate in combination at the colline–montane level. [...]

Fire Size and Burnt Area: [The authors] assumed that fires of unknown sizes were smaller than 0.1 ha because very small fires are less likely to be reported in detail. Excluding the fires of unknown sizes, the fire sizes decreased during the study period. This could be due to fire reporting becoming more accurate over time. However, the decrease in fire size over the study period could also be caused by improvements in fire fighting techniques.

Ultimately, [they] propose that the temporal development of fire sizes can be explained by the simultaneous and opposite effects of two processes: first, the continuous increase in fuel load and the expansion of the forested area since the 1950s has enhanced fire risk as well as the connectivity of potentially burnable areas. This could be the reason for the increase in median fire size during the period 1941–1970 and the more frequent occurrence of extreme events since the end of the 1970s. Second, fire-fighting techniques and equipment have improved, and previously unreachable areas have become more accessible due to the use of helicopters and the strong expansion of the road network. These certainly allowed large fire events during the period 1941–1970 to be better contained (there were no fires >65 ha during this period). These measures have also reduced median fire size, but since the end of the 1970s the fuel build-up has led to more very large fires. This interpretation is supported by the fact that seasonal weather conditions for burnt areas larger than 19 ha and monthly conditions for fires larger than 10 and larger than 50 ha were wetter during the second part of the study period. This implies that other factors besides temperature and/or precipitation must have influenced the annual burnt area during this second period.

Fire size classes were also characterized by clear differences in spatial distribution. A majority of the fires larger than 4 ha were concentrated in the continental part of Valais, and most of the fires larger than 10 ha were concentrated in a region affected by strong foehn winds (Bouët 1972). This geographical distribution emphasizes the role of foehn in explaining the occurrence of ‘‘large’’ fire events in certain areas, although the effect of drought periods may overlap with the foehn effect.

Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Fire frequency, extent of burnt area, and fire seasonality on various spatial and temporal scales

Climate is one of the major drivers of fire regimes and has a multi-faceted effect on fire activity. It can act indirectly by enhancing biomass production and therefore increasing fuel supplies. It can be a predisposing agent if it causes water stress and dries out fuel, or acts directly by igniting fires, for example, through lightning, or modulating fire behavior, for example, through wind. Climate influences fire activity on very different spatial scales: from the global, when phenomena such as the El Ninõ-Southern Oscillation affect the occurrence of large fires on subcontinental scales to the local, when winds influence fine-scale fire activity patterns [see references in the study].

Spatially, the relative importance of climatic variables varies greatly according to geographical region and ecosystem type [see exemples in the study]. Thus, region-specific analyses of the driving forces of fire regimes are required to understand local fire regimes better and to enable projections of the future fire regimes under changed environmental conditions.

In regions where the fire regime is climate-driven, climate change may have already provoked changes in fire activity [see exemples in the study]. Little is known, however, on possible recent changes in the fire regime in central Europe.

Reconstruction of the Fire Regime:
Based on documentary evidence from the archives of the forest service of the canton Valais, [the authors] built a database of forest fire events covering the period 1904–2006. This database integrates data from previous inventories covering the periods 1973–2000 (Bochatay and Moulin 2000) and 1904–2003 (Gimmi and others 2004). [...] The resulting forest fire statistics are presented in two forms, one including and the other excluding events whose size is not known. To compare the median fire sizes of three different time periods (1904–1940/1941–1970/1971–2006), a one-sided Wilcoxon rank-sum test were performed. [...]. No information on fire intensity is available. The analysis of the fire regime is therefore limited to frequency, season of occurrence, and extent of the burnt area.

Determination of Climatic Controls on the Fire Regime:
The relationship between fire frequency and climatic variables was analyzed by cross-correlating the number of fires with the sum of precipitation (mm) and mean temperature (°C) for every fire season (March–October) of the study period. [...] As several time series (temperature and fire frequency according to elevation zones) were autocorrelated, they were pre-whitened by detrending the series using kernel smoothing. Then the cross-correlations (Pearson coefficient) between fire frequency and temperature/precipitation time series at various lags were calculated.

The effect of drought on the fire regime was evaluated by comparing the changes in fire regime with changes in a drought index, calculated according to Thornthwaite (1948), over the study period. [...] The Thornthwaite index (DRI = P - PET) requires monthly mean temperatures and precipitation sums, with P equal to the precipitation sum from March to October, and PET equal to the sum of estimated potential evapotranspiration from March to October. The potential evapotranspiration was calculated based on the temperature data by taking into account day length and the sun angle [...].

Meteorological data were available from the Swiss Federal Office for Meteorology and Climatology (MeteoSwiss). Monthly data from the meteorological station in Sion for the entire study period (temperature and precipitation) were used to calculate monthly and annual water deficit values (Thornthwaite drought index) and for the cross-correlation analysis.

To determine the impact of foehn winds on fire activity, the authors superimposed the geographical distribution of fire size classes on Bouët’s (1972) map of foehn occurrence and the areas affected by the foehn according to Schreiber and others (1977). They also compared fire seasonality with the corresponding mean monthly wind speed and mean monthly water deficit (compare Thornthwaite index) in the areas with different foehn occurrence.

To determine an altitudinal limit for selecting the fire events to be included in the analysis, they opted for the boundaries suggested by Schreiber and others (1977). The meteorological variables (mean wind speed, temperature, and precipitation) were calculated using data from the MeteoSwiss meteorological stations in Aigle and Visp for the period 1981–2006.


(4) - Remarques générales

In this article, the authors analyze the fire regime in Valais (Switzerland), a central Alpine valley with a continental climate similar to that of Briançonnais in France and the Val d’Aoste and Vinschgau in Italy. Although the Valais is characterized by a rather modest fire activity in comparison to other regions in Europe, such as the Mediterranean basin, considerable changes in the fire regime have been forecast in association with a projected future climate (Schumacher and Bugmann 2006). However, apart from two previous rather descriptive studies (Bochatay and Moulin 2000; Gimmi and others 2004), a detailed reconstruction of the fire regime of Valais during the past century and an empirical understanding of the relationships between this regime and climatic variables are still largely lacking. Furthermore, only a few studies about the fire history of entire regions in the European Alps have been published so far (for example, Buresti and Sulli 1983; Stefani 1989; Cesti and Cerise 1992; Conedera and others 1996), and most of them cover only short study periods (two to three decades).

Thus, the main goals of [this] study are: (1) to reconstruct the forest fire history of Valais during the 20th century using documentary evidence (forest service reports); (2) to determine the relationship between the fire regime and the local climatic variability; and (3) to evaluate whether past climatic changes resulted in corresponding changes in the fire regime. Specifically, we want to assess if the fire regime reflects the spatial, seasonal, and temporal (1904–2006) changes in the patterns of rainfall, temperature, and the dry wind system (foehn).

[Other resuts]:

Fire Distribution: There were 906 fires (100 fires/100 km² forest) in the study area between 1904 and 2006. Most fires occurred in the central and eastern parts of the study region. There was a higher fire frequency (129 fires/100 km² forest) at the colline–montane level (elevation <1400 m a.s.l.) than at the subalpine level (40 fires/100 km² forest).

Fire Seasonality: The fire season in Valais lasts from March to October (90% of all fires), with two major peaks in March–April and in July–August. Fires in winter are very rare. This ‘‘double-peak’’ pattern results from the combination of the seasonal fire distribution in the colline–montane versus the subalpine level, that is, a high fire activity in March–April at low elevations and in August at higher elevations. [...] It seems [...] that fire seasonality at low elevations is mainly conditioned by the foehn in areas where it blows, whereas drought is the decisive factor in areas without foehn. In areas where both phenomena are relevant, spring foehn seems to play a much more important role than summer drought.

Causes of Fire: For 42% of the fire events, the causes of ignition are known. According to forest service reports, about 85% of these fires were caused by humans (negligence, accident, or arson), which suggests that the fire regime in the study area is dominated by anthropogenic influences. The relevance of lightning as an ignition cause (15% of the fires with known causes) is spatially and temporally limited. Indeed, lightning-caused fires were restricted mainly to July–August and to high elevations (mean altitude 1700 m a.s.l.). Furthermore, the mean size of the fires caused by lightning was about 0.2 ha compared to 5 ha for fires of human origin.


(5) - Syntèses et préconisations

Conclusions:
On the basis of the reconstructed fire history of Valais in the 20th century, [the authors] were able to distinguish sub-regions with different fire regimes depending on altitudinal or geographical location, even though Valais is rather small. The altitudinal gradient was mainly reflected by fire frequency and seasonality, whereas the geographical location showed differences in fire seasonality and in the distribution of fire size classes.

[This] study demonstrated the occurrence of different fire regime patterns and driving forces on small spatial scales. The occurrence of large fire events seems to be favored by the limited amount of precipitation due to continentality in combination with foehn winds, which are regionally constrained. The diversity in fire activity was additionally influenced by the local climatic variability along altitudinal gradients. In the subalpine zone, the fire regime appeared to be mainly driven by temperature and precipitation, but these two variables played only a secondary role in the colline–montane zone. Here, the influence of the foehn and, probably, other non-climatic factors, such as fuel load and human population density (ignition sources), were more important. Thus, this local complexity of fire activity requires locally differentiated approaches, for example, for the implementation of prevention measures.

During the 20th century, the fire regime has also changed. The annual burnt area has noticeably changed and there has been an increase in large fires in recent decades. [This] study suggests that temperature and precipitation played a major role in shaping both fire frequency and burnt area in the first half of the study period, but they lost their importance after the mid-20th century. Thus, it appears that the temperature change clearly evident from the meteorological records in Valais has not caused an increase in fire frequency and burnt area. Temperature was no longer a limiting factor for forest fires in this dry valley in the second half of the 20th century.

These findings have practical implications. For example, because other factors than climate change are shaping today’s fire regime, these have to be considered carefully in the development of effective fire prevention and management measures. Additional analyses will help to further pinpoint the crucial factors affecting the fire regime in Valais in the second half of the 20th and the early 21st century. In particular, special attention should be given to (1) the increase in fuel availability due to changes in forest use and management, (2) improvements in fire suppression techniques, and (3) the increased potential for humans to start fires.

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