Forest fires: European scale
Fire regimes are commonly characterized by burn frequency and severity within a given area. Severity is often estimated as the proportion of overstory trees killed by fire. In general, as frequency increases, fuels have less time to accumulate, reducing intensity and subsequent tree mortality. Increased frequency and size of large, severe forest fires are expected in Australia, the Mediterranean Basin, Canada, Russia, and the United States (1). The critical issue is whether tree mortality patch sizes (and their temporal and spatial frequency) allow recovery of the same or similar vegetation types. If high-severity patch sizes are too large, microclimates and regeneration mechanisms (e.g., seed abundance and dispersal) can limit tree reestablishment This may result in undesirable ecosystem changes. Rising temperatures, related drought stresses, and increased fuel loads are driving high-severity patches to extraordinary sizes in some areas (2).
In Southern Europe, fire frequency and wildfire extent significantly increased after the1970s compared with previous decades (6) due to fuel accumulation (7), climate change (8) and extreme weather events (9) especially in the Mediterranean basin (10). Future wildfire risk is projected to increase in Southern Europe (11), with an increase in the occurrence of high fire danger days (12) and in fire season length (13). The annual burned area is projected to increase by a factor of 3 to 5 in Southern Europe compared to the present under the A2 climate change scenario by 2100 (14). In Northern Europe, fires are projected to become less frequent due to increased humidity (15).
Wildfire changes the hydrologic response of watersheds, increasing the potential for runoff and erosion relative to unburned conditions (17).
Changing wildfire regimes in Mediterranean Europe
Human activities versus climate change
Higher temperatures and more droughts not necessarily lead to an increase in the number of wildfires or the area burnt annually. Human activity may alter fire regimes to such an extent that climate change impacts are completely overruled. These human alterations may both increase and decrease fire probability (18). An increase may result from land-use changes or more outdoor activities (including tourism) (19). A decrease may result from more effective fire suppression. Changes in land-use and fire suppression policy seem to have exceeded the strength of climate change effects on changing fire regime in southern France during the period 1976-2009. The complex interaction of climate change and human effects, which may vary regionally and from one season to another, makes regional predictions of future fires highly challenging (18).
Climate effects: fuel-limited versus drought-driven fire regimes
With respect to climate effects, the situation is even more complicated because climate affects wildfires in two opposing ways. In dry ecosystems wet conditions may be so rare that not enough fuel accumulates to start large fires, and fire activity is limited. These areas have a fuel-limited fire regime. In moist ecosystems on the other hand, dry conditions may be so rare that fuel doesn’t get sufficiently dry to sustain fire spread. These areas have a drought-driven fire regime (18). The alternation of wet and dry conditions, however, favours fires by increasing the amount of fine fuel in litter, grass and shrub layers during wet periods, which burn more intensely in subsequent dry periods (20). In Mediterranean ecosystems both fuel-limited and drought-driven fire regimes are present (21).
Human alterations of fire regimes
Human activities directly modify the fire regime In the Mediterranean Basin by setting or suppressing fires and by changing patterns of vegetation in the landscape (22). Moreover, the fuel build-up following agricultural land abandonment as a result of the rural exodus has created an increasing fire hazard in Mediterranean Europe (23). On the other hand, changes in fire suppression policy over the last few decades have probably induced sharp decreases in fires (24). Hence the functional relationships linking fire to climate have been partially modified by human activities (25), decreasing or increasing the fire activity independently of climate change.
The example of southern France
In southern France three pyroclimates can be discriminated (18): (1) the Mediterranean mountains, characterized by a high seasonality in precipitation and fires (dry summers and wet winters), and the highest burned area fraction, fire season length and the strongest increase in fire danger over the last four decades, (2) the Temperate mountains, characterized by wet and cold conditions, and the shortest fire seasons and the lowest fire activities, and (3) the Mediterranean lowlands, with the driest and warmest climates, and strong and dry winds (the Mistral and Tramontane) that favour fire spread in summer. Human presence and activities decrease from the Mediterranean coast to the rural hinterland, with anthropogenic ignitions accounting for 90% of the number of fires and 96% of the total burned area (26).
During the period 1976-2009 the climatic influence on fires in southern France was not restricted to the occurrence and duration of drought during a particular year: a mixture of drought-driven and fuel-limited fire regimes operated, emphasizing the lagged effects of warm or moist periods on fire (27). Higher fire activity was related to wetter conditions in the last three years. This illustrates that fuel abundance is an important constraint on Euro-Mediterranean (and other) fire regimes, even when drought is the main driver.
With respect to human alterations of fire regimes, contrasting short-term impacts of changes in land-use and fire suppression policy have been found: more fires where fuel biomass is high as a result of land-use change, and less fires in fire-prone ecosystems due to more effective fire suppression (28). Indeed, in the Mediterranean lowlands, which are densely populated and highly susceptible to extreme fire weather due to strong winds (29), both winter and summer fire activity were strongly suppressed, suggesting a gradual increase in the efficiency of fire suppression policy (30).
Fire policy that focuses on suppression only delays the inevitable, promising more dangerous and destructive future forest fires. In contrast, land management agencies could identify large firesheds (20,000 to 50,000 ha) where, under specified weather conditions, managed wildfire and large prescribed fire are allowed to burn, sometimes after strategic mechanical fuel treatments (3). Acknowledging diversity in fire ecology among forest types and preparing forests and people for larger and more frequent fires could help reduce detrimental consequences. New strategies to mitigate and adapt to increased fire are needed to sustain forest landscapes. The following strategies have been suggested (5):
- Landowners should follow “Firewise” guidelines (www.firewise.org/) for houses and other infrastructure. Increased development in fire-prone landscapes has increased suppression costs, exacerbated risk to human safety and infrastructure, and reduced management options. People living in these forests must be prepared rather than relying solely on fire departments. Some places may be so hazardous that building should be prevented, discouraged, or removed (e.g., by regulation or insurance and/or tax incentives).
- Fire managers should avoid trying to uniformly blacken wildfire landscapes through burnout and mop-up operations, especially in burn interiors. As wildfire sizes have grown in recent decades, direct attack has been replaced with indirect attack, where fire lines are placed some distance from the active fire front, and then the area between is intentionally burned, often with high-severity fire, to reduce fuel and create a wider fire barrier. Unburned or partially burned patches are critical refugia that aid postfire recovery in forests of all fire regimes and should be conserved whenever possible.
- Land managers could anticipate changes using models of species distribution and ecological processes and should consider using assisted migration (4).
- Strategies should be based on a forest’s historical fire regime; in forests with historically high-frequency, low- to moderate-severity fire regimes the resilient forest structure should be restored similar to historical patterns that survived during past high-fire periods (and those anticipated in the future).
- Forest restoration should be funded.
Estimates of potential increase in annual burned areas in Europe under a high-end scenario of climate change (the so-called A2-emissions scenario) show an increase of about 200% by 2090, compared with 2000 – 2008, when no adaptation measures are taken (16). With respect to these estimates, the potential effectiveness of two adaptation options was assessed: (1) fire prevention through fuel reduction via prescribed burnings, and (2) active response through better fire suppression. The results of the assessment indicate that application of prescribed burnings may keep the increase in annual burned areas in Europe below 50%. Improvements in fire suppression might reduce this impact even further; e.g. boosting the probability of putting out a fire within a day by 10% country wide would result in about 30% decrease in annual burned area for that particular country. Additional adaptation options, such as using agricultural fields as fire breaks and behavioural changes, can potentially reduce the size of burned areas in the future even further (16).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.
- Attiwill and Binkley (2013); Moriondo et al. (2006); Flannigan et al. (2009), all in: Stephens et al. (2013)
- Attiwill and Binkley (2013), in: Stephens et al. (2013)
- North et al. (2012), in: Stephens et al. (2013)
- Iverson and McKenzie (2013), in: Stephens et al. (2013)
- Stephens et al. (2013)
- Pausas and Fernández-Muñoz (2012), in: IPCC (2014)
- Koutsias et al. (2012), in: IPCC (2014)
- Lavalle et al. (2009), in: IPCC (2014)
- Camia and Amatulli (2009); Carvalho et al. (2011); Hoinka et al. (2009); Koutsias et al. (2012); Salis et al. (2013), all in: IPCC (2014)
- Marques et al. (2011); Pausas and Fernández-Muñoz (2012); Fernandes et al. (2010); Koutsias et al. (2012), all in: IPCC (2014)
- Carvalho et al. (2011); Dury et al. (2011); Lindner et al. (2010); Vilén and Fernandes (2011), all in: IPCC (2014)
- Arca et al. (2012); Lung et al. (2012), both in: IPCC (2014)
- Pellizzaro et al. (2010), in: IPCC (2014)
- Dury et al. (2011), in: IPCC (2014)
- Rosan and Hammarlund (2007), in: IPCC (2014)
- Khabarov et al. (2016)
- Kean et al. (2016)
- Fréjaville and Curt (2017)
- Ganteaume and Jappiot (2013), in: Fréjaville and Curt (2017)
- Fréjaville et al. (2016), in: Fréjaville and Curt (2017)
- Batllori et al. (2013), in: Fréjaville and Curt (2017)
- Moreira et al. (2011), in: Fréjaville and Curt (2017)
- Moreira et al. (2011); Pausas and Fernández- Muñoz (2012), both in: Fréjaville and Curt (2017)
- Pezzatti et al. (2013); Moreno et a.l (2014), both in: Fréjaville and Curt (2017)
- Higuera et al. (2015), in: Fréjaville and Curt (2017)
- Curt et al. (2016), in: Fréjaville and Curt (2017)
- Keeley (2004); Pausas (2004); Meyn et al. (2007); Zumbrunnen et al. (2009); O’Donnell et al. Pausas (2011), all in: Fréjaville and Curt (2017)
- McWethy et al. (2013), in: Fréjaville and Curt (2017)
- Ruffault et al. (2017), in: Fréjaville and Curt (2017)
- Turco et al. (2014), in: Fréjaville and Curt (2017)