Fire is a natural component of many ecosystems. Often, it is the fire regime (frequency, intensity and timing) rather than drought that determines primary productivity as well as plant community (Pyne, 1997; Bond et al., 2005). Nevertheless, dry weather enhances the risk of biomass burning. For example, the severe drought of 1994 that damaged large amounts of woody plants in central and southern Spain
(Penuelas et al., 2001) also resulted in major forest fires, which burnt approximately 1.6% of the national forest area.
It is likely that wildfires will become more common in the future worldwide (Bond et al., 2005). The IPCC Third Assessment Report states that the higher the maximum temperatures, the more hot days and heat waves are very likely to occur over nearly all land areas, increasing the risk of forest fires (IPCC, 2001). Pereira et al. (2002) simulated the impact of future climate change on the meteorological risk of fire in Portugal. They found a significant increase in fire severity and length of fire season under the future climate, which resulted from a temperature increase and a decrease in precipitation in spring-summer. Likewise, Brown et al. (2004) found that prospective drying in the western United States created a future climate scenario with an increase in the number of days of high fire danger.
Vegetation fires are always possible because plant biomass is a good fuel in our oxygen-rich atmosphere. Live biomass, however, does not burn easily because it has a high moisture content. Drought interacts with fires, increasing dead branches and leaf shedding. These materials (dead biomass or necromass) represent the fine fuels, which once dehydrated in hot and dry weather, become highly inflammable and increase the risk of fire. Although drought and wildfires share common causes, it cannot be concluded that more or larger fires will occur in more arid regions. For fires to occur and expand, adequate amounts of fine fuel must be present. Wind, topography and human activities (often as the source of ignition) will also play a role (Pyne, 1997). The Iberian Peninsula may serve as a good case study. Fire frequency is highest in the hilly provinces of central and northern Portugal and Galicia (Spain), not in the more arid south (European Commission, 2003; Pereira & Santos, 2003). Wildfires occur where highly productive periods alternate with a hot dry weather, which facilitates ignition. The Mediterranean vegetation 'could... stand as a dictionary definition of a fire-prone environment. Annually, it undergoes a rhythm of winter wetting and summer drying, over which beats a cruder rhythm of drought. Almost always there is fuel in abundance - combustibles that lack only a properly timed spark to burst into flame' (Pyne, 2005). Likewise, tropical savannas, where a highly productive rainy season alternates with a dry season, are the major contributors for biomass burning globally (Dwyer et al., 2000). In more arid climates, primary productivity is lower, decreasing the amount of fuel and fire incidence (Lloret, 2004).
Extreme events can override the climate tendency. For example, in 2003 Portugal experienced its worst fire season, with a total burnt area of about 5% of the countryside (~4000 km2; Pereira & Santos, 2003). But 2003 was not a very dry year as the annual precipitation exceeded the 1951-1980 30-year average. The exceptional fire season resulted from a heat wave, i.e., daily temperature maxima rising 5°C above the daily average (period of reference 1961-1990) for at least 6 consecutive days.
Droughts may have dramatic effects in ecosystems where water deficits are uncommon, as happened in the tropical rain forests of Southeast Asia in 1997/1998 where widespread wildfires were triggered by the droughts associated with the El Nino Southern Oscillation (ENSO) phenomenon (Roberts, 2001). Likewise, it was estimated that during the 2001 ENSO period of drought approximately one-third of Amazonian forests became susceptible to fire (Nepstad et al., 2004).
In regions where fire has been present for a long time, such as where a Mediterranean type of climate prevails, the vegetation has evolved under a strong fire influence (Lloret, 2004; Pausas et al., 2004; Bond et al., 2005). Plant traits responsible for post-fire persistence operate either at the level of the individual (resprouting) or by stimulating germination from the soil seed bank. Nevertheless, the regeneration depends largely upon environmental conditions before and after the fire as well as the fire regime (Lloret, 2004).
The post-fire persistence plant traits are often associated with differences in drought resistance. Morphological drought-avoiding traits (e.g. higher root/whole-plant biomass, deeper root systems) are more common in resprouters than in non-resprouters (Pausas et al., 2004). Furthermore, fire-induced sprouting does increase drastically the ratio of root to canopy biomass and will promote drought avoidance after fire (Lloret, 2004). On the contrary, woody non-resprouters (e.g., germination stimulated by fire) tend to be more drought-tolerant (e.g. higher xylem resistance to cavitation and embolism) and survive on drier sites than do resprouters. It appears that a greater drought resistance may be only coincidental and not causally related.
Fires may induce changes in soil hydraulic properties and nutrient availability, which may exacerbate the impacts of a drought. The effects depend largely on type of biomass burnt and on soil characteristics (type and moisture content), fire characteristics (intensity and duration), as well as on post-fire precipitation (Chandler etal., 1983). In general, low to moderate severity fires may promote a transient increase of pH and available nutrients as well as the enhancement of hydrophobicity, lowering the capability for the soil to soak up water (Certini, 2005). Severe fires, however, may have a much stronger impact. They may cause removal of organic matter, the creation of water-repellent layers, which may decrease markedly water infiltration rates, the deterioration of the soil structure and the increase in bulk density, which will result in further decreases in permeability and in water-holding capacity of the soil (Certini, 2005). One consequence of these changes in soil hydraulics is increased run-off and surface erosion, which, in turn, may induce a decline in nutrient availability, enhanced by volatilisation losses due to heating (Lloret, 2004; Certini, 2005). However, fire may improve nutrient availability, especially in cases where primary productivity is stagnant due to the immobilisation of nutrients in plant biomass or slow-decomposing litter and soil organic matter. In such cases fire may function as a rejuvenation factor at ecosystem level that will stimulate post-fire primary productivity, although this effect may be short-lived (Briggs & Knapp, 1995; Van de Vijver et al., 1999; Santos et al., 2003a).
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