Article | Fuel Material and Vegetation

By: Francisca Yunis Richter

Published on July 26, 2023

Concerning climate change, Chile has experienced a rainfall deficit for more than ten years, known as mega-drought, favoring the flammability of the vegetation

(González, 2020)

Wildfire’s causes are related to a combination of factors, such as weather and vegetation, and the source of ignition related to the fire. There are wildfires of natural origin and human causes; the first ones ignite due to volcanic eruptions or dry electrical storms without rain presence. The second is caused by humans, whether intentional or accidental (CONAF, 2011). Depending on weather conditions, embers can be spread by the wind from one place to another, causing new sources of fires (CSIRO, 2019).

The concept of fuel material describes the flammable living and dead vegetation consumed in wildfires. Fires can be identified by the predominant type of fuel burned, such as grasslands and low vegetation, forests, among others (Sullivan et al, 2012). Vegetation, including trees, grass, shrubs, and leaves, act as fuel for a wildfire, the more abundant and drier this fuel is, the more intense the fire will burn. Fuel material can be improved by fuel management in the territory, which can reduce the probability of ignition and the initial rates of fire propagation in high-risk areas, if its implementation is adequate and before the wildfire season.

Source: Consequences of the “Black Saturday” forest fires of 2009. 173 deceased, 445,000 hectares burned, and 2,100 homes destroyed. Credit: Elizabeth Donoghue / Flickr.

Mediterranean landscapes are prone to forest fires, especially when climatic conditions, such as strong winds, high temperatures, and low humidity, favor the ideal conditions for the spread of fire. Likewise, the risk of fires increases when the predominant vegetation cover in the territory is highly combustible, homogeneous, and includes pyrophilic vegetation (De la Barrera et al, 2018).

Long-term seasonal weather patterns, such as periods of drought or high rainfall, can affect the availability and moisture content of vegetation. Living fuels have a humidity percentage regulated by the composition of vegetation and the amount of precipitation in the territory. The moisture content of dead fuels tends to reflect atmospheric humidity: relative humidity and moisture level in the soil. These depend on recent rainfall and temperatures (Nolan & Thornton, s.f).

Source: Smoke an flames in Australia, European Space Agency – contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO
Source: Australian burnt extent map for the 2019–20 bushfire season, from the National Indicative Aggregated Fire Extent Dataset (DAWE, 2020)
Source: Niche diagram for potential fire regime along with surface fuel types. For example, eucalyptus forests were differentiated between the monsoonal tropics, the semi-arid tropics, and the temperate zone (Murphy et al, 2013).

The cases of Australia and Chile show different types of wildfires depending on their source of ignition. Although the origin of Australian fires ranges from lightning to human activities, natural causes represent only 6%. Likewise, more than 90% are by human activities of intentional or accidental origin (Bryant, 2008). On the other hand, the risk of bushfires in Australia is also determined by the availability of fuel vegetation and its context, such as an ignition source such as lightning or fires from human activities, and a suitable climate for a wildfire to develop.

Multiple factors contribute to the wildfire climate in Australia, such as low rainfall and humidity, high winds and temperatures, all of which improve wildfire risk but also can increase moisture stress on vegetation in the run-up to the fire season (DAWE, 2020). Likewise, the climate crisis and the existing conditions of the Australian climate have intensified droughts, changes in wind gusts, and precipitation patterns, favoring the spread and intensity of wildfires.

In Australia, rainfall patterns shifts and low humidity have increased vegetation growth before the wildfire season, mainly due to rising temperatures, low humidity, and precipitation. Therefore, vegetation dries up and becomes more flammable (WildfireX, 2022). Nationwide rainfall in Australia during 2019 was 40% below average, making 2019 Australia’s driest year since records began in 1900. Much of southern and eastern Australia is in a long drought, which can affect the growth rate of vegetation and its moisture content (CSIRO, 2019).

Source: Combustible Vegetation Intensity Types (Department for Environment and Water, n.d.).

The wildfire regime in Chile is associated with intense fires during the months with higher temperatures and low precipitations, where the climate crisis has increased drought and heat waves, improving the ideal conditions for extreme and destructive wildfires. The causes of wildfires in Chile are mainly due to human activities, whether intentional or accidental. Therefore, the occurrence of fires is directly associated with the proximity of cities and urban infrastructure such as road systems, railway lines and recreational areas (González et al, 2020). Although the climate crisis has increased the intensity and frequency of wildfires, more than 90% of the fires registered by the National Forestry Corporation (CONAF) are caused by humans.

On the other hand, one of the possible causes of the increase in fires during the last decade is the intensive transformation of the natural landscapes of native forests into exotic species and pyrophilic forest plantations, such as pines and eucalyptus trees, which could affect the intensity and frequency of wildfires (De la Barrera et al, 2018).

Source: Area affected according to vegetation and other land uses by wildfires between the regions of Coquimbo and La Araucanía (CONAF, 2017)

During the 2017 mega-fires, the findings determined that close to 70% originated in areas with a high load of fine vegetation, which favored the spread of wildfires. Likewise, the 2023 wildfires originated mainly in wildland-urban interface areas near forestry plantations. Regarding the 2023 fires, at the regional level, Biobío had 116.145 hectares burned with plantations for industrial purposes out of a total of 154.000. Concerning the native forest affected by the fires, the numbers were different since only 7.934 hectares were affected (UdeC News, 2023). Currently, the south-center of Chile has extensive forestry plantations areas, which can form continuous units of 300.000 hectares. Therefore, there is a latent risk for settlements near the wildland-urban interface. Likewise, population growth and urban expansion towards these interface zones, mainly due to the construction of second homes, change in land use, and scarcity of land in urban sectors (CONAF, 2023), increase the risk of wildfires, being mainly caused, either intentionally or accidentally, by humans.

Source: Urban Resilience Plan in Wildland-Urban Interface (PRUIR, 2017)


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