The fire triangle
A fire must have three elements in order to start and spread: fuel, heat, and oxygen. The “fire triangle” is made up of these three elements. Simply said, “fuel” is any flammable substance that ignites and burns; in the context of a forest fire, this often refers to plants. Although certain plants have leaves that have been specially evolved to burn rapidly, dead, dry vegetation like leaf litter or dead timber is typically more combustible than living tissue. Oxygen, constantly present in the air, makes up the second part of the fire triangle. The third element, heat, can be created by lightning or by people, such as when a match or cigarette is carelessly thrown away. For a fire to start, all three parts of the triangle must be present. If one of the elements is removed, the fire will go out. When attempting to put out a forest fire, firefighters benefit from using this theory. For instance, they might halt the spread of a fire by removing possible fuels using a “fire line” an area surrounding the fire’s perimeter that has been cleaned of vegetation or fuels and cannot be crossed by the fire. They can smother the fire with a chemical, a fire retardant, or water (this is known as a wet line) to rob it of oxygen.
The fire ecology
Since the first anaerobic microbes tainted the atmosphere with oxygen and multicellular plant life migrated to the land, fire has been a crucial component of terrestrial life. As we have already stated, heat, fuel, and oxygen came together to form fire for the first time on Earth. In fact, a large portion of the planet’s ecology depends on fire, making it more than just another evolutionary hurdle that life must overcome. Fire ecologists understand that fire is a natural activity that is essential to the operation of many ecosystems, albeit not all. Understanding the mechanisms connecting fire behavior and ecological effects was the primary goal of the study of fire ecology. This includes the interactions between fire, living things, and the natural world.
Many plants have evolved to thrive in conditions where fire occurs due to a long history of recurrent fire in the terrain. Even some plants rely on fire to help them spread and develop. To survive and coexist with fire, plants have developed a variety of adaptations. According to their adaptations, plant species can generally be divided into five groups, but some can fit into more than one. i) The species known as resisters are those that can survive fires of moderate to low intensity with little to no harm. Resisters have various fire-resistance adaptations, such as thick bark to protect them from flames, deep roots that are fire resistant, the shedding of their lower branches to stop the fire from spreading, and moist, short needles or leaves that are difficult to burn. Poderosa pine (Pinus ponderosa), sugar pine (Pinus lambertiana), and Douglas-fir (Pseudotsuga menziesii) are a few examples.
ii) The species that can survive fire are sprouters. After a burn, sprouters can regenerate from their roots, trunks, limbs, and crown. There are several sprouting shrubs. Some of these species also produce seeds with tough shells that must be cracked open by fire. In a fire, the parent plant may suffer damage, but the new sprouts can flourish in nutrient-rich soil with less competition. Madrone (Arbutus sp), aspen (Populus sp), and oak (Quercus sp) are a few examples.
iii) By releasing several seeds that germinate after a fire, seeders have evolved to avoid being burned. The abundant nutrients that are recycled back into the soil help these sprouts flourish. A plant is most likely to spread its seeds and begin to grow just after a fire because there is more room for growth and less competition for resources like sunlight, water, and nutrients. The ecosystem required for many seeders’ seedlings to sprout and develop is created by fire. Seeders are not invaders because their population does not spread as quickly as invaders’ does because they were already present in the area before the fire. Buckbrush (Ceanothus sp), lodgepole pine (Pinus contorta), are a few examples.
iv) Invaders take over areas that have recently burned. Prior to the fire, their populations were either limited or unknown. Invaders have seeds that are easily dispersed by wind, animals, or humans. Many invaders are noxious weeds that colonize areas following disturbances like a fire, flood, or development. Star thistle (Centaurea sp), fireweed (Chamerion sp), and scotch broom (Cytisus sp) are a few examples.
v) Because they grow in places where the fire is not typically present, avoiders are the least fire-adapted species. They frequently live in areas with high elevations or close to water. Because avoiders are a species of late succession, they are not found in places that have recently burnt. Avoiders can aid in the spread of a fire because they have thin bark, shallow roots, and a lot of resin.
But in fire-prone environments, trees have adopted fire as a way of life in the mesic and/or cool conifer forests in western North America, which are accustomed to wet and dry spells, as well as more or less regular fires. Other forests, which are found in high areas and are less prone to flames, are typically home to species that cannot survive and regenerate when the fire breaks out. Pinus sp, Arbutus sp, Quercus sp, Pseudotsuga sp, and Populus sp, are a few examples of tree species with adaptations to fire. To mention one, the closed cones of cones require high temperatures and low humidity, two conditions that are typically present during a fire, in order to open and release the seeds. The cones break open and release their seeds, sowing the calcined earth when the fire decimates the forest. Seedlings are born wherever if the following season’s rainfall is adequate. As a result, while fire kills, it also creates life. On the other hand, no defenses against fire are present in other species. Late winter or early spring is when Scots pine (Pinus sylvestris) and black pine (Pinus nigra) release their seeds. They lack a supply of seeds to withstand the pain when the summer arrives and the fires follow. Only those specimens that have endured or are still living on the edges of the charred area can support its renewal. Here’s where a problem arises: although some resilient pine forests require fire for regrowth, others see it as a fatal foe. The research indicates that forest fires are becoming more frequent and extensive every year, an acceleration that could endanger the dynamics of forest regeneration even though it is fire resistant.
Climate change and fire incidence
It is anticipated that the changing climate will alter fire regimes. Several studies have predicted that there would be more regions susceptible to wildfires, that future flames will be more intense, and that there will be more fires in places that are already prone to fire occurrences (Figure 1). Through the increased release of CO2, soot, and aerosols during combustion as well as the elimination of vegetation that would have otherwise functioned as a CO2 sink, greater wildfire activity has the potential to operate as a driver of climate change. If the prediction of increased fire frequency and intensity is accurate, then these fires will contribute to the global trend of rising temperatures, which has the beneficial effect of increasing the likelihood of wildfires, especially in areas with favorable environmental and climactic conditions for wildfire ignition and spread.
Figure 1. Concerns concerning postfire management practices on publicly owned forests have been raised due to the frequency and severity of forest fires in the western United States growing. Credits: Arnav Kainthola, from www.pexels.com
In the past, lightning served as the primary source of ignition for fires, which followed a natural progression. Nowadays, however, most global fires are caused by human activity. Our existence in most ecosystems means that the world has changed significantly as a result of humans, and so has our connection with the environment. The impacts of fire vary in size, length, and severity as well as the season it happens and how long it has been since prior fires (frequency). Unusually intense fire effects are having a significant impact on wooded landscapes as a result of these effects and recent changes in fire regimes brought on by the effects of climate change. Large fires have occasionally resulted in widespread vegetation type conversion (forest to non-forest, for example), which could have disastrous effects on the local biota and basic ecosystem processes.
Mesic and/or cool conifer forests in western North America are prone to infrequent stand-replacing wildfires, and regeneration of the forest within high-severity burn patch interiors can be sluggish but effective over a lengthy period of time (decades to centuries) (Figure 2). However, under a warming climate, increasing fire frequency and the magnitude of high severity burn patches could complicate post-fire forest recovery and encourage landscape-level changes in the composition, distribution, and structure of forests. Large, intense, and frequent fires may therefore completely overpower the reproductive characteristics of obligate seeding conifers and prefer resprouting angiosperms (plants with flowers) in their place.
Figure 2. For mesic and/or cold high-elevation forest types, big and severe infrequent fires are within the natural range of variation; yet, lately observed and expected increases in fire activity may also hinder post-fire tree development. Credits: Kevser from www.pexels.com
Fire refugia
Remaining live tree islands contained within fire perimeter boundaries, also known as fire refugia, can be a crucial fortress for forest persistence and recovery over time where large, severe, and/or frequent wildfires kill a significant number of live trees within a continuous area, with stand-replacing patch sizes exceeding tree species’ dispersal abilities. Fire refugia (i.e., remaining seed sources) may impact forest recovery trajectories and prospective forest state transitions, which is crucial to delaying and/or preventing this shift (Figure 3). Some fire refugia survive on the landscape for extended periods of time, remaining unburned after numerous succeeding fires. These persistent fire refugia can be found in topographically defined microclimates or areas encircled by inflammable landscape elements, regions that are shielded from fire. The preservation of rare species linked to microclimates absent from the nearby burn mosaic and old-growth dependent ecosystems may depend especially on persistent refugia. Other, more sporadic fire refugia develop by random coincidence with stochastic elements like fluctuation in fuel continuity or erratic processes like wind shifts during burning. Despite being less resilient, they nonetheless perform crucial ecosystem tasks.
Studies on fire ecology benefit from the ability to analyze conditions and track changes over broad geographic areas provided by remote sensing. Biophysical measurements of the ground’s state before and after a fire are provided by remote sensing equipment. These measures have been applied to fuel mapping, active fire detection, burned area estimation, burn severity assessment, fire refugia, and vegetation recovery monitoring. For example, researchers of Portland State University created fine-grain maps of fire refugia through remote sensing and carried out field-based assessments of post-fire conifer tree establishment largely originating (i.e., dispersed) from fire refugium in the Central Cascade Range of the Pacific Northwest United States to examine how fire refugia attributes (i.e., extent, composition, and structure) interact with local climate and environmental conditions to determine post-fire forest recovery responses.
According to the researchers, fire refugia areas amid high-severity burns that are unburned or that burn less severely may help withstand change and play a crucial role in the resilience of forests. In addition to giving individual plants and animals a place to live, they also hold relics (such as canopy shade and seed supplies) that permit repopulation of nearby areas and ecosystem regeneration. For mesic and/or cold high-elevation forest types, big and severe infrequent fires are within the natural range of variation; yet, lately observed and expected increases in fire activity may also hinder post-fire tree development. Therefore, to encourage fire-resistant ecosystems, managers must be able to recognize fire refugia, comprehend their biological roles, and make the greatest use of this information.
Figure 3. After a fire of high intensity, the frequency and distribution of small forest remnants can predict how quickly and how far the forest will recover. Credits: Society of American Foresters
References
- Cochrane, M. A. & Ryan, K. C. Fire and fire ecology: Concepts and principles. in Tropical Fire Ecology: Climate Change, Land Use, and Ecosystem Dynamics (ed. Cochrane, M. A.) 25–62 (Springer, 2009). doi:10.1007/978-3-540-77381-8_2.
- Aponte, C. et al. Forest fires and climate change: causes, consequences, and management options. Int. J. Wildland Fire 25, i–ii (2016).
- Downing, W. M. et al. Influence of fire refugia spatial pattern on post-fire forest recovery in Oregon’s Blue Mountains. Landsc. Ecol. 34, 771–792 (2019).
- Szpakowski, D. M. & Jensen, J. L. R. A Review of the Applications of Remote Sensing in Fire Ecology. Remote Sens. 11, 2638 (2019).
- Busby, S. U. & Holz, A. Interactions Between Fire Refugia and Climate-Environment Conditions Determine Mesic Subalpine Forest Recovery After Large and Severe Wildfires. Front. For. Glob. Change 5, (2022).
- Shatford, J. P. A., Hibbs, D. E. & Puettmann, K. J. Conifer Regeneration after Forest Fire in the Klamath-Siskiyous: How Much, How Soon? J. For. 105, 139–146 (2007).
