Understanding how a forest fire progresses is not just a matter for technicians: It helps us to prevent, to make better decisions and to protect lives and ecosystemsFar from being a simple process, fire responds to clear physical rules, the configuration of the terrain and the amount and type of vegetation available.
Furthermore, fire is part of many ecosystems and, at the same time, is largely driven by human activity. In countries like Spain, the vast majority of fires are caused by human activity., while its propagation dynamics depend on a handful of well-known factors: fuel, weather, and topography.
Fire as an ecological factor: natural and anthropogenic
Fire is also a ecological factor with direct and indirect effects in species distribution, plant succession, and habitat stability. In some systems, it even stimulates the regeneration and reproduction of certain species.
Although it can start naturally (for example, from lightning), Most fires in Spain are caused by human actions., intentional or due to negligence. Their impacts are manifested in the microclimate, the soil and the vegetation, also in Biosphere reserves.
- Microclimate: The loss of vegetation cover after a fire increases incident radiation, thermal amplitude, ground-level wind speed, and evaporation; This favors more xerophytic and pyrophytic communities and reduces diversity.
- Ground: The risk of water erosion increases, permeability decreases, pH rises due to ash, and nutrients are mobilized abruptly. decreases the activity of soil microorganisms.
- In vegetation, the immediate impact is the destruction of the aboveground parts; afterwards, herbaceous plants and pioneer species usually appear. initiating a secondary succession that may stagnate or regress if the recurrence of fires is high.
Causes, origin, and factors that govern behavior
A forest fire is considered to be any fire that It spreads uncontrollably through forested land.For combustion to occur, the three elements of the fire triangle must come together: heat, oxygen, and fuel.
Once started, some of the heat dissipates into the environment, but another portion preheats and starts new fuelsallowing the process to continue without the initial source. In the forest, the dominant heat transfer is convection: hot air rises, heats material at higher altitudes, and can promote fire jumps to the treetops and the appearance of secondary fires.
The behavior of the fire is explained by the “fire behavior triangle”: fuel, meteorology and topographyEach vertex modifies the intensity, speed of advance, and direction of the front, according to the severity of forest fires.
- Forest fuel: It includes live vegetation and flammable debris. Its structure, load, continuity, thickness, and moisture content determine the start of the fire. the energy released and the difficulty of control.
- Meteorology: solar radiation, precipitation, temperature, relative humidity, wind, and atmospheric stability They have a decisive influence on the spreadThe “Rule of 30” (T > 30 °C, RH < 30%, Wind > 30 km/h) indicates an extreme danger scenario.
- Topography: The slope accelerates uphill propagation due to preheating, while the relief and orography They channel winds and create microclimates which can intensify the advance of the fire.
Physical mechanisms of heat propagation
The fire spreads by transferring energy to nearby fuels until they ignite. The key mechanisms are radiation, convection, and conduction., with two additional phenomena of great operational interest: preheating and the dispersion of incandescent particles.
Radiation
Radiation heats neighboring fuels without direct contact and is closely linked to the length of the flames; It can trigger ignitions at a certain distance of the main front when the radiative flow is intense.
Convection
Columns of hot air rise due to differences in density and, combined with the wind, They carry embers and heatThis mechanism is responsible for numerous secondary outbreaks that jump barriers and make containment difficult.
Preheating
The heat emitted by the flames raises the temperature of fuels not yet involved (and of rolling stock), shortening the time to ignition and accelerating the spread, especially on hillsides.
Driving
Transmission through the material itself is less relevant in surface or crown fires, since wood is bad driverHowever, in underground fires, driving becomes crucial.
particles
Embers and other incandescent fragments, propelled by wind or the convective column, They plant new hotspots hundreds of meters awayEven stones heated by rolling can start points outside the perimeter.
Fuel models and their applications
To anticipate fire behavior, the following are used fuel models that group vegetation with similar featuresThe 13 Rothermel models adapted by ICONA are arranged into four families: pastures (1-3), scrubland (4-7), leaf litter under trees (8-10) and remains of cutting or silvicultural operations (11-13).
Assigning a model to each zone allows to zone the territory and predict the evolution of the fire according to the structure and continuity of the fuel, facilitating both preventive planning and decision-making in fire suppression.
Types of fires and operational phases
Every fire goes through three stages: initiation (ignition), propagation and extinctionFor tactical purposes, it is also useful to distinguish the typology according to the affected plant stratum.
- Ground or subsurface fire: It progresses through decomposing organic matter and roots; it usually combusts slowly and with little flame due to lack of oxygen.
- Surface fire: It advances through leaf litter, herbaceous plants, scrubland, and fallen fine wood; It is the most frequent type.
- Fire of cups: It can occur as torching (localized crowning), as passive crowns (dependent on surface fire) or as active crowns (independent propagation in crowns, usual with winds > 30 km/h and continuous canopy).
Depending on the extent, it is referred to as an attempt (less than 1 ha), normal fires (between 1 and 500 ha) and large forest fires (FFF, more than 500 ha), the latter responsible for a large part of the area burned annually by high flame lengths, high speeds and cup activityThese events are expected will increase in the coming years.
Among the GIFs, the following have been described: sixth generation firesfueled by extreme conditions and abandonment of the mountain, with convective columns capable of generating multiple secondary foci and deceptive behaviors (they intensify when it seems that they would tend to subside).
In addition to the classic approach by fuel, weather, and topography, some analyses include categories such as wind-driven, topographic, or gas-dominated firesuseful for interpreting propagation patterns and adapting the attack maneuver.
Meteorology, relief and biomass: the combination that rules
The slope and solar orientation, along with valleys, hills and canyons, They accelerate the uphill advance and channel the wind.creating a “chimney effect.” The continuity and state of the biomass (whether it is alive or dead, its moisture content, and its structure) determine the energy and speed of propagation. Furthermore, droughts and heat waves increase the danger.
The weather is key: wind, temperature and relative humidity They define windows of opportunity or scenarios of extreme risk. With hot, dry air and plenty of fuel on the slope, the fire tends to gain intensity and complexity from the start.
In grasslands, with fine, dry fuel, very rapid advances have been documented; in stands of conifers or eucalyptus, The propagation can easily exceed 9-10 km/h with wind and dry fuels. The embers, in turn, explain line breaks and new hotspots ahead of the front.
Recent examples illustrate this complexity well: in the fire at Casas de Lázaro (Albacete, 2022), the sloping topography and the dry wind They increased the speed of the front; in the area of Las Médulas (León), with irregular terrain and heat, the transport of embers complicated the anchoring of lines and containment at a distance.
Prevention and preventive silviculture
Prevention aims to reduce flammability, to increase the fire resistance of masses and provide safety for operationsIt focuses on the available fuel and minimizing the probability of ignition; measures include fire prevention strategies.
- Preventive defense areas: strips of 40-100 m (up to 150 m depending on slope and mass height) in woodland where the fuel model is modified to reduce biomass and facilitate the control of ground fires.
- Firebreaks: strips of 20-30 m with exposed mineral soil, associated with reforestation or scrubland where large areas are not viable; They serve as support for lines of defense..
- Preventive support belts: of variable width, supported by infrastructure, with clearing and pruning for lower fuel load.
Regarding the three main factors, we can act on biomass (firebreaks, containment areas, security perimeters, silvicultural management and stand conservation) and, indirectly, anticipate the weather with AEMET forecasts and fire hazard maps to plan uses and restrictions.
Social awareness, improved use of the forest, extensive livestock farming and prescribed burns during low-risk periods They are part of the preventative toolbox. Legislation and the effective prosecution of arsonists also deter and reduce the number of fires.
Detection and surveillance
Early detection, accurate location, and rapid communication to headquarters are vital: The “fire alarm” reduces critical minutes between ignition and the arrival of emergency services. Most alarms are based on smoke, which can be false, legitimate (authorized burning) or illegitimate (with the potential to cause a fire).
- Fixed ground surveillance: towers and guardhouses in high-value or high-risk areas, located for maximize visibility and minimize blind spots.
- Mobile ground surveillance: 4x4 surveillance and first attack vehicles with radio, motor pump and tank (400-500 l), hoses and lances, fire extinguisher backpacks, pulaskis and other equipment; They provide a deterrent effect and initial attack capability.
Modern systems incorporate forest sensors that monitor CO, CO2, temperature, humidity and wind in near real time, feeding detection algorithms and propagation models and showing progress in early warning modelsThe data is integrated into the cloud and helps with decision-making and operational safety during a fire.
Extinguishing and operational safety
Direct attack acts on the burning material (water, foam/retarder, asphyxiation or fuel separation), while indirect attack prepare lines and remote treatments from the front (contingency firebreaks, backfires, soaking with unburned fuels).
Ground resources (brigades of agents and forest firefighters) work in coordination with aircraft to anchor lines, protect assets, and secure perimetersExtinguishing forest fires is a high-risk task: sudden changes in direction, convective collapses, heat and smoke generate disorientation and lethal dangers.
Speed is everything: in Spain, on average, about 20 minutes to move ground vehicles From detection, 64 minutes to control and 120 to extinction (average values that vary according to terrain, access and conditions).
Causes, percentages and ignition conditions
The immediate causes are varied, but two major factors converge: abundant vegetation and periods of droughtWith soil moisture below ~30%, plants do not recover lost water, they dry out and emit ethylene (a combustible compound), making the vegetation and surrounding air flammable.
- Intended: Nearly 54% in Spain. The main causes are unauthorized or uncontrolled agricultural burning, as well as pyromania, hunting practices, vandalism, revenge killings, scaring away wildlife, and land speculation. or economic motivations.
- Negligence and accidents: ~26%. Authorized burns that get out of control, cigarette butts, poorly extinguished campfires, machinery, power lines, forestry work and other.
- Lightning: 4-5% of cases.
- Unknown: ~15%.
- Plays: ~2% due to reignition of embers hidden in previous fires.
Overall, human activity causes more than 80% of fires in both Spain and the US, and in Spain Most of the burned area comes from these eventsThe percentages vary by region and year, but the anthropogenic trend is clear.
Operational levels and coordination
For management purposes, a scale is used that classifies emergency levels between 0 and 3. Levels 0 and 1 These are managed by the regional authorities with their own resources. Level 2 involves the activation of state resources, and level 3 raises the emergency level to National scope with the State assuming control.
Impacts, recovery and pyroecology
Fires affect soil, water, air, biodiversity and the economy. With the vegetation cover gone, erosion skyrockets and nutrients are lost; the soil is impoverished biologically and chemically by the high temperatures.
The fauna suffers direct mortality (especially invertebrates and young), in addition to displacements and habitat loss; Human health may be affected due to smoke and pollutants. Infrastructure and property are damaged, and there are communication disruptions and remediation costs. The CO2 emitted and the loss of carbon sinks contribute to climate change.
A forest can take between 30 and 50 years to recover, depending on climate, species, and severity. To accelerate recovery, the following are used: soil protection against erosion (mulching, straw), inoculation of symbiotic fungi and, when natural regeneration is not viable, selective reforestation and projects to restore the forests.
Many species have evolved with fire: serotinous pineapples, epicormic shoots, and lignotubercles They allow ecosystems to persist or regenerate after the flames have passed. Pyroecology studies this fire-ecosystem relationship in grasslands, savannas, chaparral, and coniferous forests, where fire can create mosaics and maintain diversity.
In recent decades, prevention and firefighting efforts have succeeded in reducing the annual area burned globally, although climate change and rural abandonment continue to pose challenges. They threaten to reverse the trend.Hence the importance of maintaining forestry work, infrastructure networks, surveillance, research, and a co-responsible citizenry.
If we put all of the above together, it becomes clear that the advance of fire responds to physical laws and an operational triangle (fuel, meteorology, topography) shaped by human activity; Prevent with silviculture and planning, detect earlier and attack with sound judgment It makes the difference between a minor fire and a major forest fire, while post-fire management and pyroecology teach us to live with fire without letting our guard down.
