Drought and Wildfire Risk

Prolonged drought makes tropical forests more susceptible to wildfire by turning normally moist vegetation and litter into dry, combustible fuel. It does not itself ignite a fire, but it weakens the forest's natural moisture barrier, allowing lightning or human-caused ignitions to spread more readily.

Drying and accumulation of fuel

Persistent rainfall deficit reduces soil moisture and raises water stress in vegetation. This increases the amount and flammability of available fuel.

• Low moisture in leaves, litter and woody debris reduces the heat required for ignition and sustained burning.
• Water stress can cause leaf shedding, branch dieback and tree mortality, adding dead biomass to the forest floor.
• A longer dry period extends the time during which fuels remain capable of carrying fire.

Faster ignition and fire spread

A humid, closed canopy normally shades the forest floor and helps retain moisture. Drought-induced canopy thinning permits greater heating and air movement near surface fuels.

• Higher temperatures and lower humidity accelerate fuel drying and increase fire intensity.
• A drier understory allows flames to spread across areas where fire would normally extinguish.
• Continuous litter, shrubs and fallen branches improve fuel connectivity, enabling fire to move between patches.

Self-reinforcing degradation

Fire can make a drought-stressed forest more vulnerable to subsequent fires, creating a positive feedback between drying, burning and forest degradation.

• Fire-induced tree mortality and canopy opening expose the forest floor to more sunlight and wind.
• Repeated burning can favour flammable grasses and open vegetation, making future fires easier to spread.
• Susceptibility varies with forest type, drought severity, landscape fragmentation and the availability of ignition sources.