Wildfires are a natural feature of many ecosystems, shaping landscapes and clearing the way for new growth. While images often show widespread destruction, many trees survive the blaze. This survival is the result of specific physical properties and environmental circumstances that prevent combustion or shield the tree from lethal heat. A tree’s ability to withstand a wildfire is determined by its internal composition, its outward structure, and the nature of the fire itself.
The Role of Tree Moisture Content
The most immediate defense a living tree possesses against fire is the water held within its tissues. Combustion requires a substance to reach its ignition temperature, but water acts as a significant energy barrier. The fire’s heat energy must first evaporate all the internal water before the plant material can pyrolyze, or thermally decompose, into flammable gases.
Trees with naturally high water concentrations, such as aspen or some broadleaf deciduous species, are more resistant to ignition. High internal moisture means the fire must boil off a large volume of water, which cools the plant material and delays burning. For a living tree to ignite, the energy delivered by the flames must be sustained long enough to overcome this vaporization process.
The measurement known as Live Fuel Moisture Content (LFMC) is a primary factor in predicting a tree’s flammability. Fresh foliage, for instance, can have a moisture content value of 300% relative to its dry weight, making it highly resistant to burning. When a tree is stressed by drought, its LFMC drops, meaning less energy is needed to reach the ignition point, drastically increasing its vulnerability to fire.
Physical Defenses: Bark and Structure
Beyond internal water, a tree’s physical structure provides multiple layers of insulation against external heat. The most obvious of these is the bark, which acts as a thermal shield to protect the living tissue just beneath it, known as the cambium. If the cambium is heated to a lethal temperature, the tree will die, but thick bark slows the transfer of heat from the fire to this vulnerable layer.
Species evolved in fire-prone regions often exhibit remarkably thick bark, serving as a highly effective insulator. The Coast Redwood, for example, can develop fibrous bark up to 12 inches thick, which is a major reason for its resilience. Bark composition also matters, as non-resinous or exfoliating barks shed burning material and prevent heat buildup on the trunk.
Structural adaptations also play a role in preventing fire from reaching the crown. Many fire-tolerant species practice self-pruning, where lower branches are naturally shed as the tree grows taller. This creates a large vertical gap between the ground fire and the main canopy, eliminating “ladder fuels” that a surface fire would use to climb. Furthermore, trees that contain low amounts of volatile resins and oils are less flammable, contrasting with species like pines that rely on these highly combustible compounds.
How Fire Type and Environment Dictate Survival
A tree’s fate is often decided less by its inherent defenses and more by the specific characteristics of the fire event. Wildfires are generally categorized by the section of the forest they consume, with surface fires being the least destructive to mature trees. Surface fires burn low to the ground, feeding on fallen leaves, twigs, and small vegetation, and typically move with low intensity.
In contrast, a crown fire is a high-intensity blaze that spreads rapidly from treetop to treetop, consuming the entire canopy. The transition from a surface fire to a crown fire is a critical point that dictates tree mortality. This transition is influenced by external factors, such as the height of the tree crowns and the density of the vegetation.
Environmental conditions dictate the fire’s behavior and intensity, affecting whether a tree is bypassed or incinerated. High wind speed pushes flames and embers, increasing the rate of spread and intensity. Topography also matters, since fires move slower across flat ground and accelerate up steep slopes. Finally, fuel arrangement, such as gaps in the vegetation, can prevent continuous spread, forcing a crown fire to drop back down to a less threatening surface fire.