Dead wood, often called deadfall or snags, absolutely burns, but the quality of that burn varies based on its condition. The difference between wood that ignites easily and wood that only smolders depends on a few fundamental scientific principles. Understanding how wood converts its stored energy into heat requires looking at its internal structure, water content, and degree of decomposition.
The Science of Wood Fuel
Wood represents stored solar energy, chemically locked within its structure through photosynthesis. Wood is composed mainly of three complex organic polymers: cellulose, hemicellulose, and lignin. Cellulose (40–50% of the wood’s mass) and hemicellulose are the primary energy components that fuel a fire, while lignin acts as the structural binder.
The actual burning process, known as combustion, is initiated by pyrolysis. When wood is heated to temperatures between 390°F and 570°F (200°C and 300°C), it thermally decomposes in the absence of oxygen. This heating releases volatile gases, such as methane and carbon monoxide, which ignite and produce the visible flame. Once these gases are consumed, the remaining solid material, a carbon-rich residue called char or charcoal, glows and burns at a slower rate.
The Role of Moisture in Preventing Combustion
Moisture content is the most significant factor determining whether dead wood burns effectively. Wet wood requires a large amount of energy before it can reach the necessary pyrolysis temperature to ignite. This energy is needed to heat the water to its boiling point of 212°F (100°C) and convert it into steam.
This conversion process absorbs a substantial amount of heat, known as the latent heat of vaporization. This heat is drawn directly from the fire, cooling the wood and preventing it from reaching the temperature required for the release of flammable gases. Consequently, wet wood smolders, produces excessive smoke, and delivers significantly less net heat output. The water within the wood exists as free water, which is easiest to remove, and bound water, which is chemically held within the cell walls.
How Decomposition Alters Flammability
The decomposition of dead wood significantly changes its physical structure and energy content over time. Fungi and bacteria consume the wood’s structural components, primarily targeting the energy-dense cellulose and hemicellulose. This biological action reduces the wood’s density, leading to the soft, crumbly material often referred to as “punky” wood.
Decomposed wood may dry out faster due to its reduced density and increased porosity, but the overall mass of potential fuel is diminished. Rotten wood, even if dry, ignites quickly but produces a fast, weak burn with less heat and a higher percentage of ash residue. In contrast, “sound deadwood,” which is recently dead and has not yet undergone significant decay, retains its high density and full potential energy content, making it excellent fuel once dried.
Identifying Optimal Dead Wood for Burning
To select the best dead wood for burning, look for signs of low moisture and minimal decomposition. Wood that has remained off the ground, such as standing dead trees (snags) or logs propped up by other debris, tends to dry faster and retain its structural integrity. Ground-contact wood, conversely, acts like a sponge, absorbing moisture and accelerating decay.
A simple field test for soundness involves striking the wood. A hard, ringing sound indicates solid, dense wood, while a dull thud or a soft, crumbling surface signals advanced decomposition. Optimal firewood is relatively light for its size, suggesting low moisture content, and exhibits intact bark, which acts as a protective barrier against moisture reabsorption.