The question of whether tree sap is flammable often arises from a misunderstanding of what a tree excretes. The answer depends entirely on the specific substance produced. The highly flammable material people associate with ignitable tree exudates is not the nourishing fluid that circulates through the tree, but a different, protective compound. Understanding the distinct biological roles and chemical makeup of these two substances provides the explanation for their varying reactions to fire.
Defining the Difference: Sap versus Resin
The fluid known as sap is the lifeblood of a tree, moving water, sugars, and nutrients throughout its vascular system via the xylem and phloem. This substance is predominantly water, containing dissolved sugars, hormones, and mineral elements. Because of its high water concentration, true sap is resistant to ignition; heat applied will cause the water to evaporate before the remaining solids can catch fire.
Conversely, the sticky, aromatic substance that oozes from trees like pines, firs, and spruces is called resin. Resin is a secondary metabolic product, meaning it is not involved in the tree’s primary processes of growth and nutrient transport. Its function is defensive, acting like a natural bandage to seal wounds, deter pests, and prevent fungal or bacterial infection. The chemical composition of this viscous material is the source of its flammability.
Chemical Composition and Ignition Mechanism
The reason resin is flammable lies in its complex chemical makeup, which is rich in volatile organic compounds (VOCs). These VOCs are mostly a class of hydrocarbons called terpenes, such as pinene, produced in specialized resin canals within the tree. Terpenes are energetic molecules with a low flash point, meaning they do not require a high temperature to begin producing flammable vapors.
For combustion to occur, a substance must release a flammable vapor that mixes with oxygen in the air. When heat is applied to resin, the volatile terpenes rapidly vaporize, creating a concentrated, ignitable gas cloud above the substance. The high calorific value of resin, often exceeding 38.0 megajoules per kilogram, confirms its capacity to release significant energy when it burns. This vaporization and subsequent gas-phase combustion allows tree resin to ignite and burn vigorously.
Factors Influencing Flammability
The ease with which a particular sample of resin will burn is influenced by several variables beyond its base chemical composition. The water content of the material is a major factor, with fresh resin being less flammable than aged, cured material. As resin ages or cures, the water and lighter terpene components evaporate, leaving behind a concentrated, less hydrated mass of heavier compounds.
The concentration of resin within the wood or bark also determines flammability; species rich in resin, such as pine, are more flammable than low-resin species. The physical form of the resin also plays a role in ignition. Thin layers or threads ignite faster than a large, thick glob because a greater surface area is exposed to heat, allowing for faster terpene vaporization. Ambient temperature also affects the process, as warmer conditions increase the rate at which VOCs vaporize, lowering the energy required for the material to reach its flash point and ignite.