Cedar, particularly species like Western Red Cedar (Thuja plicata), has been a prized building material for centuries due to its longevity. This wood is naturally resistant to decomposition, even in wet environments where other woods quickly decay. Cedar’s remarkable durability is not due to its physical structure, but rather an internal chemical defense system developed by the tree. This natural protection comes from specialized, biologically active compounds stored within the wood. These chemicals turn the timber into an inhospitable environment for the organisms that cause rot and decay.
Understanding Wood Rot and Decay
Wood rot is a biological process where microorganisms, primarily fungi, break down the structural components of wood. These fungi, including brown-rot, white-rot, and soft-rot species, treat the wood as a food source. They secrete enzymes that dismantle the complex polymers that provide wood its strength, such as cellulose, hemicellulose, and lignin.
For this process to occur, fungi require a food source, oxygen, and sufficient moisture. When wood remains consistently wet, its moisture content rises above the fiber saturation point, creating the ideal habitat for fungal spores to germinate. Fungal hyphae penetrate the wood cells, releasing enzymes to consume the cell wall material. Brown rot fungi, for instance, primarily target cellulose, leaving behind a brittle, crumbly brown residue of lignin.
The Chemical Source of Cedar’s Durability
The substances responsible for cedar’s resistance are known as extractives, which are non-structural chemical compounds deposited in the wood. These extractives are metabolic byproducts synthesized by the tree’s living cells. They are not part of the wood’s cellulose or lignin structure but fill the cell lumens and cell walls.
The most potent defense chemicals are the tropolones, specifically the various forms of thujaplicins (alpha, beta, and gamma). These molecules are toxic to the microbes that cause wood decay. Other extractives, such as the lignan plicatic acid, also contribute to the wood’s long-term resistance. These chemicals give cedar its distinct, pleasant aroma and characteristic reddish-brown color, which serves as a visual marker of the defensive compounds.
How Cedar’s Extractives Fight Decay
Thujaplicins and other extractives inhibit decay by interfering with the fundamental biological processes of decay fungi. Their primary mechanism of action is acting as powerful antimicrobial agents. They disrupt the cellular machinery of the invading fungi, effectively poisoning the organisms within the wood.
This disruption involves chelation, where the extractives bind tightly to specific metallic ions. Decay fungi require trace metals, such as iron, copper, and manganese, to power their metabolic enzymes. By chelating these essential ions, the extractives make them unavailable to the fungi, inhibiting their ability to decompose the wood. Furthermore, these compounds act as natural insecticides, deterring wood-boring insects like termites and powderpost beetles.
Heartwood vs. Sapwood: Concentration of Resistance
The protective extractives are not distributed uniformly throughout the cedar tree, which impacts the wood’s durability. The outer layer, known as sapwood, is the living part responsible for transporting water and nutrients. Sapwood contains few protective extractives and is therefore highly susceptible to decay, similar to non-resistant species.
The more durable section is the heartwood, which forms the dense, inactive core of the tree. As the tree grows, the inner rings of sapwood die and transform into heartwood. During this process, the tree deposits the protective extractives, including thujaplicins and plicatic acid, into the cells. This concentration of defense chemicals makes the heartwood highly decay-resistant, while the vulnerable sapwood must be removed for maximum longevity in exposed applications.