Cedar wood is widely recognized for its natural resilience and enduring quality, particularly its ability to withstand decay. This inherent durability makes it a preferred material for outdoor applications like decking and fencing. The resistance of cedar to breakdown stems from specific biological mechanisms and chemical compounds produced within the tree itself. Understanding these natural defenses reveals the science behind cedar’s long-lasting nature in various environments.
Understanding Wood Rot
Wood rot is a biological process of decomposition, initiated by fungi that consume and break down wood fibers. These fungi reproduce through spores, which are abundant in the air and become active when they encounter suitable conditions. Fungal growth requires specific environmental factors: moisture, oxygen, a food source, and a favorable temperature range.
For fungi to thrive and cause decay, the wood’s moisture content needs to be above 20%, with optimal germination often occurring between 27% and 30%. Fungi secrete enzymes that degrade complex molecules within wood, such as cellulose and lignin, dissolving parts of the wood cell walls. Active fungal growth occurs in temperatures ranging from 65 to 95 degrees Fahrenheit (18 to 35 degrees Celsius).
Protective Compounds in Cedar
Cedar’s resistance to rot is primarily attributed to natural chemicals called extractives. These organic compounds are produced by the tree as a defense mechanism against biological threats. The specific composition of these extractives varies by cedar species, contributing to their distinct properties.
Western Red Cedar, for instance, contains potent extractives including thujaplicins, lignans like plicatic acid, and other terpenoids. In Eastern Red Cedar, the main components are sesquiterpenes such as cedrene, cedrol, and thujopsene, which contribute to its characteristic aroma.
How Cedar’s Compounds Deter Rot
The extractives in cedar wood deter rot through fungicidal and insecticidal actions. Thujaplicins, found in Western Red Cedar, are toxic to many decay fungi, inhibiting their growth. Cedarwood oil from some species, containing compounds like cedrene and cedrol, exhibits fungicidal properties. Plicatic acid, an extractive in Western Red Cedar, also contributes antifungal activity.
These compounds also act as natural deterrents to wood-destroying insects. Cedar’s distinctive aroma, due to volatile oils like cedrol and thujone, is unpleasant to insects such as moths, carpet beetles, ants, termites, and mosquitoes. These compounds interfere with an insect’s sensory systems, disrupting their ability to find mates or suitable egg-laying sites. Some cedar compounds also possess anti-feedant properties, discouraging insect larvae from consuming the wood.
The Role of Heartwood
The distribution of protective compounds within a cedar tree is not uniform. These rot-resistant extractives are predominantly concentrated in the heartwood, the dense, non-living inner core of the tree. Heartwood forms as older sapwood cells die and become filled with these natural chemicals, contributing to its darker color and enhanced durability.
In contrast, the sapwood, the lighter-colored, living outer layer, contains fewer protective compounds. Sapwood is less resistant to fungal decay and insect attack than heartwood. The proportion of heartwood within a tree increases with age, so older cedar trees yield wood with higher concentrations of these decay-resistant substances.