Wood is an incredibly durable and complex organic structure, making it one of the most challenging materials in the natural world to consume and digest. The difficulty stems from the tree’s cell walls, which are primarily composed of two polymers: cellulose and lignin. Cellulose, a long chain of glucose molecules, is tightly bound within a matrix of lignin, a highly cross-linked aromatic polymer that lends wood its rigidity and resistance to decay. Because most animals lack the necessary enzymes to break these complex molecules apart, a specialized biological strategy is required to access the stored energy within woody tissue.
Large Herbivores That Consume Trees
Large vertebrate herbivores, especially mammals, have developed feeding strategies to exploit the structural components of trees. North American beavers consume the cambium and inner bark, or phloem, which are nutrient-rich layers situated directly beneath the tough outer bark. Their powerful incisors allow them to fell trees and access this food source, particularly during winter when other forage is scarce.
Moose and deer rely on “browse” during cold seasons, feeding on the twigs, buds, and young branches. An adult moose may consume 30 to 45 pounds of woody material daily to sustain itself through the winter months. Elephants also strip bark and break branches, incorporating this fibrous material into their massive daily diet.
Smaller herbivores, like the North American porcupine, primarily eat the inner bark and terminal twigs during winter. This inner bark is a concentrated source of carbohydrates and minerals. These animals selectively consume material lower in indigestible lignin content.
Wood-Boring Insects and Larvae
Invertebrates, particularly the larval stages of various insects, represent the most pervasive and ecologically impactful consumers of wood. Termites are the most famous example, consuming vast quantities of woody debris and playing a substantial role in global carbon cycling. They mechanically process the wood, relying on a symbiotic relationship with internal microorganisms for digestion.
The larvae of wood-boring beetles, often called woodworms, cause significant structural damage as they tunnel through wood for years. This group includes metallic flat-headed borers and longhorned beetles, which bore into living, stressed, or dead trees, initiating decomposition. Bark beetles typically feed and reproduce in the phloem layer just under the bark, often killing the tree.
Caterpillars of certain moths and larvae of wood wasps also specialize in consuming wood. These invertebrates target trees weakened by drought or disease. They contribute to nutrient recycling by turning solid wood into fine sawdust-like waste, or frass.
Fungi and Microbes: The Decomposers
Fungi and specialized microbes possess the chemical tools required to break down the highly recalcitrant lignin polymer. The two main types of wood-decay fungi, white rot and brown rot, employ fundamentally different strategies. White rot fungi are the only organisms known to completely degrade all components of wood. They use powerful extracellular enzymes, such as laccases and peroxidases, to break down lignin. This comprehensive degradation leaves the wood a pale, soft, and stringy mass of residual cellulose.
Conversely, brown rot fungi primarily target the cellulose and hemicellulose components. They use a non-enzymatic process involving the generation of highly reactive hydroxyl radicals to modify and fragment the lignin structure. The resulting wood is brown and crumbly because the tough lignin matrix is left behind, chemically altered but relatively untouched.
Other specialized microbes, including certain species of anaerobic bacteria and fungi, contribute to decomposition in oxygen-deprived environments. These organisms are found in places like the guts of herbivores and deep in submerged wood. They are adapted to operate without oxygen and utilize different biochemical pathways to access complex carbohydrates. Their combined action is responsible for the slow, steady return of carbon and nutrients from woody biomass back into the ecosystem.
The Biological Challenge of Cellulose Digestion
Symbiotic Digestion
The ability to digest wood hinges on breaking the \(beta\)-1,4-glycosidic bonds in cellulose, a reaction requiring the enzyme cellulase. Since animals do not produce this enzyme themselves, they rely on a symbiotic partnership with microorganisms. This mechanism involves housing a specialized community of bacteria, fungi, or protozoa in a modified chamber of the digestive tract, allowing the host to access stored energy.
Mammalian Fermentation
In ruminant mammals like deer and moose, fermentation occurs in the multi-chambered stomach, particularly the rumen, which acts as a massive microbial vat. These microbes produce cellulase, breaking down cellulose into volatile fatty acids that the host animal absorbs for energy. Similarly, non-ruminants like porcupines and rabbits utilize an enlarged pouch called the cecum, housing a dense population of symbiotic microbes for hindgut fermentation.
Insect Strategies
Insects like termites rely on a similar strategy, hosting flagellated protozoa and bacteria in their hindgut to perform cellulose breakdown. Some lower termites must regularly acquire these symbionts through social behavior, such as feeding on the feces of their nestmates. The efficiency of nutrient extraction is maximized by some animals, like rabbits and beavers, through coprophagy—re-ingesting their feces to pass the material through the microbial-rich gut a second time.