Why aren't forests buried in dead wood, and did fungi once fall behind?

Forests are not buried in dead wood because organisms, above all fungi, break down even the toughest part of wood: lignin. Lignins give rigidity to cell walls, especially in wood and bark, and do not rot easily. Lignin makes up 20 to 35 percent of the dry mass of wood, and 30 percent of terrestrial non-fossil organic carbon on Earth. So wood is a big, stubborn meal.

Unlike proteins, DNA and even cellulose, lignin resists degradation. It is cross-linked with the other cell wall components, which limits the access of microbial enzymes to the cellulose and hemicellulose inside. Few organisms can crack it. Bacteria lack most of the enzymes fungi use for it, and lignin derivatives inhibit bacterial growth. Some white rot fungi can degrade the lignin, mostly with secreted peroxidases, though others lack this ability. Even in a compost heap, fungi matter, because they decompose lignin, which many other composting microorganisms cannot.

Now the puzzle that makes this an expert question. The name Carboniferous means coal-bearing, and refers to the many coal beds formed globally during that time. Coal forms when organic matter builds up in waterlogged, oxygen-free swamps and is buried and compressed. Why did so much coal form then? The delayed fungal evolution hypothesis proposes that trees developed lignin before fungi evolved the means to degrade it, so vast amounts of lignin-based material could pile up. Genetic analysis of basidiomycete fungi suggests they evolved in the Permian, which supports this view.

But this is debated. Significant coal deposits formed after lignin-digesting fungi were well established, and fungal degradation of lignin may have evolved by the end of the Devonian. A second theory says the Carboniferous geography and climate alone could account for the peak. Scientists have not settled it.