What Is the Shelf Growing Out of That Tree? Mostly Something You Cannot See

The shelf projecting from a trunk is a fruiting body, whose function is producing and releasing spores, and it is a small and temporary part of an organism whose main body is a network of fine threads permeating the wood inside. That network may have been growing for years or decades before any bracket appeared, which means the visible structure is a sign of an established infection rather than the beginning of one. The underside carries the spore-producing surface, arranged as pores in most bracket species rather than as gills, and a single bracket releases spores in quantities measured in billions. Some brackets are annual and decay after a season, while others are perennial and add a new layer each year, which can be counted like growth rings.

Species differ in which components of wood they can break down, and the difference is visible:

• •White rot fungi break down lignin as well as cellulose, leaving pale fibrous stringy wood
• •Brown rot fungi break down cellulose and leave lignin, leaving dark wood cracked into cubes
• •Soft rot affects the surface layers under wet conditions
• •Some species attack only dead wood and are purely decomposers
• •Others attack living heartwood, which is already dead tissue within a living tree
• •A few attack living sapwood and kill the tree directly

Wood-decaying fungi perform a function almost nothing else can. Lignin is a complex and resistant molecule that gives wood its rigidity, and very few organisms can break it down, with white rot fungi being the principal ones, so without them woody material would accumulate rather than being returned to circulation. The evolutionary appearance of that capability is argued to explain why the enormous coal deposits of the Carboniferous formed and why comparable deposits are rarer afterwards, though the timing is disputed. Hollowing of living trees by heart rot creates cavities that a large number of birds, bats and invertebrates depend on for nesting and roosting, so a fungus that a forester regards as damage is a resource that much of the woodland fauna requires.

Several bracket species have practical histories worth knowing. One found on birch was carried by the preserved body recovered from an Alpine glacier, and species in that group have documented medicinal use and a structure suited to sharpening blades. Another, known as tinder fungus, was processed into a material that catches a spark readily and was a standard fire-lighting item across Europe for millennia, with the same preserved body carrying that too. Several are used in dyeing, producing strong colours from the flesh. A few are edible when young, including one with the texture and appearance that gives it a name referring to poultry. And the pharmaceutical interest in wood-decaying fungi is substantial, since organisms that break down resistant molecules produce unusual enzymes and compounds.

Identifying the species is a diagnostic skill with practical consequences. Different fungi attack different parts of a tree, so one species indicates decay in the roots and butt while another indicates it in the upper trunk, which determines what kind of failure is likely and therefore what the risk is. The rot type matters, since brown rot leaves wood that fails suddenly and brittlely while white rot leaves wood that bends before breaking, which changes how a tree is assessed. Position on the trunk indicates the extent. The presence of a bracket does not by itself mean a tree must come down, since many trees live for decades with substantial decay and compensate by growing additional wood where it is needed, and assessment weighs the decay against the target beneath.

The shelf is a temporary fruiting body of an organism whose main body has permeated the wood for years, so its appearance signals an established infection. White rot leaves pale stringy wood and brown rot leaves dark cubical wood that fails suddenly. Very few organisms break down lignin, and the hollows these fungi create are what much of the woodland fauna nests in.