A fallen trunk looks like the end of a tree's contribution to a forest. It is closer to the beginning of a second phase that supports a large share of woodland species.
Decay proceeds as a sequence, not a process
Freshly dead wood is colonised first by species that can exploit remaining sugars and moisture, including bark beetles and early-stage fungi.
As those organisms alter the wood's chemistry and structure, they make it available to the next group, which could not have used it in its original state.
The sequence continues for decades in large trunks, so a single piece of deadwood supports a changing community rather than a fixed one.
Fungi do the structural work
Wood is largely cellulose and lignin, and lignin in particular resists breakdown, which is why wood persists rather than decomposing like leaf litter.
Certain fungi produce enzymes capable of degrading lignin, and they are the organisms that make the carbon in wood available to everything else.
Different fungal groups attack different components, producing the distinct forms of decay that determine which invertebrates can subsequently use the wood.
Saproxylic invertebrates are a large and threatened group
Species dependent on dead or decaying wood account for a substantial proportion of forest invertebrate diversity, particularly among beetles.
Many are highly specialised, requiring a particular decay stage, wood diameter, tree species or degree of sun exposure, which makes them sensitive to how forests are managed.
They are consistently over-represented among threatened invertebrates in regions with a long history of intensive forestry, because deadwood was systematically removed.
Vertebrates depend on the same resource indirectly
Woodpeckers feed on the larvae inside decaying wood and excavate nesting cavities in it, and their old holes are then used by many other species.
Bats roost in loose bark and hollows, and small mammals and amphibians shelter beneath fallen trunks where moisture and temperature stay stable.
Removing deadwood therefore reduces populations well beyond the invertebrates that use it directly, through the cavity and shelter supply it generates.
Quantity, size and variety all matter
Large-diameter deadwood is disproportionately valuable because it decays slowly and maintains internal moisture, supporting species that small branches cannot.
Standing dead trees and fallen trunks host different communities, since sun-exposed standing wood dries and warms while ground contact keeps trunks damp.
Retention practices reflect this, specifying standing stems, fallen timber and a range of sizes rather than a single volume target, since a target met with brash achieves little.