This strange black network, that grew under the bark of this pine tree when it was alive, identifies the fungus that killed it - honey fungus Armillaria mellea.
Honey fungus is perhaps the most notorious of all fungal tree killers, not just of forest trees but also specimen trees of many species in gardens and arboreta.
The fungus kills the host by producing this network of rhizomorphs - bundles of hyphae that look like boot laces. They digest the living tissues between the bark and the water-conducting xylem that forms the woody core of the tree. The victim in this photo is a beech tree.
The rhizomorphs are doing their deadly work well before the fungal fruiting bodies appear. Often the first symptoms are individual branches of the tree dying, but it can take years to completely kill the tree. Often the roots are killed and then it topples in a gale. Meanwhile the rhizomorphs also grow downwards over the roots and out through the soil, until they find roots of a neighbouring tree that they can invade.