What time alone builds
A century of continuous woodland is not the same thing as an ancient one. What separates them is accumulation: of dead wood in every stage of decay, of veteran trees with deep-furrowed bark and hollow heartwood, of microhabitats — sap runs, rot holes, fungal brackets, loose bark — that take generations of a single tree's life to develop. These things cannot be transplanted or seeded in. They arrive only when disturbance stays away long enough.
The term "old growth" describes woodland, forest or other habitat that has reached this accumulated state. In temperate Europe, ecologists often look for a cluster of markers: standing deadwood above a certain diameter, fallen trunks in advanced decay, trees old enough to have developed cavities, and a lichen flora that includes species known to spread only a few metres per century. That last point matters. Organisms like the lungwort lichen Lobaria pulmonaria — sensitive to air pollution, dependent on stable humidity, capable of dispersing only over very short distances — will not simply arrive when conditions improve. They require an unbroken chain of suitable habitat stretching back in time. Where that chain was cut, they are absent, and replanting trees does nothing to restore them on any human timescale.
The invertebrate community inside decaying wood tells a similar story. Saproxylic beetles — the several thousand species in Europe that depend on dead and dying timber — include many that need specific fungal decay states in wood of specific species at specific diameters. Some need the work of earlier species before they can colonise at all, making the dead-wood community a succession within a succession. The rarest, in Britain and across lowland Europe, are associated almost exclusively with sites that have been continuously wooded for centuries.
This is why fragmentation is so damaging to old-growth character: not just because patches shrink, but because the interior — shaded, humid, wind-still — contracts faster than the boundary. Old-growth indicators disproportionately occupy that interior. A wood reduced to a narrow strip may retain its tree cover while losing everything that made it ecologically distinctive.
Walking into a genuinely ancient wood, the floor is the first signal: deep leaf litter, multiple stages of log decay, fungi threading through the substrate. The canopy is uneven, gap-broken by fallen veterans. Nothing about it is tidy, and that untidiness is the information.