Replacing felled trees with new planting is routinely presented as equivalent. In habitat terms it is not, because the features that make old trees valuable accumulate over long periods.
Cavities take a century or more to form
Tree cavities develop through decay following branch loss, injury or fungal infection, and the process requires a trunk large enough to contain the eventual space.
Many birds and mammals nest exclusively in cavities, and a woodland of young trees offers none regardless of how many stems it contains.
Excavating species such as woodpeckers create the initial holes, but they require decayed heartwood to work into, which again means old trunks.
Deadwood is a habitat in its own right
Standing dead trees and fallen trunks support fungi, invertebrates and the species that feed on them, and this community is largely absent from managed young stands.
The volume and size distribution of deadwood matters, since large-diameter pieces retain moisture and decay slowly, providing stable conditions over decades.
Removing dead and dying trees for tidiness or safety therefore eliminates a significant share of a forest's biodiversity without changing its appearance much.
Bark structure accumulates with age
Old bark is deeply fissured, and the crevices hold moisture, invertebrates and roosting bats in ways that the smooth bark of young trees cannot.
Epiphytes, including lichens and mosses, colonise slowly and some species are found almost exclusively on trunks that have been undisturbed for a very long time.
These communities are also poor dispersers, so even where old trees eventually develop nearby, recolonisation may take far longer than the tree took to grow.
Structural complexity develops through disturbance
Old forests contain gaps, fallen trunks, multiple canopy layers and trees of many ages, all created by storms, disease and individual tree deaths over time.
Even-aged plantings produce a uniform canopy with little vertical structure, which supports far fewer species than the same area of structurally varied woodland.
Some management techniques deliberately create this variation, including thinning irregularly, retaining dead stems and creating artificial cavities, though these accelerate rather than substitute for age.
Carbon and habitat run on different schedules
A young stand can accumulate carbon quickly, which is why planting is attractive as a climate measure and is often counted as compensation for felling.
Habitat value follows a different curve, rising slowly and continuing to rise long after growth rates have levelled off in the same stand.
Treating the two as interchangeable produces the outcome now visible in many places: increasing tree cover alongside continuing decline in woodland specialist species.