Old Growth Forests: The Ancient Ecosystems We Are Still Learning to Understand

An old-growth forest is not simply a forest with old trees. It is a fundamentally different ecosystem from a young or managed forest — characterised by structural complexity, biological diversity, carbon density, and ecological processes that develop over centuries and cannot be replicated by any amount of tree planting or forest management. Old-growth forests contain large trees — often with diameters measured in metres and heights measured in tens of metres — alongside large quantities of standing dead wood (snags) and fallen logs (coarse woody debris) that provide habitat for hundreds of species. They have multi-layered canopies, diverse understorey communities, complex soil ecosystems, and mycorrhizal networks of extraordinary sophistication.

What Makes a Forest Old Growth

The definition of old-growth forest varies by forest type and region, but common elements include: trees that are old relative to their species' maximum lifespan (typically several centuries or more); structural complexity including multiple canopy layers and a diversity of tree sizes; significant quantities of dead wood in various stages of decay; evidence of natural disturbance dynamics over centuries; and forest floor communities adapted to the stable, shaded, humid conditions that old forests create. In temperate forests, old-growth is generally considered to begin at 150-200 years for most species — though the full expression of old-growth characteristics may take 300-500 years to develop.

Dead Wood — The Overlooked Habitat

One of the most distinctive features of old-growth forest is the abundance of dead wood — standing dead trees (snags) and fallen logs in various stages of decomposition. In managed forests, dead wood is typically removed for timber or to reduce fire risk. In old-growth forests, dead wood can constitute 20-30% of the total above-ground biomass. This dead wood provides irreplaceable habitat: over 20% of all forest biodiversity is estimated to depend on dead and decaying wood. Cavity-nesting birds, bats, small mammals, hundreds of invertebrate species, thousands of fungal species, and hundreds of plant species all require dead wood of specific sizes and decay stages that only old, undisturbed forests provide.

Old-Growth Structure — What Makes an Ancient Forest

Old-growth forest — also called primary forest, ancient woodland, or virgin forest depending on context and region — is not simply old trees: it is a complex structural condition that develops only over centuries and involves the interaction of multiple biological and physical processes. Characteristic structural features include large-diameter trees of multiple species and ages, including individuals several hundred to several thousand years old; abundant standing dead wood (snags) in all stages of decay, from recently dead to the barely-standing; large fallen logs in various decay stages on the forest floor; canopy gaps created by individual tree deaths that create light patches exploited by shade-intolerant species; and a complex understorey of shrubs, herbs, and tree seedlings adapted to the deep shade conditions beneath a closed canopy. These structural features collectively create an extraordinary diversity of microhabitats — cavities, surfaces with specific moisture and temperature regimes, substrates of specific decay stages — that support ecological specialists found nowhere else.

The recovery of structural complexity in logged forests is extraordinarily slow, and the simplistic view that "new trees grow back quickly" profoundly underestimates this reality. Tree height and basal area can recover to old-growth standards within 50-100 years in productive forests, but large-diameter trees (>100cm) take 200-500+ years to develop; the abundance of large snags typical of old-growth takes 100-200 years to recover after a clear-cut; fallen log volumes take 150-300 years; and the full complement of old-growth-dependent species — particularly cavity-nesting birds, saproxylic beetles, and many lichens and fungi — may take 500 or more years to recover in logged stands, if they ever do in fragmented landscapes where source populations for recolonisation are absent.

Old-Growth Forests and the Carbon Cycle

Old-growth forests — stands of trees that have never been logged or severely disturbed, often containing trees hundreds to thousands of years old — are ecologically irreplaceable and increasingly recognised as climate-critical assets. For decades, forestry management assumed that old forests were carbon-neutral or even carbon sources — their respiration as high as their photosynthesis — making them "inefficient" carbon stores that could be more productive (in carbon terms) if harvested and replaced with faster-growing young plantations. Long-term direct measurements of CO₂ exchange between old-growth forests and the atmosphere, using eddy covariance towers that measure the net flux of CO₂ between the forest and the air, have overturned this assumption: many old-growth forests continue to accumulate carbon for centuries, primarily in the slow build-up of soil organic matter, coarse woody debris, and the biomass of very large trees, whose volume increases disproportionately with diameter. A single old-growth Douglas fir or redwood adds more biomass per year in its third century than in its first century, because its volume increases with the square of its radius while circumference (where growth occurs) increases linearly.