What is an Old-Growth Forest?
The old-growth forest condition is not an end point, but a continuum defined by slow ecological change initiated when forest tree growth begins to slow down (e.g., between 120 and 150 yrs. for eastern white pine). This change in growth happens when a tree starts to allocate more energy to maintaining its increasingly large size as opposed to using that energy to produce new growth. This point in time is known as the “age of onset”. Thus, as a tree ages, energy allocation to maintenance increases and allocation to producing new growth decreases.
Old trees beyond the age of onset are the most important feature of an old-growth forest, however, snags (dead standing trees), logs, and integrity (no or little human disturbance) are also important primary characteristics of old-growth forest. Some secondary and more variable features of old growth are included in the table below. Ancient forests have all or many of the same features as old-growth forests except that there is no minimum tree age and they usually cover larger areas - they are also called pristine and virgin forests.
Most old-growth characteristics continue to develop for centuries in the absence of catastrophic and/or human disturbance. However, trees do eventually die from old age, pathogens, severe wind storms, and fires. As long as some of the older trees remain after a natural disturbance, other younger trees will colonize where older trees died.
Some old-growth forests support rare, threatened and endangered plants, animals and other biodiversity (fungi, lichens, etc.). In most parts of the world, including Ontario’s temperate forests, old-growth is very rare, yet in Ontario as in most of the world, there is no legislative mandate to protect them.
There are some excellent examples of old-growth forests in and near the City of Peterborough including Mark S. Burnham Provincial Park, Jackson Park, Stewart’s Woods, and the Kawartha Land Trust’s Jeffrey-Cowan Forest Preserve. On your next visit to one of these forests, be sure to look for the features of old-growth forests, but also remember that old-growth forests can be very different from each other even within a particular forest type.
Characteristics Associated with Old-growth Forests in Ontario
(red = primary features, black = secondary features)
dead wood - logs and snags
integrity (absence of or very low human disturbance)
high tree density
multiple vegetation layers: understory, shrubs/saplings, canopy, super-canopy
high diversity in the herbaceous layer
lichen and fungus abundance and diversity
openings in the forest canopy
undisturbed soil layers
Old-Growth Forest Values
“[Old-growth forests] are living laboratories of adaptive evolution, and invaluable gene repositories. They can be used to test hypotheses about complexity, stability, resilience, and ecosystem change. ...Small incremental investments in collecting and managing old-growth data will pay manifold dividends to future generations of researchers, managers, and policy makers.”