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Image by Grant Durr

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 years 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 to produce seeds, 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 the Catchacoma Forest, 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

Primary Features

  • Old trees

  • Dead wood (logs and snags)

  • Integrity (absence of or very low human disturbance)

Secondary Features

  • High tree density

  • Multiple vegetation layers: understory, shrubs/saplings, sub-canopy, 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

Carbon Sequestration

Old-growth forests store and fix more carbon than any other terrestrial ecosystem on earth

"Large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree".

Research and Education

Old-growth forests are extremely complex ecosystems that are understudied and under-appreciated, even after decades of research. In Old-Growth Forests: Function, Fate and Value (2009), Dr. Christian Wirth notes that we still haven't located all remaining old-growth forests and we have a lot to learn about the above- and below-ground organisms and processes that make old-growth forests so unique. 


Old-growth forests contain a plethora of plant and animal communities, species, and genes. This makes them resilient and resistant to natural disturbances and invaluable to numerous industries, including medicine, biotechnology and ecotourism. 

Habitat Provision

Many species depend on cavities and crevices in large trees for nesting and roosting, and old-growth forests are a major source of these. Several bird, bat, and owl species need these habitats to carry out their life processes. Others may depend on specific conditions that can be found in old-growth forests. For example, the endangered Pale-bellied Frost Lichen, which grows on trees, requires humid, shady conditions to survive. Similarly, the threatened Cerulean Warbler requires the unique structural characteristics of older forests. There are also some species of insects, invertebrates, fungi, lichens, and bacteria whose survival depends on dead and decaying trees and logs. You'll find plenty of these trees and logs in old-growth forests. 

Wellness & Spirituality

Many studies have documented the health benefits of spending time in forests, including decreased stress and improved mental well-being. Old-growth forests are also valuable for the spiritual and cultural connections they provide. 

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