What is old growth?

A forest in the Cascade Range in northern Washington. Fire is rare or absent in this landscape. Shade-tolerant trees regenerate in small gaps created by wind and disease. In the absence of catastrophic fire or windstorm, small patches of old, young, and maturing trees create a nearly continuous old-growth forest. (Source: Tom Iraci, USDA Forest Service)

What image comes to mind when you hear the term “old growth”? To many it describes a forest that has grown for centuries without human disturbance and now is a stand of massive, towering trees with jumbles of large decaying tree trunks; deep shade pierced by shafts of sunlight; and dense patches of herbs, shrubs, and saplings that may conceal rare species. Such a forest is as awe-inspiring as it is biologically rich. It may contain the largest trees, the oldest trees, the most at-risk forest species, and the largest accumulation of carbon per acre of any forest type on earth. Given this picture, it’s not difficult to understand why old-growth forests have become charismatic ecosystems in the debate about forest biodiversity.

This image accurately depicts many old-growth forests, but it doesn’t fit all or even most of them. The basic definition of old growth is simply a for­est that is dominated by big, old trees, both live and dead, standing and fallen, and that usually contains many other smaller trees. The individual trees are irregularly distributed over the land, and their diverse sizes give rise to a layered appearance. Most true old-growth forests give an overwhelm­ing impression of diversity instead of uniformity.

A stand of old-growth longleaf pine in southwestern Georgia, with standing dead trees, regenerating saplings and wiregrass ground cover. Frequent low-intensity fire is a key element in maintaining this longleaf pine/wiregrass ecosystem. (Source: Jones Research Center)

Policymakers and forest managers are struggling to respond to various definitions of these complex ecosystems. What is certain is that older forests are reservoirs of species that are often rare or absent in younger forests and that they themselves are an important element in the biological legacies and continuity of our terrestrial environment.

If this crucial element of biological diversity is to be preserved, old-growth policy and manage­ment must be based on sound ecological concepts and principles, so that intelligent decisions can be made about where to focus our limited resources.