Educational Series: Old Growth Forests and Their Animal Inhabitants are Disappearing

By Nick Engelfried
When European colonists first arrived in North America, much of the continent was covered in ancient trees hundreds or even thousands of years old. Since the end of the last ice age, forests extended over vast swaths of what are now the United States and Canada, providing habitat for countless animal species and co-existing with the Indigenous human inhabitants. Yet, within a few centuries of colonists’ arrival, most of the original forest cover in the U.S. had been eliminated. Today, less than 5% of ancient forests in the Western United States are left, with that number dropping to below 1% in the Eastern U.S. To preserve a future for the continent’s remaining old growth forests and the species who depend on them, it is essential that we better understand and appreciate these extraordinary ecosystems.

There is no universally agreed on definition of what constitutes an old growth forest, sometimes called “primeval forest” or simply “ancient forest.” Originally, the term referred to forests that had never been logged or otherwise seriously disturbed by human activity, and which therefore existed in a relatively untouched state. Today, it is recognized that forests which were logged at some point in the past can again take on the characteristics of old growth if left to regenerate for long enough. It has therefore become common to define old growth as forest where the oldest trees have attained a certain age, regardless of whether the area was once disturbed by people. However, exactly what age of tree constitutes old growth depends on what forest you are looking at.

Because different forest types go through different natural cycles, it makes sense to accept a lower age threshold for “old” growth in some areas than others. In parts of the Pacific Northwest where trees in pre-colonial times would regularly reach an age of several centuries, the criteria for old growth may be 200 years or more. In regions where natural disturbances are a more regular occurrence, forests might be considered to have attained old growth status by a slightly younger age. However, it is generally recognized that to qualify as old growth, the most ancient trees must be no less than about 120 years old.

Once a forest has become old growth, it serves as essential habitat for a plethora of plants and animals, including some species found only in these types of ecosystems. The large, old trees themselves are the most obvious feature that makes old growth unique, and some animals are drawn here because of the shelter these giants provide. However, big trees are not all that makes old growth special. In fact, most healthy old growth forests are home to trees of all ages, with saplings springing up to take advantage of the light every time an old tree falls and leaves a gap in the canopy. Meanwhile, shade-tolerant tree species like hemlocks thrive in the darker understory. Over time this leads to the creation of a multilevel forest structure, with trees of different sizes providing different types of shelter and habitat for wildlife.

Some people are surprised to learn that dead trees are just as important as live ones in a true old growth forest. Any visitor to the remaining pockets of old growth in the Pacific Northwest will notice the ground is strewn with huge fallen logs and other woody debris, while here and there immense dead trees, or snags, stretch high into the canopy. Decaying logs and branches enrich the soil and provide habitat for animals like the Pacific giant salamander – North America’s largest terrestrial salamander species, which can reach a length of more than one foot from nose tip to tail. Snags, and the insects they attract, serve as homes and a source of food for woodpeckers. Abandoned woodpecker nesting cavities may in turn be used by owls and other birds.

One of the most famous animals dependent on old growth forests is the spotted owl, subspecies of which occur in Mexico, California, and the Pacific Northwest. In general, these birds are found only in large expanses of ancient forest and will avoid clear-cuts or areas with significant disturbance. A dramatic reduction in habitat over almost their entire range has put the species at risk, and today they are listed as threatened under the U.S. Endangered Species Act. Compounding the problem is that remaining spotted owl habitat is increasingly being invaded by barred owls, a larger, related species native to the Eastern United States that has spread westward, perhaps in response to shifting weather patterns caused by climate change. Barred owls compete with and sometimes even hunt spotted owls, and the combination of their presence and continued habitat degradation has driven the smaller species from much of its remaining range.

Another bird that is at least partially reliant on old growth is the marbled murrelet, a small seabird whose nesting habits remained a mystery for many years. In the 1970s, scientists discovered they nest in the canopies of coastal forests, sometimes miles inland. The best marbled murrelet nesting sites tend to be large, old trees whose thick branches make good platforms and tend to be covered in the mosses and other epiphytic plants they use for nesting material. Like spotted owls, marbled murrelets populations declined precipitously as their preferred habitat shrank. Today, they are considered a threatened species by the U.S. federal government and Washington State lists them as endangered.

Other animals whose fates are tied to old growth include the red cockaded woodpecker, which is found in remaining pockets of old forest in the Southeast United States; the marten and fisher, tree-dwelling members of the weasel family that live in the Pacific Northwest; and goshawks, which use old growth trees for their nests. Many other species are not strictly dependent for their survival on old growth, but still benefit from and become more abundant in these forests. For example, surveys have found population densities of common Northwest bat species to be 3-6 times greater in old growth than in nearby forests that suffered from recent disturbance.

Scientists once considered old growth to be a “climax ecosystem” that barely changed once it matured, unless some dramatic and unexpected disturbance occurred. Today, our understanding of these ancient ecosystems is more nuanced; natural disturbances like fire and landslides have always been part of the cycle of life, even in the oldest forests. The main reason old growth is at risk today is not because these forests can’t regenerate after a disturbance. Rather, it is that they are being logged much faster than they can recover. Even timber harvesters who replant after logging seldom allow forests to return to anything like their original, old growth status before clear-cutting again.

The fact is that forests need over a century – sometimes more – to fully recover from disturbance and become old growth, and few loggers are willing to let the landscape remain undisturbed for that long. The only way to preserve at least a vestige of the ancient forest cover that once stretched across much of North America is to strictly protect those stands of ancient trees that still exist. Old growth logging has no place in a sustainable future, and at least some previously logged forests that are approaching old growth status should be allowed to continue regenerating. Only if surviving ancient trees are left standing can we ensure a future for the animal species who rely on old growth to survive and thrive.

Photo credit: U.S. Forest Service

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Nick Engelfried Writes About Animals, the Environment, and Conservation for the ForceChange network

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