Dean M. Chriss

Old Growth Forest Ecosystems
A Nonrenewable Resource
October 3, 2020

Old Growth Forest and Devil's Club, Landscape Photograph
Old Growth Forest and Devil's Club

We always hear that our forests are a renewable resource. Like a crop of corn or soy beans, we cut them down, replant them, and continue the cycle over and over again. It makes perfect sense if you don't bother to think about it.

Old growth forests are incredibly diverse and consist of a lot more than trees. More than 5028 species of invertebrates, 407 species of mammals and birds, and 1000 plants including mosses, lichens, and fungi live in old growth forests and most of them can live nowhere else. Looking at exactly what an old growth forest ecosystem is, and where these things live in the forest, explains why.

An old growth forest ecosystem is one that simultaneously meets these four criteria:

1. Large living trees
In the Pacific Northwest, old growth forests are dominated by large conifers that range from 250 years to well over 1000 years old. Heights of the mature trees in the forests that have not yet been logged range from about 100 feet to 367 feet, but used to range from about 100 feet to about 415 feet. Diameters of these trees range from about 6 feet to 40 feet. Old and young trees grow together in a diverse mixture of species. They are covered by a thick growth of mosses and lichen that provides a home for many insects, birds and small mammals. These trees usually survive fires because they are reservoirs holding thousands of gallons of water protected by a thick insulating bark. Redwood bark, for instance, can reach 12 inches thick and sequoia bark can be 31 inches thick.

When forests are logged only one specie of commercially valuable trees is replanted, if replanting is done at all. Like a corn crop, this is known as a monoculture, meaning that just one specie exists. These monocultures are biologically sterile because they lack the diversity to support most species that lived in the original forest. We know that 70 years after a forest is logged and replanted most of its original inhabiting species are still absent. This is true even when those species still exist in nearby forests that were not logged. 1700 different species depend upon the crowns of mature living trees in old growth forests. See the table at the end of this article for a breakdown species utilizing different parts of an old growth forest ecosystem.

2. Large Standing Snags (Standing dead trees)
Dead trees typically stand for well over 200 years. As their branches slough off, sunlight reaches the forest floor allowing other plants to germinate. Insects and woodpeckers open up the dead wood providing habitat for many other species. These in turn become food for larger predators like the northern spotted owl, the marten, and black bear. 629 different species depend upon the these standing dead trees for survival. After a forest is logged these standing dead trees do not exist, and they cannot exist until a new forest grows and matures enough for such large trees to die. That takes hundreds of years, and don't forget that we are still dealing with a biologically sterile monoculture.

3. Large Down Trees
In an old growth forest ecosystem roughly fifty tons per acre of dead logs crisscross the forest floor, helping to hold the loose soils in place. They also serve as nurse logs, providing a place for new trees to take root. If you are in an old growth forest and take the time to notice, you will see that many sets of trees are growing like fence posts in a straight line, as if they were planted. That is because many of the trees took root on top of a fallen tree after it rotted. These fallen trees take two to five hundred years to decay and release tons of nutrients back into the soil for new trees to use. As they do this hundreds of other species use them for food and shelter. Down logs are also huge reservoirs of water that are important in times of drought, when water can often be wrung from the rotting wood. Remember that these trees can be 10, 20, and even 40 feet in diameter, so it's a deep and well insulated reservoir. 4106 different species depend on these large fallen trees for their existence, making them the most biologically valuable resource in an old growth forest.

4. Large Fallen Trees In Streams
This criteria affects species that live far beyond the forest. Fallen trees lie in random patterns in headwater streams. These enormous logs form pools that hold woody debris long enough for 70 percent of it to be processed as food and shelter by insects and bacteria. Studies demonstrate that populations of large fish like Coho salmon and cutthroat trout are directly related to stream pool volume. Larger volume means more fish. Logging of old growth forests is a major reason for a severe decline of several fish species in the Pacific ocean. Young fish benefit from insects in the wood, which are an important supply of food. The pools provide shelter from violent storm run off, and the larger pools stabilize water temperature. All of these factors affect the number and health of fish. This again shows that fallen dead wood is the most biologically valuable resource in an old growth forest. It should be noted that logging causes large amounts of erosion, fills streams with silt, and enormously increases peak storm runoff volume. These factors often eliminate fish from a stream.

When one thinks about the four criteria above it becomes obvious that old growth forest ecosystems are not renewable. The thousands of species that rely on old growth forests disappear as soon as the forest is logged. They can't wait 500 to 1000 years for a new forest to grow even if that was possible. For perspective, the United States has existed for less than 250 years and has logged over 93% of its old growth forests in only the last 120 years. Outrageously the last 7% is still being logged. When it is gone there is no more, ever.


Different species depend on each part of an old growth forest ecosystem
Mature Tree Crown Standing Dead Tree Fallen Logs
Invertebrates 1500 300 3228
Mammals and Birds 100 129 178
Plants including
mosses, lichens, fungi
100 200 700