Gigantic fog enshrouded trees, a moist and cool refuge, an ecosystem of incredible diversity, America's greatest renewable resource... But hold on a minute, these forests are not as renewable as you might think. In fact, no one has ever restored any logged forest to an old growth condition, and they may not be renewable at all. Climate changes that have occurred and are occurring, together with nutrient depletion and erosion caused by logging operations may in fact make it impossible for subsequent forests to gain "old growth" status. If these forests are indeed renewable, it would take between 500 and 1000 years for them to once again attain the characteristics of an old growth or climax forest. For perspective, the United States has been around for less than 250 years, and we have logged over 94% of our old growth forests in just the last 100 years. That's one fifth the minimum time they may take to regenerate.
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Some companies replant the forest after cutting the old trees. While this makes the landscape look green and might soothe a few consciences, it does little good environmentally. Much like a field of corn, only a single tree specie exists in such a forest. These monocultures lack the biological diversity required for a healthy forest community. They do little more than turn the forest into a green money making machine. Studies show that even seventy years after such a forest is planted, many animal and plant species that inhabited the original forest are still absent. 

Scientists have found that four characteristics define an old growth forest. Only if all four exist in the same forest simultaneously does the forest qualify as "old growth":
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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. Twenty-five species of conifers are found in these forests. As you travel down the coast from Alaska to California, the dominant tree changes from Sitka Spruce, to Douglas fir, to Coast redwood. The height and diameter of the dominant trees also changes with location. Heights range from about 100 feet to 367 feet. Diameters range from about 6 feet to 40 feet. Old growth forests are extremely diverse. Old and young trees grow together in a mixture of species. They are covered by a thick growth of mosses and lichen which 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 up to 31 inches thick.
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2. Large Standing Snags (Standing dead trees)
Dead trees typically stand for more than 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.
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3. Large Down Trees
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. The logs take two to five hundred years to decay. As they do, hundreds of species use them for food and shelter. Down logs are also huge reservoirs of water that are important in times of drought. Water can often be wrung from the rotting wood. 
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4. Large Fallen Trees In Streams
Fallen trees lie in random patterns in headwater streams. These 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. 
Recent studies show that populations of large fish like Coho salmon and cutthroat trout, are directly related to stream pool volume. The Pacific Coast fishing industry has been in a state of crisis recently. The logging of old growth forests is a major reason for the decline of their catch. Fish benefit from insects in the wood, which are an important supply of food, the pools provide shelter from storm run off, and the larger pools stabilize water temperature.