The Truth About Biodiversity and Old-Growth Forests

Many environmentalist groups are totally against any form of timber harvests in the US, ignoring the fact that an acre and a half of trees are planted for every acre that is harvested. If they are pressed about this point, they often claim that they want to protect old-growth forests in order to increase biodiversity. What is the truth about the biodiversity of old-growth forests, though?

First, it is important to understand that an old-growth forest is just another phrase for a mature forest. Biodiversity is a term that describes the number of species present. Would it surprise you to learn that biodiversity actually goes way down as a forest matures?

This isn’t hard to understand. Forest succession is the process from no forest to fully mature forests. When there is no forest, sunlight strikes the ground abundantly, so there are a huge number of plant species, including many kinds of grasses, wildflowers, and so forth. There are also a large number of animal species, due to the large number of plant species.

Bushes begin to grow. These shade out some species, but there is still a lot of biodiversity because there are many species of bushes. 

The first trees begin to grow. These are mostly fast-growing species like cottonwoods, alders, aspens, and poplars. These rapidly begin to lower the biodiversity because they shade more of the forest bottom. Plants that require full sunshine begin to disappear.

Fast-growing trees give way to slower-growing trees like firs, pines, spruces, and hemlocks. These often take 30-65 years to reach maturity. Many of these produce a great deal of shade on the forest floor, so even plants that can tolerate partial shade begin to die out. These trees also change the soil, restricting the biodiversity even more. 

In an old-growth forest, most species of plants are gone. The animals that ate those plants, and the animals that ate them, are also absent. Although many people don’t even think about it, trees also have lifespans. In an old-growth forest, even the slower-growing trees begin to die. Biodiversity is nearly absent and there are normally only one or two dozen plant species living in the forest. Without the food, there are also fewer animals, except those that are just moving through.

Eventually, the forest dies and the whole cycle starts over. 

The amount of time that passes between a mature, low biodiverse forest and the beginning of a new forest with high biodiversity can often be measured in centuries.

What happens in high-yield forestry, though, is that mature trees are removed and used. In the process, more of the forest is opened up to allow an increase in biodiversity because of increased sunlight and resources. The truth is that a managed forest that includes forest harvesting and replanting is far more biodiverse than an old-growth forest. There can easily be 100 times more plant and animal species in a managed forest than in an old-growth forest.

None of this is rocket science and most people can understand it with only a little thought and a basic understanding of plants. If someone tries to tell you that an old-growth forest is more biodiverse, don’t fall for it. That is a falsification that is nearly the opposite of the truth. The key to biodiversity is actually responsible forest management that includes forest harvesting and replanting.

Notice the difference between the number of plants evident in the new forest and in the old forest, in the picture at the top of this article.


What do you think?


Written by Rex Trulove

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    • We’ve had the same thing happening here. It isn’t rocket science. When they wait until the forest dries out before doing a controlled burn and only check on the burn once every day or two, they are begging for the fire to get out of hand.

      • Balance takes effort and knowledge, but the USFS used to achieve it. It can be done. It takes substantial planning, a lot of tree planting, and finding the middle ground between people who want to log every tree standing and people who don’t want to log any tree.

      • See, that is the problem, Ghostwriter, not everything needs to be managed all the time. Some things just need to breathe. Of course we can err in the other direction as well. But I like Rex’s concept of a middle ground.

    • That was formerly the goal of the US Forest Service, until the 1970’s and 1980’s. An area was selectively logged and replanted one year. The next year, another area was. This continued until they got back around to the original area and the planted trees were mature, with more new growth from the new trees that were seeded from the mature trees they left behind as seed trees.

      Unfortunately, they got away from that when they started caving in to lobbyists who frankly didn’t know their facts.


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