We know the drill. Cities are gray pollution farms and forests are verdant planetary saviors. We also know it’s more complicated than that. Here’s an interesting way a seemingly “green” forest can be a source of pollution.

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Approximately 30 percent of forested land in Japan consists of plantations. Some of them are new, well-managed, and environmentally sound. Others are old, and have fallen out of use. The older plantations tend to be cedar and cypress farms, which were first planted over forty years ago, when demand for these kinds of wood were high. It has dropped since then, and the farms aren’t being managed anymore, but the trees are still around.

Like animals, plants have a youth during which they need certain kinds of nutrition. Young trees and plants take up a great deal of nitrogen. One of the major problems for farmers, before commercial fertilizers and crop rotation, was the fact that new-planted crops would suck all the nitrogen out of fields. Each successive generation of crops would leave the soil more and more impoverished until harvests failed and people starved. Growing plants need nitrogen.

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Old plants shed more nitrogen than they use. Every time the cypress trees in these old forests shed their needles, they drop NO3 on the ground. In normal forests, these needles would be broken down by bacteria, mix back into the soil, and provide nutrients for newly growing plants. The commercial tree plantations don’t have young growth. At first, new trees and competing plants were cleared out, and the canopies of older trees make it less likely that new growth will come in. What these plantations have, then, is a build-up of nitrogen in the topsoil, which, when it rains, gets washed into local streams.

Nitrogen pollution in water starts a whole new chain of events. Because nitrogen is such a vital nutrient, having an excess of it in the water supply causes a rapid increase in the algae population, which then depletes the water of oxygen. This can literally suffocate the fish living in the water. Nitrogen pollution and algal bloom are common problems in waterways near commercial farms, making the forests, in this respect, no better for the environment than big farms.

The point of this is not to depress people. It just lets us know that just because something looks both green and pristine doesn’t mean it’s an unqualified environmental boon. And just because something is, temporarily, a problem doesn’t mean it has to remain so. A little undergrowth management and these forests can have complete nitrogen cycles again.

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[Source: A Nitrogen-Saturated Plantation is a Large Non-Point Nitrogen Source]

Image: Laitche