At the Roots of the Rainforest

 

When you took your virtual field trip, you probably thought about plants, animals…and perhaps even fungi or protists. But did you imagine reaching down for a handful of soil? Down there you can find important clues to “what’s up.”

 

Ecologists get the dirt on how an ecosystem functions by looking at the soil. On your virtual field trip, if you were in Georgia, the soil was probably bright red; in Florida, sand and limestone were from the old Appalachians. In the prairies, it would have been light and clumped. In that Illinois forest, the soil would have been black and acidic. And in the rainforest, it was likely to have been reddish clay, and surprisingly thin. You might have seen the scars of vehicles and human use on the shore, and certainly noticed the eroding clay from the Andes in the river. In this environment, roots of trees must be shallow, with trunks often beginning to branch above the surface to provide extra support in the thin humus layer when there is too much moisture.

 

Soil is a complex mixture of bits of rock, mineral nutrients, decaying material, fungi and decomposers. In many ways, it’s an ecosystem in itself. Slight variations in the chemical or physical composition of soil can create major changes in the way organisms survive and adapt in an ecosystem. Pure sand or finely broken minerals might look like soil from a distance, but without a rich, organic content these wouldn’t qualify as “soil.” Only decomposers can take bits of organisms and break them down to chemical forms that can be re-used by other organisms. And almost all decomposers respire like animals—needing oxygen to release nitrogen, potassium and other minerals for future use.  Remember, minerals must cycle through ecosystems and be used over and over again.

 

The litter in a rainforest is thick with the remains of all the organisms that have lived and died above. It’s wet—because it rains every day-- so there is far less oxygen available than you might think. The remains of organisms don’t break down well; some “mummify” because there’s too little oxygen for effective decomposition. That’s why rainforest soils are often acidic and poor in free nutrients that could be easily used by living things. (They are a lot like bogs where some of the plants resort to eating insects to get their mineral doses.) When a rainforest is cleared and/or burned, crops grow poorly. The precious humus of the rainforest often sits on a relatively impermeable layer of clay. (You'll learn Lessons from Latrines in Week 3). That means that the tread marks of a logging machine or truck can cut through virtually all of the useful soil, sending it into the river and away forever.

 

 

Fragile and Failing

 

Soils are often divided into three horizons. Horizon A, the top, is the root zone. It often contains partially decayed plant and animal tissue called humus. Only that top few inches of rainforest soil are actually usable by plants. Where there is enough oxygen, the intense biological activity in the “brown food web” at the forest floor can decompose biomass in an amazingly short time. Horizon B is subsoil, with weathered mineral matter that has been leached deeper by rains. Horizon C lies directly over bedrock, and is poor in free minerals.

In most rainforest soil, horizon A is thin and infertile. In the best rainforest soil, biological activity extends down to about 30 cm. By contrast, the biological activity in the soil of a prairie farm might extend 50 cm (20 inches) or more. Beneath that is the impermeable clay--almost like a layer of visqueen. When that rainforest is cleared, the hot tropical sun quickly destroys the ability of decomposers to do their job.

 

Clear a rainforest, and you’ll likely get land that’s only suitable for the most basic grazing land. In an area like the Amazon basin, only around 4% of land is suitable for agriculture of any type. (In Week 3 we'll talk about some exceptions--Mesoamericans developed great techniques for bioengineering areas of much better soil.) Yet some 75% of the land today is so poor that cultivators are unlikely to get more than a single crop before it is completely exhausted. Yet two factors keep up the farming pressure on rainforests:

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Th There is a commercial advantage to cheap grazing land for beef. (Much of the fast food beef that Americans eat comes from cleared rainforest land.) In many tropical countries, the land use laws don’t allow poor or indigenous peoples to will their land to their heirs. These people, often called “shifted cultivators,” move into land they can only use for a short time. Since they can’t get full title to the lands they’ve farmed for generations, the farmers have little impetus to conserve for the children who can’t inherit the land. This is being changed in Peru. Watch for signs of new land ownership as you travel along the Amazon. You'll learn more in Week 3. In this NASA photo of Bolivia, there’s a key to the year that an area was cleared. The color code shows photosynthesis now—the brighter is more productive. Note that the longer the soil has been used, the less fertile it is. (The grey area has not been color coded because it is wild.)

Photo credit: NASA

 

 

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