Worlds apart: the soils and souls of temperate and tropical forests

The difference between tropical and temperate forests is vast. And crucial.  I’m not talking about the number of trees or the number of species of trees or the height of trees or any of that.  I’m talking about how nutrients cycle through a forest, how the forest eats and grows and dies and rots and grows and eats and grows and dies and rots and rots and rots.

Temperate Forests

temperate forests hold carbon in the soils

The large roots from that tree are a good 10 feet long (+3m), the feeder roots would be longer.

In a temperate forest, where I live and have lived my whole life, the trees take up the nutrients, including the carbon.  Folks extol the carbon sequestration capacities of a tree, but although the tree picks carbon up and forms it into wood, it’s not the wood itself that is valuable to the temperate forests. The two vital bits are that the tree forms the carbon into roots and that a tree rots very slowly.

The roots stay in the ground.  Yes, the root ball itself will tip up with the tree when it falls, snapping off somewhere in the zone of rapid taper (where transport roots turn into buttress roots), but the vast majority of the root system stays in the soil and rots to form soil carbon.  The soil is the amazing carbon sink, not the tree.

The trunk itself works more like a second chance for the carbon to get into the soil: the trunk will rot slowly enough that insects will chew up and poop out the wood pulp. Mosses and fungi and the sheer physical exertion of swelling up with rainwater and then drying back out (or not, depending on the local climate) will grind the carbon of the wood into a young but rich soil.

Seeds will land in this coarse wealth and begin to grow, taking up the carbon into a new plant, with new roots.  When that plant dies, the roots will return the carbon to the soil, the trunks and stems will create a new medium for new seeds to land on and start growing in.  In a temperate forest, half the forest is the soil.  Not “in the soil”, “the soil”.  Half of what makes the whole thing tick is the constant tug of nutrition downward, being banked for safe keeping in the soil.

Tropical Forests

dry tropical forest in Africa

(the Sahel region in Africa by Michelle Kovacevic, CIFOR)

In a tropical forest, there is no banking, just thieves. The heat of the sun causes carbon and other nutrients to ‘burn off’ into the air.  With any luck, a coating of plants or debris or mulch or whatever else will block the amazing reach of the sun’s rays, but in a dry and dusty place, this is not a guarantee. The dry penumbras of the deserts have it rough.

Rain would help, one thinks, surely the rainforests are in better shape- just look at them!

tropical rain forests have thin soils

This huge beautiful chunk of Brazil rests on the thinnest imaginable bed of nutritious soil.

But a rainforest has another thief: rain.  The rain pounds or drips, showers or leaks…  Once water is there, it leaches the nutrients deeper into the soils.  By the time a forest is getting 75 or 120 inches (2 or 3 meters!) of rain each year, the result is the same: what the sun doesn’t burn off, the rain sucks down deep.

So what’s a tropical forest to do?  They grow up.  There is no banking.  If you want the carbon sloughing off that decaying tree over there, you’d best get on it, stat.  Nothing sleeps, nothing waits a few months for spring.  Everything eats voraciously, converts the meals into material, stores all the wealth tucked up in their carboniferous petticoats, and eats some more.

Burn down a temperate forest and it’s a tragedy- there was so much invested in there that will take so long to regain, and some delicate species may as-good-as-never come back.  The soil will be exposed to the sun, vulnerable to erosion and gullies, but if a steward acts fast, and wisely, the centuries of investment that have built that soil will be there to jump start the next iteration.

Burn down a tropical forest, and there’s nothing to come back.  The entire wealth of the tropical forest was horded in those logs that burned.  There is the barest of economies occurring in the top few inches of soil, but those roots often don’t go very deep- it’s such an intricate web of life there that a tree can count on its neighbor to help it stay up.  Exposed to the sun and the rain, the nutrition in the soil will be gone soon too.  Cut the forest instead of burning it, and now whatever boost the ash would have given is gone too.

It’s not the trees that make the temperate and tropical forests so different, it’s the soils.  One of the biggest flaws in most of the 20th century approaches to helping advance tropical agriculture was a complete failure to grasp this vital difference.  You can’t just transplant what works well in Kansas over to Nigeria.  Temperate and tropical ecological health are predicated on different functions.

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2 responses to “Worlds apart: the soils and souls of temperate and tropical forests

  1. Your description of tropical forests is simplistic and does not meet reality. There are some tropical soils that act as you describe, but, there are also millions of acres of tropical forests that are similar to temperate forests with well-developed O, A and B horizons.

    • And there are some temperate forests on very very thin soils- they go from O to A to bedrock. Nothing is everywhere, but there’s a lot of supposition that the ideas and techniques that work in one place will work in another, especially when it comes to the world powers deciding how they want to go about helping poorer countries. My point was that what works great in Missouri can’t necessarily be airlifted to the Ivory Coast. Knowing that other nutrient paradigms exist is the first step to realizing they might be a factor somewhere you want to go.

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