Before leaves, and before coal, there lived the Lepidodendron.
Most of the coal deposits humans use today come from the great Carboniferous forests of 350 million years ago. These forests spread over much of the world, and lasted through 80 million years of stable climate. By comparison, the age of the human race is barely a fifth of a million.
How did these gigantic trees turn into the coal reserves we know today?
Most people tell us that the land was very swampy then, so when the trees fell into the water, the bacteria couldn’t get at them so easily. Instead of rotting away the usual way, these trees remained lying in the water until they got covered by earth. And then, over millions of years, they were compressed into the carbon-rich form of coal.
But that’s only part of the story.
The forests of the Carboniferous era were nothing like the forests we humans are so used to destroying. Those were the days when life was just beginning to get a foothold on land. Even the word “foot” didn’t make much sense, because there weren’t really many feet around at the time. And in any case, the animals couldn’t move out of the sea until the plants had gotten the land ready for them.
Living on land is not as easy as living in the ocean. In the ocean, you can just let your body float on the water, but on land you need a skeleton to hold you up. In the ocean, you can simply reach out and catch hold of a passing nutrient, but on land you have to actively search for them. Most importantly, in the ocean, you get the water you need to keep your cells alive. On land, you have to drill for it.
The Psilophytes were the first plants to reach out to the land. They still grew from inside the water, but stuck their tops out to catch more sunlight. They were also one of the first to develop stiff cells for stalks that could stand upright on their own. Water-plants rely on the water to support them — and so do water animals. That’s why beached whales get injured or even die when they get stuck on land too long: their bodies simply aren’t strong enough to hold them up.
While the Psilophytes make their first innovations for land-growing plants, it was another group called the “club mosses” that really took over. And the most successful of these was the Lepidodendron.
The Lepidodendron was unlike any tree alive today. It didn’t have all the advanced technologies like leaves and branches. Instead, it used to shoot straight up from the ground, like a two-metre-wide rod of wood, and grow rapidly, becoming as high as the Leaning Tower of Pisa in just ten to fifteen years. That’s an incredibly short time for a tree!
Lepidodendra didn’t have leaves or branches. Instead, their main trunks were covered with photosynthesising scales. These scales did the same job leaves do today: taking energy from the Sun and converting it into energy for the plants to use.
But because there were no leaves or branches, Lepidodendron trees could grow much closer together than most trees of today. It is thought that over 800 trees could fit in just one acre of soil!
Only when the trees reached their full height did they do something close to leafing. Right at the top, they would spring out a few small branches, with sun-absorbing spikes sticking out. Then they would sprout small, spore-filled cones from the tips of their branches. The spores were what the trees grew out from, just like mushroom spores. It was the closest Lepidodendra ever got to seeds.
Once the spores were ready, the now old tree would let the cones fall to the ground, thus preparing for the next generation of scaley trees before it died.
Because of the Lepidodendron, the life-and-death cycle of the Carboniferous forests was much faster than any forest today. Trees were dying so fast that they didn’t have time to decompose and return to the soil. Why?
For trees, decomposition usually happens when insects, animals, and bacteria start feeding on the dead wood. Over time, it finally gets converted back into soil. Or maybe not “back”. Some places start off as unusable wasteland. It’s only when some things come and bacteria start working on them that the wasteland gets slowly covered with nutrient-rich soil for plants to grow in.
Before the Carboniferous, all land was wasteland.
It was some time in the early Devonian, 400 million years ago, that the Psilophytes made their first move out to land. And with them came the bacteria that they lived with, the bacteria that lived on them, and the bacteria that fed on them when they died.
These bacteria were used to just a few plants scattered here and there. Or maybe to some small areas where the trees grew thick. But the bacteria were in no way prepared for layers upon layers of Lepidodendron tree-trunks, all waiting to be decomposed. It’s like trying to finish your food while someone’s constantly serving more onto your plate.
When you food gets left over, it’s eaten up by the bacteria. But what happens when the bacteria’s food gets left over?
In the case of the Lepidodendron, the “food” just got compressed by all the layers of trees on top of it. And then again, over millions of years, it got compressed by the earth that covered it up. Some of it was too far buried for the bacteria to reach. And, especially in flooded areas, there was no oxygen for bacteria to breathe even if they did manage to reach there. It’s like trying to finish your food while someone’s constantly serving food into your nose.
Of course, this compressing of ‘food’ happened to other trees as well. But not so much, because no other tree grew as densely as the Lepidodendron.
Some of the trees got filled with mud and compressed to rock fossils, like the stone trees in Scotland, Arizona and Tiruvakkarai. But most of them turned into the black coal we mine today.
All the energy in that coal comes from the energy of sunlight, trapped by the Lepidodendron and other trees all those millions of years ago.
The Lepidodendra finally died out beacuse the climate became too warm for them. New trees with new technologies of leaves, roots and seeds took over. When our coal reserves run out, it will be like a delayed reaction from the dying of the Lepidodendron.
A very delayed reaction.
An earlier version of this article was published in Sirius #221 12–25 June 2016 “The Scaley Trees”.
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