Trees Weren’t Always Biodegradable

by hridika ahire

Trees Weren’t Always Biodegradable

March 5, 2021

Trees are such an integral part of the ecosystem. They provide us with so much and cause no harm to the environment. In fact they help keep the atmosphere clean. But did you know that trees weren't always biodegradable and led to massive coal formation?

Before all this urbanization happened, much of Earth was covered with trees, water bodies and animals. Then came humans, who started using trees for satisfying our own needs. Trees provide us with our basic necessities like food and shelter as well as papers, furniture etc. But, there was also a time when trees were a major source of raw material for coal formation. Coal has more than 50 per cent carbonaceous matter that is produced by the compaction of plants and trees over centuries.  Coal was littered around the Earth in great numbers in the late Carboniferous and Permian times, which was over 300 million years ago.

scool buzz Marine life in the carboniferous period. Wood engraving by F Wellcome
Marine life in the carboniferous period. Wood engraving by F Wellcome

The Coal-bearing period

The Carboniferous period is a geological period that spanned from the end of the Devonian Period to the beginning of Permian period. The name Carboniferous means Coal-bearing. Interesting, isn’t it? Many coal beds were formed during this period globally. The reason for coal being abundant at the time was the non-appearance of some microbes which eat away the dead trees and make them biodegradable. Therefore, the trees from the Carboniferous period were not biodegradable. 

Some of the trees from the Carboniferous time were as tall as 160 feet above the ground! This was due to the higher concentration of oxygen present in the atmosphere back in the day. They had delicate fernlike leaves on top of trunks that were as thin as a pencil. In this period, the plants were just starting to evolve and climb higher using cellulose and lignin, which is a fiber to stay upright. Can you imagine having trees so tall and walking among them? We would look like mere ants from up top.

carboniferous 56a0097a5f9b58eba4ae926f

Why was decomposition a tough task

Lignin is a massive component of wood that the modern trunk is made of. The only organisms that can eat away at this fiber easily is the white rot fungus that lives on dead trees. White rot fungi are important in biodegradation, it even mineralizes lignin present in the trees, to water and carbon dioxide. This fungi however, was not present till after the Carboniferous period. This was one of the reasons why the trees were not able to decompose.

The downfall of these trees was that they had shallow roots. Due to this the roots could not support the massive height of the trees and its weight and soon enough, they would break and fall. A whole lot of trees grew in warm and moist areas and took in the carbon from the air. As time passed and they fell, they released oxygen into the atmosphere.

Peter Ward and Joseph Kirschvink wrote in their book, A New History of Life, that the bacteria, fungi, and other such microbes that eat away the dead wood, were missing at the time. The absence of the microbes resulted in no decomposition of tress. The trees would fall on top of each other and pile up. The weight of those humongous trees piled on top of each other would lead to compression of trees into peat, also known as turf, and eventually coal was formed.

Coal is formed when dead trees are drowned in moist areas or swamps and are subject to heat and pressure over hundred of millions of years. As we saw in this case, the trees were available in large numbers along with moist, swampy environment. The matter from the tree transformed from moist, low-carbon peat, to coal, a black or brown sedimentary rock which is widely used in almost all aspects of life.

Plant fossils in carbonaceous shale from the Pennsylvanian of Pennsylvania, USA.
Plant fossils in carbonaceous shale from the Pennsylvanian of Pennsylvania, USA.

There are four types of coal available, which are:

  1. Lignite coal: This is a brown, combustible coal, formed by naturally compressed peat and has low heat content.
  2. Sub-bituminous: This is a brownish-black coal with 42 to 52 percent carbon present in it.
  3. Bituminous: It is a black coal that contains tarlike substance called bitumen or asphalt. It is higher in quality than the Lignite and sub-bituminous coal and it has around 45 to 86 percent of carbon in it.
  4. Anthracite: It is also known as hard coal as it is a compact variety of coal with very few impurities and has 86 to 96 percent of carbon in it, which is the highest among them all.

All this coal came from the Carboniferous Period

Nick Lane, a biochemist says that the rate of coal formation was 600 times the normal rate back in the carboniferous period. Ward and Kirschvink say that 90 percent of the coal that is used today came from this period.  “The Carboniferous period was the time of forest burial on a spectacular scale,” the writers Ward and Kirschvink say.

Had it not been the absence of the microbes, we would not have had such huge amount of coal available to us today. The abundance of coal makes it very economic for industrial as well as domestic use. Thanks to the trees that couldn’t decay fast enough, we have this multi-purpose fossil fuel ready to use.


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