The eternal dance of carbon
Autumn colours and leaf drop
At this time of year the trees are starting to change colour and we're reminded of the cycle of life and of the nutrition of nature as leaves fall to the ground.
The beginnings of leaf drop, also known as abscission, start when a layer of cells is formed between where the leaf stalk joins the stem. This layer, known as the abscission layer, is formed in the spring during active new growth of the leaf, even as it's starting to form the leaf is also being prepped to fall from the tree.
In autumn, hormones within trees begin to change. One of these of note is a hormone called auxin. During the active growing season the rate of production of auxin in the leaves are consistent with the rest of the tree, when these rates are even, the cells of the abscission layer remain connected which keeps leaves on the tree. {1}
When the days get shorter and cooler auxin production in the leaves starts to drop as well. The drop in levels of this hormone causes elongation within the abscission layer, this elongation of these cells creates fractures until the leaf breaks away from the plant, eventually falling or being blown off by autumn winds or rain.
There are a number of reasons why this happens, leaves are the how the tree both 'breathes' and 'sweats' loosing moisture which means that without leaves the tree can remain in stasis throughout the winter, also a lack of leaves means that the tree is less likely to be blown over in storms and strong winds.
A recap on photosynthesis
Let's consider how these leaves are formed in the spring and summer.
A tree requires energy to grow, as does any other living organism, they get this energy from carbohydrates formed through the process of photosynthesis. Light is the energy source that is used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.
The process of photosynthesis is commonly written as:
6CO2 + 6H2O → C6H12O6 + 6O2.
This means that the reactants, six carbon dioxide molecules and six water molecules, are converted by light energy captured by chlorophyll (implied by the arrow) into a sugar molecule and six oxygen molecules, the products of the reaction. {2)
What's key to note here is that trees provide us with two very very important functions, they remove carbon dioxide from the atmosphere and give us oxygen.
Back to autumn
At the end of the growing season the tree releases these stores of complex sugars as leaves fall to the forest floor for the next part of the nutrient cycle to begin.
The leaves are disassembled by a vast number of bacteria, fungi, mites, nematodes, earthworms, insects and even small mammals but the most important are the bacteria. Bacteria use these stored sugars to live and break down the carbohydrates in the leaves by combining them with oxygen to produce energy and CO2, the process breaks the energy bonds created in photosynthesis undoing the process.
Fungi spread out filaments into the decaying matter and break down the sugars in a similar process to bacteria, extracting energy to live and returning some of this stored carbon back to the atmosphere.
The cycle of carbon is the cycle of life
Carbon that started in the atmosphere as Carbon Dioxide was converted into complex sugars by the tree, some of it will be locked into the woody growth to be released decades or even hundreds of years laster when the tree finally dies and is it's self decomposed into the forest floor, some of it falls as a leafs to be consumed by living things that release this carbon back into the atmosphere in a dance of energy that has existed for at least 500,000,000 years but what is more mind blowing is that it is the same atoms of carbon that have been involved in this plant to atmospheric dance for half a billion years and most of the carbon we use today came from a collision with another smallish planet about 4.4 billion years ago. {3}
Meditation
This month when you look up the changing colours of a tree, meditate on your part of this process as you breathe in the life giving oxygen produced by that tree and breathe out carbon dioxide that has existed since the dawn of time and wonder about the path of each atom you breathe and the near infinite possibilities of it's path as you marvel at the living world that you are a part of.
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1. https://www.woodlandtrust.org.uk/blog/2020/10/why-autumn-leaves-change-colour/
2. https://www.britannica.com/science/photosynthesis
3. https://nature.berkeley.edu/garbelottoat/?p=1679