Two things about some trees tell you that autumn is here: the leaves change their colour, and they fall.
But who tells the tree that autumn is here? And why only some trees have their leaves changing colours? What are the colours actually?
You may already know what makes leaves green. It's the pigment chlorophyll. They absorb light to photosynthesise, which is a process where they convert the atmospheric carbon (carbon dioxide) into the food carbon (glucose). This process is so important because it is ultimately where all the energy in an ecosystem comes from. This captured energy passes down the food chain and fuel the entire biosphere. That's why plants are also called producers.
Chlorophyll and leaves look green because they absorb mainly blue and red light from the sun, and thus bounces off the green light to our eyes. That's why we can see their green colours. However, there are actually another layer of pigments in leaves that are hidden by the bright green chlorophyll. These pigments include yellow flavanols, orange carotenoids, and red to purple anthocyanins.
Carotenoids should be no stranger to you - you can find them in many foods like carrots, egg yolks, and tomatoes!
<aside> 💡 Different tree species have different combinations of various levels of these colourful pigments. When the green chlorophyll is gone, the unique combinations give each species a unique leaf colour.
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Just like any good tools, chlorophyll gets damaged when it's used for too long (think about all those hazardous UV light from sun!). They are constantly breaking down but also being produced and replaced. However, this process slows down in autumn, when chlorophyll is broken down at a faster rate than it is replaced. Ultimately, the green chlorophyll is gone and these other pigments are exposed.
What are these pigments' use, you may ask?
There are two main theories:
First, they help absorb the green light that is reflected by chlorophyll. Not only does the leaf maximise the solar energy it captures, it also helps protect other important parts of the leaf from light damage. Light carries energy and it can cause many hazardous chemicals to form, which can damage the leaf.
Second, carotenoids are an important starting material for the synthesis of plant hormones. One of them is called abscisic acid, which leads us to our second question.
As it slowly goes into autumn and eventually winter, two things happen: less water is available and temperature drops. Water and heat are two main factors that keep the metabolism in all living organisms running. Without them, trees have to think of some ways to go dormant, so they can survive through the harsh winter.
If the leaves are kept on, the little pores on the leaves, called stomata, where they "breath" will continue to carry gas in and out of the leaves, and also water vapour. When less water is available via the roots and water is going out through the leaves, the tree will eventually die from dehydration.
Abscisic acid is the plant hormone which regulates leaf abscission. It triggers a seal at the base of the leaves, as such, water and nutrients could not flow to the leaves and they die and fall off. The nutrients in the leaves will then be recycled in the soil and taken up by the tree again for the next year's growth.
However, not all trees have this strategy to preserve water. Trees that do this are called deciduous. In contrast, there are many trees that remain green throughout the year, they are called evergreen. As you may guess, deciduous trees are dominant in temperate zones which get colder in winter, while evergreen trees are dominant in tropical zones which don't have cold winter.
<aside> 💡 Each tree has its own strategy to survive through changing weather. That's why we can appreciate the biodiversity of trees.
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What triggers all these reactions? To simply put it, it is a combination of day length and night temperature. As days shorten, leaves produce less sugar by photosynthesis and change the balance to trigger leaf aging. Cold nights accelerate this process.
You may have heard of "Indian summer", which is warm and dry day followed by colder nights. It is a usual phenomenon between September to November in the temperate zone of Northern Hemisphere. It keeps the leaves working but also exhausts them faster, so they turn red late (or even not completely red) but shed really quickly. In contrast, a normal mild wet autumn will make the whole process much slower so the colour changes completely before the leaves fall off.
Watch this video from the Oxford Spark's 'Lockdown Walks' series to learn from Morgan Wade about autumn colours.