I am sure most of you have noticed that 2020 has been a glorious year for the annual fall colours.
Every year, trees and shrubs put on a wonderful show prior to losing their leaves and preparing for winter storms. This year has presented one of the most spectacular shows in decades.
Every week, when you think the show is over, another species comes into colour. What is happening here?
Most plant leaves are green due to the presence of a chemical called chlorophyll. It is used in a process called photosynthesis, which uses energy from the sun to metabolize food by turning carbon dioxide and water into sugars for food. In the process, oxygen is released into the air, which is used by animals, such as ourselves, to breathe.
Chlorophyll is green because it absorbs all colours except green, which it strongly reflects. Thus the leaves look green. Chlorophyll is an expensive molecule to make and the plants often try to break down the chemical and absorb it into the plant so the resources can be reused the next year. As these chemicals are absorbed by the plant, the pigments in the leaf can be intensified or allowed to show.
As the days grow shorter and the temperatures get colder, the leaf no longer has the chlorophyll to reflect the green light and the natural colour of the leaf starts to show. Leaves come in many colours, but the most common ones are red orange and yellow. Differing amounts of these colours in the leaves account for the shades of colour that we see.
Yellow and orange leaves have those colours because of chemical pigments called carotenoids. The main one is called carotene and we are most familiar with it in the orange colour of carrots.
The bright red of many species of maple trees is due to the presence of pigments called anthocyanins. The reason for this colour is a bit strange as nature tries to perform its functions with the least amount of effort. Constructing anthocyanins takes a great deal of energy from the plant and it seems odd that chemical would be created in a leaf that is about to be discarded from the tree.
One theory suggests that when the chlorophyll breaks down, the leaf becomes vulnerable to the sun and can be damaged by its rays. The red colour could act as a sunscreen for the leaf to allow the plant to extract as much energy from the leaf while it still can without suffering damage from sunburn. This helps the plant store more reserves for the long winter. There also may be an adaptation to the cold that we don’t exactly understand.
Red maples, which turn bright red in Canada, barely change colour in warmer climates, such as the deep south. As well, studies in Greece suggest that leaves that turn red suffer less insect damage.
Whether it is the red colour or the fact that red leaves contain less nitrogen than other colours, and are therefore less nutritious, is yet to be determined.
We still have a lot to learn about the autumnal display that is so common around us. Why are some years almost bereft of colour and other years we get a wonderful display?
It seems that the colours we see in the autumnal displays are caused by multiple factors. Extremes of temperature can affect the colours produced as can periods of drought. This means that each year gives us a different autumnal display and we can never be sure of what kind of display will be on offer.
This year has been a difficult one. The fall display has been spectacular. Let’s hope it is a harbinger for better days to come next year.
Tim Philp has enjoyed science since he was old enough to read. Having worked in technical fields all his life, he shares his love of science with readers weekly. He can be reached by e-mail at: firstname.lastname@example.org.