It’s that time of year again where nature gives us one last spectacular show before we head into winter. But what causes this amazing display of colour? Well, it’s time to get geeky with the science behind autumn colours.
Believe it or not leaves contain several different pigments throughout the year, but during the spring and summer we see them as green and this is because the chlorophyll, which is responsible for the green colour, is the dominant pigment and is constantly replaced during the summer months.
As trees begin this preparation for winter, chlorophyll production ceases and the existing chlorophyll in the leaves begins to break down. The essential nutrients in the leaves that can be reused are transferred out of the leaves into the branches and this is when we start to see the green colours fade and other colours such as the yellows and oranges come through. As the trees cut off contact with the leaves by the formation of a cells across the base of the leaf stalk in preparation for leaf shedding, this restricts the movement of sugars and these trapped sugars become concentrated in the leaves and promote the production of anthocyanins which produce the vibrant red pigments we see, these are only produced in the autumn when it’s bright and cold.
Different trees have different proportions of the different pigments and the colours we see come from the following:
Green is from chlorophyll
Yellows come from carotenoids and flavonoids
Oranges come from carotenoids
Reds from anthocyanins and carotenoids
- this is a chemical contained within the chloroplasts in the leaf cells and is an essential component of photosynthesis, the process by which plants (and some other organisms) convert the sun’s energy into chemical energy which is used to convert water, carbon dioxide and minerals into energy-rich carbohydrates. These sugars are then used to by the plants to grow, but as summer turns to autumn and temperatures begin to drop and light begins to fade the trees begin to prepare for winter.
Flavonoids and Carotenoids
- this is a large family of chemical compounds and are actually present in leaves along with chlorophyll but we don’t see these colours as they are present in lower levels than the chlorophyll and are masks by the dominant greens. As chlorophyll degrades the green colours fade and the yellows and oranges become more noticeable. Flavonoids contribute to the yellow colours with the carotenoids contributing yellows, orange and red colours. Although these compounds also degrade they do some at a much slower rate than chlorophyll.
– these compounds are actually members of the flavanoid family but are not commonly present in leaves during the spring and summer months. As the days shorten their synthesis is initiated by an increase in sugar concentrations in the leaves. Although their precise role is unclear it is thought that they may have some light-protective properties allowing the tree to protect its leaves from light damage and extend the length of time they are on the tree before being shed. The anthocyanins produce the vivid reds, purples and magentas that we see and it is thought that these colours may also be affected by the acidity of the tree sap.
So why do we see differences in colours and intensities of colours from year to year?
The differences in colours and intensity we see are influenced by the combination of weather conditions and the chemical processes taking place in the trees. There are three main conditions that influence the colours.
Cold Nights – low temperatures destroy the chlorophyll in the leaves causing the green colour to fade, if temperatures stay above freezing the production of anthocyanins is enhanced producing more red colours.
Dry Weather – during dry weather the sugars become more concentrated in the leaves enhancing anthocyanin production leading to redder colours in the leaves.
Bright/Sunny Days – As autumn starts, the production of new chlorophyll stops, but if there are bright sunny days during this time photosynthesis can still occur which uses up the remaining chlorophyll which increases the sugar concentrations and therefore colours.
In the years we have warm, sunny early autumns we see more intense colours in the leaves and a more spectacular display. So why not head out and check out the changing colours this year for yourself.