A look at what's behind the display of vivid autumn colours this year
Although the days are getting shorter, the nights longer and there is an increasing chill in the air as we head relentlessly on towards winter, not to mention the country increasingly locking down again in the face of a pandemic, the changing leaf colour of trees at the moment will hopefully help uplift one’s mood on a sunny day. Leaves of the Acer genus, commonly known as maples, turn a fiery red; while leaves of beech trees become golden, while other trees see their leaves turn yellow or purple.
Leaf colour comes from four pigments which are produced by leaf cells to help them produce food for the tree or shrub. These pigments are chlorophyll (green), xanthophylls and carotenes (yellow and orange) and anthocyanins (reds and purples).
As we head through autumn and the days get cooler and shorter with less sunlight, the production of chlorophyll slows and eventually halts. Chlorophyll still present in the leaf fades as a result, which allows the yellow/orange carotene pigments to be unveiled which aren’t normally seen in spring and summer as they are masked by the green chlorophyll.
In some species and then in certain conditions, leaves can turn a vibrant red too. Although xanthophylls and carotenes are present in leaves throughout the growing season, anthocyanins (reds) are not and tend to be manufactured in the leaf in years when lots of sunlight and dry weather have increased the concentration of sugar in tree sap, sugars become trapped in the leaves, triggering the tree to release anthocyanins as the tree prepares to shed them.
However, anthocyanins are only present in around 10% of tree species native to temperature regions, including the UK, northern Europe and North America. Though some areas in these temperature regions, such as New England in the USA, can have a high proportion of these tree species that produce anthocyanins, such as maples which are dominant, hence why New England is famous for its autumn colour displays.
After the leaves change colour they eventually die and the trees shed their dead leaves because the leaves are no longer able to photosynthesize with lack of sunlight and colder temperatures. By shedding their leaves, trees are able to survive the winter in a dormant state and make room for new growth in the spring.
Trees are good at recycling. The fallen leaves contribute to healthier soil after they fall, through decomposition. Some carbohydrates and sugars remain in the leaves. On the ground and as the leaves decompose, these carbohydrates and sugars give nutrition for 'feeders' to flourish. Once fungi and bacteria have begun to break down the leaves, insects feed on them too. The broken down leaves also provide a source of fibre which helps the soil retain moisture and improve drainage.
Thanks to prolonged dry and sunny weather in September, vibrant autumn colours are appearing in many areas this October.
A prolonged spell of dry sunny weather preceding October, when most trees change colour, is often a good precursor for a good autumn display. The dry weather allows sugars to become more concentrated in the leaves, thus more anthocyanin is manufactured in the leaf which makes them redder. While sunshine also increases sugar concentration in the leaf, even when chlorophyll production stops, and thus increases anthocyanin production leading to redder leaves in certain species of tree or shrub. An often cloudy and rainy autumn tends to lead to dull and muted autumn colours.
However, we have also had some cold nights recently, which favour a quick change in leaf colour, because low temperatures destroy the chlorophyll, so the green leaf fades to yellow quicker than under mild conditions, though temperatures above freezing enhances anthocyanin production to allow the leaves to take on a red/purple colour.
Trees start to turn colour in autumn first in the far north of the UK where it is colder, in September in the north of Scotland then by mid-to-late October in the far south of the UK. For information on some of the best places in Britain to see autumn colours, here are a few links:
