There have been spectacular Autumn colours in our trees this year. I’m often asked why one year can be better than another, and about the science behind leaf colour (“fall color” in the US).
The science behind Autumn leaf colour
Our broadleaved trees are largely responsible for the colour in our gardens, parks and forests, although some deciduous conifers, such as Larch, are also colourful. The change from summer green to autumn glory is triggered mainly by shortening day length, and partly by cooler temperatures. Inside the tree, these environmental changes result in biological changes in the junction between stem and leaf. The cells divide to create a cork-like (abscission) layer that acts as a barrier to materials such as carbohydrates that previously passed from leaf to stem, and minerals that were transported from the roots to the leaf.
What about the colour? During the growing season the green (chlorophyll) masks all the other colour pigments in leaves (except in plants such as the Copper Beech). When the corky barrier forms between leaf and stem no more chlorophyll can reach the leaves and sunlight gradually breaks it down. As it fades the other colours become visible and we see the glorious Autumn shades of purple/red (anthocyanins), orange (carotenoids)and yellow (xanthophylls). Eventually these pigments are broken down in the leaf and all that remains are the waste products (tannins).
Why are some years better than others?
The timing of the show of Autumn colour does not vary much from year to year because the process, as we’ve seen above, is largely dependent on day length. In terms of the colours, these can be influenced by the amount of sunlight, temperature,and rainfall. Plenty of sunny Autumn days, if they come with low temperatures, will destroy the chlorophyll quickly revealing the other colours for longer. These conditions also promote the formation of more anthocyanins (purples and reds).
Unfortunately though, if it gets too cold anthocyanins are not produced, so an early frost means a premature end to colourful leaves. Another factor that can reduce colour is drought stress during the growing season as this can lead to the early creation of the corky layer, meaning that leaves may drop before they have a chance to show off their colours. Of course another impact comes from heavy rain or strong winds, both of which can damage and remove leaves from the trees.
Our love of Autumn leaf colour
Autumn colour is appreciated across the world where deciduous forests dominate but the display varies greatly from country to country, and in their different regions. In Western Europe, where Autumns can be mild and cloudy, colours are modest in comparison to some areas of Scandinavia, Japan and North America. In the United States, the Appalachians are famous for a long colour display where the high diversity of tree species can lead to a show of a month or more. In New England, where maples dominate, the display is intense but often short-lived.
Our appreciation of Autumn tree colour is recognised, in some parts of the world, with dedicated names and can be important for local economies thanks to Autumn colour tourism. Here are some examples:
- Leaf peeping in the US and Canada, supported by networks such as the The Foliage Network
- Ruska – Lapland region of Finland
- Momijigari in Japan