Autumn colours – the fact and the fantasy
The colours of autumn have inspired many – in literature, song, folklore and art.
Legend has it for example, that mystical Jack Frost nips leaves with his fingers, painting them bright with his brush as he moves silently through the forest.
But what are the facts behind the ‘fall', and why are some year's colours more vibrant than others?
With their many broad leaves, deciduous trees display the most noticeable autumn leaf colour; their intensity affected by a combination of genetics, climate and geography. Early colonial painters were accused of exaggeration in their bright portrayals of New England's ‘fall' when, in fact, it is that perfect combination of nature and location that produces the region's renowned and startling hues.
In truth those bright red and gold colours are also present in spring and summer; they are just masked by the green of chlorophyll which, essential for photosynthesis, is in greatest abundance during the growing season.
As autumnal days shorten and temperatures drop, the rate of photosynthesis declines and the dominant green chlorophyll starts to decompose, allowing the yellow and orange pigments of carotenoids to be revealed.
Red, blue and purple pigments of anthocyanin also develop – usually when excess sunlight and dry weather prevail. These allow the tree to recover any last remaining nutrients before the leaves fall and winter takes its hold.
Why do we see differing hues each year?
An old wives’ tale claims rainy days wash the colours from leaves. And indeed, cloudy, rainy days or warm nights do reduce the intensity of the autumn colours.
Conversely, bright, sunny days and cool nights enable photosynthesis to continue, even when chlorophyll levels are declining. The yellow and orange carotenoids become more powerful, and as night temperatures fall below 45oC they convert to anthocyanins enabling the red pigment to reach maximum development and thus create the most colourful fall.