The Science Behind Fall Colours

Colour change in the leaves of trees is a succession signaling the termination of the growing season. It starts out as streaks or blotches on the leaves which eventually cover the entire leaf... leaf after leaf after leaf. Until before you know it, you have an amazing spectical of beauty before you. Greens turn to yellow, rusty orange, scarlet, deep red, or shades of brown and tan.

Temperature changes signal the beginning of the foliage colour change. Hence, an early period of cool weather brings about an early colour change. The combination of decreasing length of day and cooler nights trigger cells between the stem and leaves - the abcission layer. The cells of this layer eventually cut off the leaf from the stem, depriving it of water and minerals.

The green pigment chlorophyll is not the only pigment in leaves. Other pigments in leaves are overpowered by chlorophyll in the growing season when it is abundant. When chlorophyll breaks down however, these other pigments are revealed. Carotenoids and xanthophylls give leaves of many trees the yellowish and brownish colours we see in Autumn. As chlorophyll and other colours break down the result is a brown leaf (the result of drying up).

Some Fall colours are byproducts. The red colour seen in Sugar Maple, Sumacs, and other species are caused by anthocyanin, a pigment produced by leaves with a high sugar content. When the abcission layer forms, sugar is cut off from the leaves and is gradually converted to anthocyanin. This is how the leaves turn red and then yellow. Anthocyanin is only briefly produced to decompose and reveal other pigments.

Abnormally dry weather can significantly reduce the intensity of Fall colours, as parched dry leaves do not produce enough sugar to make anthocyanin. WIth the dry conditions we've had this year, the vividness of Autumn leaves will most likely be subdued, at least on the orange and red side of the spectrum.

FUN FACT...

What do autumn leaves and ripening bananas have in common?
The green colour in unripe bananas comes from chlorophyll, the same pigment that gives green leaves their colour. As bananas ripen, the chlorophyll breaks down and disappears, revealing the yellow colour which has been there all along. The yellows and oranges of autumn leaves are also revealed as their chlorophyll breaks down. Of course, other changes also occur as bananas ripen: the starches change to sugar and the flesh softens as pectin (a carbohydrate) breaks down.