There are three kinds of pigment in a usually green leaf: carotenes which are yellow, red and pink anthocyanins, and chlorophyll, which is the green that masks the other colours until autumn.
When the days get shorter and there is less daylight, the leaf stops making chlorophyll. The chlorophyll that is already in the leaf breaks down and disappears, the greens fade and the yellow, red and pink colours show through.
The shorter days and falling temperatures trigger the formation of a corky layer of cells between the leak stalk and the twig it grows on. This blocks the transfer of sugars from the leaf to the tree; the trapped sugars in the leaf break down and form more red-coloured anthocyanins so that as the autumn progresses leaves become increasingly orange, red and purple. The layer of corky cells becomes fragile as the chemistry of the leaf changes and eventually the leaf is blown from the tree or falls under its own weight.
There are advantages for a tree that sheds its leaves. Winter in temperate zones is a time of wind and storms and trees are safer without leaves, we have seen that in this summer’s stormy winds. The Great Storm of 1987, which felled an estimated fifteen million trees in the UK, happened in mid-October, before the leaves had fallen.
Autumn is brief and beautiful; walk in the park and enjoy its colours.
Pictures taken in the park in 2018 by DKG