Why Leaves Turn



I’ve just finished reading descriptions of what goes on inside a leaf at the end of summer – dying cells, clogged arteries,  altered chemical.  Who wants to hear about the complicated inner workings of a tree’s foliage that go on to produce the autumn spectacular?  It makes the process sound as if trees were going through the menopause, their flaming colors the result of severe hot flashes.

Every morning I look across at Canaan Mountain, wondering when it will don its cloak of many colors.  What is it waiting for anyway?  Actually, it is waiting for cool nights.  Not killer frosts, which many people think are needed to produce the gold and crimson autumn colors, just temperatures that have dipped down into the 40s. A lot of different elements go into making Mother

Cool nights are what stop the corky layer between leaf stems and branches that develops to blows the whistle on food production.  The trees may like a few extra weeks of good green chlorophyll, but that green is so strong it covers up the yellow pigments, carotene and xanthophylls which are waiting eagerly to turn the woods and roadsides into shimmering gold.

The scarlet and purple pigments, anthocyanins, require high light intensities and high sugar content for their development, so they, too, need bright sunny days and cold nights.   These pigments will turn a sun-drenched (and sad to say, salt-drenched) roadside maple into a flaming orange torch. The woodland maples, who’ve been unable to trap sufficient sugar in their leaves, however, will turn a soft butter yellow.

Jack Frost is not responsible for the autumn extravaganza.  He’s the culprit who puts an end to the show by filling the woody fibers that hold the leaves to the twigs with ice crystals so that they break. If his timing is poor, Jack can ruin the entire performance, forcing the leaves to drop before they’ve had a chance to show off their colorful costumes. 

Autumn turns the dowdy, shapeless sumacs into raspberry-robed partygoers, but makes the vibrant green apple tree look more dingy than a derelict. It decorates the delicate birches with a thousand gold coins, and eventually replaces them with copper ones. 

An ash tree keeps its golden crown for only a few days, but a beech will hold its fluttering leaves, as pale as winter sunlight, until wind and snow finally tear them away.  The oaks cling to their leaves even more tenaciously.  Red oaks deepen to a beautiful burgundy, but the white oaks merely fade to brown with oxidation, the same way the flesh of an apple does when exposed to the air.

Locust Hill has no trees that become really colorful each fall.  The photo of my sugar maple at the start of this column was taken years ago.  This is what it looks like now.   And the Carpathian Walnut tree beside it. riddled with sapsucker holes, always has ugly brown leaves by October.  Even Canaan Mountain’s autumn colors are not worth a photo this year.    I hope, unlike me,  you’re surrounded by gold and scarlet trees.