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Earth Dr Reese Halter's Blog


Story ran in the Toronto Star October 13, 2012

That’s some palette Ma Nature is using to turn out these days of brief but splendrous color. Her secret is in the complex pigments at work, so to speak.

To understand autumn colors we must examine the leaf. Leaves spend the winter tightly wrapped in a cover of weather-resistant scales formed the previous summer; they emerge from these buds in spring.

The buds burst as they absorb the sun’s energy. Young leaves expand and endeavour to survive the onslaught of insects, strong winds and hot, sometimes scalding, sun.

Leaves are filled with cell sap containing millions of green chlorophyll molecules. They take energy from the sun, water from the ground and carbon dioxide from the atmosphere and, in return, give back oxygen to the air. They also make sugars, needed by all parts of the tree to grow.

A magnesium atom at the center of the chlorophyll molecule makes it unstable. If it’s artificially replaced by a copper atom, then the green color becomes permanent. That’s exactly what we find in the store when buying green toothpaste, shampoo and many food products.

Not all cells in leaves are chlorophyll (green). Some cells, called chromoplasts, contain non-light-making pigments, like carotenes (oranges of carrots) and xanthophylls (yellows of corn). They give a leaf its yellow color when it loses chlorophyll.

Some oak leaves have tannins — bitter to taste and used as a plant defense against insects. These create a brownish leaf color in autumn.

A third class of pigments in tree leaves called anthocyanins produces brilliant reds and purples of apples, grapes and most maples. They are not found in the chromoplast cells that make carotenes. They form in leaf cell sap when there’s a chemical reaction between accumulating sugars and organic compounds. The more acidic the cell sap, the more dazzling the red. Purples and blues, on the other hand, occur when the cell sap is less acidic.

Substances other than pigments also occur in leaves. Resins help repel sap-sucking or leaf-munching bugs. Leaf hormones govern growth and protect against water stress and cold temperatures.

As September progresses, our days get shorter. Soon layers of waterproof cork cells form between the leaf stalk and the twig. The leaf is now sealed off from its tree, incapable of receiving water from the sap stream and unable to export leaf sugars to the tree. The leaf cell sap begins to accumulate. The greens of the chlorophylls are destroyed and yellows are unmasked. The cool fires of fall have been ignited!

Alder leaves don’t display much, if any, autumn color. Cottonwoods, birches, aspens and larches are rich in carotenes but not anthocyanins; they paint the landscape with yellows and golds. Maples are heavy with anthocyanins and produce deep wines and reds.

Some species have a lovely mix of both reds and yellows.

Sunny days and crisp cool nights are just the right combination for majestic fall colors. I recommend hiking any of the high country trails in Enchantment Lakes basin southwest of Leavenworth in the Alpine Lakes Wilderness, Washington or Mt Frosty in British Columbia’s Manning Provincial Park at least once in your life to experience the sublime subalpine larches; they display a brilliant autumn golden-yellow for about a week or so before they, too, shed their needles.

The more autumn sunshine, the more spectacular the autumn leaf colors.

So make it a point to get outside this weekend, become a “leaf-peeper” and celebrate the majesty of nature.

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Earth Dr Reese Halter is an award-winning broadcaster and distinguished biologist. His latest books are The Insatiable Bark Beetle and The Incomparable Honeybee

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Text © by Dr Reese Halter 2012. All rights reserved.


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