April 02, 2020

In Autumn, red maple is most brilliant of all tangible things

It’s the time of painted leaves again. There are better years and worse years for color, but the red maple beside my driveway never misses. It’s always one of the first to show fires, just as it’s first to bud in spring, and every fall it blazes even when its fellow poplars and ashes are simply fading to yellow.

No one is sure how to predict the intensity of autumn tints. It’s well-known that the principal sparks of leaf-flame are lessening light and cooling air. It also seems clear the weather – dry or wet, cloudy or sunny – affects the chemical changes in the leaves. So does the tree’s location. But how these parts fit together, exactly, has not been scientifically established. Are the trees brighter after a wetter September, or after a drier one? Debatable and debated. And why this maple is so reliably the most brilliant of burning bushes among the chokecherry, raspberry, dogwood, birch and huge pine surrounding it, seems nigh-on inexplicable.

The leaves are green in summer because they’re flush with chlorophyll, a pigment that soaks up light the tree uses to photosynthesize into energy. (To a tree, light is food.) Chlorophyll absorbs red and blue light, and reflects back green light, making the leaves green. Chlorophyll is a somewhat unstable compound, and so the tree has to manufacture it continuously all summer. To do this, it needs a lot of light and warmth.

At the tag end of summer, the daylight dwindles, the sun strikes less directly, and the air cools. It’s harder for the tree to maintain its chlorophyll levels.

Meanwhile, the leaves contain other pigments as well. Carotene absorbs blue and blue-green light and reflects back yellow light, and anthocyanins absorb blue, blue-green and green light, and reflect back red. As the chlorophyll levels diminish, the carotene and anthocyanin levels stay the same or even increase, and so less green light, and more yellow and red light is reflected from the leaves. They turn red, yellow, orange, copper and purple according to the measures of carotene and anthocyanins in them. A red maple has a lot of anthocyanins and its leaves turn scarlet. A sugar maple has more carotene and so its leaves go red and orange.

Because of the shorter periods of sunlight and the colder air, the deciduous trees gradually shut down their energy-making processes, and part of that shutdown includes shedding leaves. As the sunlight declines day by day, the production of growth hormones slows, and the leaves begin to age and die, a process called senescence. A layer of dead cells builds up between the leaf stem and the twig. Eventually that layer becomes brittle and breaks, and the leaf falls. A full-grown oak tree will shed a quarter of a million leaves.

How quickly a tree closes down its chlorophyll-manufacturing process has to do with variabilities of moisture, temperature and light, but no one knows a formula for it. The red maple by my driveway apparently knows it, but reveals it in flames rather than explains it by equations. It is what it is, and somehow seems superior to the sum of its chemical parts. A revelation beats an explanation every time, in my book.


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