senescence

Washington may be known as the Evergreen State, but the fact of the matter is that many of our native plants do not stay perpetually green, and instead turn brown and shrivel up when sunlight grows scarce in the fall. This is the harbinger of winter: sun giving way to clouds, greens giving way to browns, and the nights waxing long and dreary, often to the accompaniment of rain. It is an inexorable shift; as the planet turns, so the leaves.

Our northerly latitude provides us with discrete transitions from season to season, where marked differences in day length and temperature dictate the cadence of life. Fall represents the declension of daylight—a time when plants and animals must adapt to the sun’s ebbing rays. Fall’s onset is an event engaging all the senses, a change all but impossible to ignore. And nowhere is this change more apparent than in the foliage of our deciduous trees.

Like all green plants, trees are predominately autotrophic, meaning that they make, or “synthesize” food for themselves via photosynthesis. Inside the cells of every leaf are disk-shaped structures called chloroplasts, which contain the pigments necessary to turn solar energy into chemical energy. Photosynthesis—the process by which plants convert sunlight and carbon dioxide to sugars within their chloroplasts—relies almost entirely on chlorophyll, the green, light-absorbing pigment that gives most leaves their color. During a tree’s growing season chlorophyll is the dominant pigment, working literally all day to capture sunlight while the getting’s good.

But chlorophyll is a frangible compound, inclined to close up shop and dissolve when there’s little light. After the autumnal equinox, the photoperiod—or length of day—shortens by almost two minutes per diem until December 22 or so, when night reigns for nearly 14 hours at its peak. More influential than extremes of temperature or precipitation, this decrease in daylight is the tree’s signal to power down its chloroplasts and hole up for winter. Cells begin producing sugars and amino acids in lieu of chlorophyll, to act as antifreeze agents. Nutrients are drawn from the leaves down into the branches and roots for storage. The tree is undergoing “plant senescence”, a gradual paring down of its metabolic processes in preparation for months of cold, dark dormancy.

First and foremost is the matter of the leaves. Deciduous trees carry leaves that are fair-weather fans: thinly clad, built for maximum light absorption and therefore extremely useful in sunny months. Come autumn, however, the leaves become a liability, siphoning precious fluids through their veins and finding less and less photosynthetic work to do. Frail and unarmored, they are at the mercy of the wind and cold. Chlorophyll breaks down completely by this point, exposing the gaudy carotenoid, anthocyanin, and xanthophyll pigments hidden beneath. (These compounds, responsible for the reds, yellows, purples, and oranges so fancied in fall foliage, act as a sort of sub-dermal sunscreen for the leaf during periods of growth, protecting it from harmful solar rays.) Seeking to cut its losses, the tree forms a layer of cells at the petiole, or base, of each leaf, clogging its veins. Soft parenchymal cells adhere to the leaf side of each stem, while waxy, impermeable suberized cells stick to the tree side. This cellular cork, called the abscission layer, is built upon until the desiccated leaf hangs by a mere wisp of tissue, poised to tear free and drift down on the breeze.

Thus denuded, the tree bides its time through the too-short days and frigid nights, living sparingly on its sugar stores until spring. We marvel at the variegated exuberance of fall—the brilliant reds and lambent yellows of a globe-girdling conflagration—but it is simply a wardrobe change between seasons, as the raiment of the sun is cast off and the Spartan coat of winter shrugged on in its stead.

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