Leaves & the Secret of Life

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So the leaves have to go. As autumn approaches most of the minerals and nutrients stored in each leaf are sent back to the winter pantry, the tree itself. The juncture between leaf stem and tree branch slowly seals itself off, forming a corky layer called the abscission zone.

Photosynthesis and chlorophyll production cease. Beginning at the edges of the leaf send proceeding inward toward the major veins, the green pigment fades away like nest from a mirror, revealing the other pigments.

These are chlorophyll's shy cousins. the yellow- pigment xanthophyll and the redorange carotene. They emerge alone or in varying combination, depending on the type of tree, and are responsible for striking canary-yellow poplars and aspens, bronze beeches, golden hickories and sweet gums.

In many other trees such as black gum, sassafras, dogwood, and some maples and oaks, the pigment anthocyanin adds red to the palette. Just how much red depends not only on the kind of tree but also on the weather, because anthocyanin is formed only when sugars trapped in the leaf during cold snaps are exposed to direct sunlight.

It's anthocyanin that varies most from year to year, making some fall leaf displays more colorful than others. A sunny, early autumn punctuated by crisp, cold nights paints a landscape bright with scarlets, purples, reds and oranges. Cloudy days and mild weather, or an extended summer drought that inhibits sugar production, result in a more monochromatic scheme of yellows and browns.

Eventually all leaves lose their brighter pigments and turn brown. For a few fleeting weeks though, each tree contributes its own characteristic color to the fall patchwork. In their last hurrah the leaves cling to the branches only by the barest of threads, their abscission zones weakened. One by one, each leaf is torn from its parent tree by a gust of wind or an autumn rain and spirals to the ground.

Once returned to Earth, the season's leaves gradually decompose, conditioning and adding their minerals to the soil and supporting countless unseen creatures—earthworms, fungi and more—vital to the life of the tree above. Eventually their nutrients will be drawn back up through roots and living plant tissue and finally into leaves again, green and growing until the coming of another fall, another turn in the forest cycle.

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