Paul Simon's lyrics to "Leaves That Are Green" seem particularly apt as summer gives way to autumn.
"Leaves that are green turn to brown,
And they wither with the wind,
And they crumble in your hand."
Those green leaves were remarkably productive during the growing season, capturing the sun's energy and converting it into the carbohydrates a tree needs to grow.
But, as Simon reminds us, time hurries on.
Long before we notice the brilliant colors of autumn, the tree has already begun preparing for the change.
As the days grow shorter, the leaves send much of their sugars into the trunk and roots for storage, while valuable nutrients such as nitrogen and phosphorus are reclaimed and tucked away in the branches, ready to fuel next spring's growth.
Then the tree quietly begins to cut ties with its hardworking leaves by creating an abscission layer — a thin layer of specialized cells where the leaf stem meets the twig.
That layer is doomed to fail.
Enzymes gradually weaken the connection until the leaf is sealed off from the tree. No more water enters. No more sugars leave. The leaf's work is finished. You might say the leaves are ready for retirement.
Except that's not quite true. The tree still needs its leaves.
Though withered and brown, fallen leaves are essential to the health of the forest. They feed fungi and countless soil organisms. Curled beneath the snow, they shelter queen bumble bees, butterflies and moths waiting out the winter — also salamanders and other small creatures.
As the leaves slowly decompose, they become the fertile soil that nourishes the next generation of trees.
The tree lets the leaves go, not because they are useless, but because they have another purpose waiting on the forest floor.
