The woods were very still yesterday morning.  There had been a killing frost the night before and the rising sun was quickly wiping it away.  A few ice crystals sought refuge in the long shadows.  Most of the trees had already shed their sheltering leaves, so the sun advanced with unencumbered rapidity on its daily mission. Though the branches of the sassafras, walnut, willow, ash and elm were bare, the stubborn red maples and oaks were still desperately clinging to their wardrobes.

  Walking through this scene I was impressed with the sense that nature was holding her breath for a moment.  A Red-bellied Woodpecker chortled in the distance and a Fox Squirrel set about with a series of guttural barks, but otherwise the place was hushed.  There was no breeze. Only the soft crunch of my footsteps disturbed the scene. I stopped for a moment when a movement caught the corner of my eye.  A single maple leaf slowly descended to earth via a random butterfly like path.  It gently bounced off a black cherry twig and slid over the surface of a spicebush leaf before reaching the ground.  Each aerial contact gave off a light ticking sound but the final landing was silent. Soon a small flock of leaves started to fall.  Prompted only by the invisible forces of gravity each leaf took its own flight path.  One fluttered like a small bird while another maintained an even keel as it dropped straight down. Soon the place was raining maple leaves and the woods came alive with horizontal motion. Then, after only a few dozen seconds, everything stopped and the stillness resumed.

  I believe it was my presence that set off that morning flight.  No, I’m not saying that the trees were actually aware of my nearness but that I set up some vibration that triggered the release of one leaf. That leaf in turn triggered the others and so on. Yoda would claim that I tripped the wires of “the force,” but I know not of such things. I do know that the wind took over the disturbance role by day’s end and that a good portion of the maple leaves made their final journey yesterday.  The oaks, of course, held firm.

  Witnessing the fall of a single leaf, let alone causing it, is a small experience in a big world. We all know that deciduous trees shed their leaves but may take the whole process for granted.  Being there when an individual leaf dies is a bit humbling.   True, the death of a leaf is not as heart wrenching as that of a favorite pet, but it does cease to be living tissue and so fits the definition. The tree needs to release the leaves so that it can continue living. The tree is in control of the whole situation and leaves (no pun intended) only the final moment to the whims of chance.

  It’s all about abscission.  Over the course of the year a maple leaf – or any other tree leaf for that matter – hosts chlorophyll which manufactures food for the tree.  Triggered by the shortening days, reduced light intensity and lower temperatures, the parent tree begins to prepare for the onset of winter. In the waning days of summer and early fall, the leaf can’t convert the sun’s energy as efficiently as it used to. The tree, in other words, uses up the additional food as fast as it’s made.  Also, since water evaporates off the leaf surface, the tree starts to loose more water than it can afford. The drying effect of winter is the reason trees shed their leaves.

  At this point, the tree starts to cut its losses and draws essential liquids and food reserves back down into the winter-safe realm of the roots.  All the nutrients, minerals and sugars are pulled back from the leaves.  The point where the leaf connects to the stem is called the abscission layer (take a look at this micro photo of a maple leaf abscission layer).  Normally the cells in this area act like a sponge full of tiny vessels to transport fluids to and from the leaf.  In the fall, these cells start to swell and exude a waxy substance which cuts off the vessel flow. Deprived of the tiny lifelines, the chlorophyll dies and the bright autumn colors are exposed (we’ve been through this part before).  The leaf is gets old and literally enters its golden years. It is a brief retirement.

  Instead of a gold watch, the parent company forms a corky layer to cover the scar created by the dying leaf and essentially locks the leaf out for good.  The abscission layer eventually breaks down completely leaving only the skeletal transport tubes to hold the leaf to the stem. The rest is up to chance.

  All it takes is a puff of breeze, a footstep vibration, or a squirrel’s sneeze to create a force load that exceeds the strength capacity of the frail tubes.  The moment this last connection is compromised is the moment a leaf is sent to earth. Think about that next time you see a leaf die.