Autumn Leaf Drop is a Living Process

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Watching autumn leaves turn color and then fall to the ground may seem like a dying process, but it is very much a vital, living process. In fact, leaf drop is only possible if the leaves are attached to a plant or branch that is alive.

Have you ever seen a tree with a section of dead leaves clinging to one of the branches, or observed an entire tree full of dead leaves holding tight to the branches, never to fall? That branch or whole tree is surely dead itself if it holds persistently onto its dead leaves after they die. Eventually, wind, rain and storms will batter the dead leaves until they are ripped off the branches by weather and time, but they will never slip elegantly off the branches of a dead tree or plant like they do from living plants and trees each fall.

Plants that shed their foliage at the end of the growing season are termed deciduous, and people travel every year to watch the parade of leaf colors as groves of deciduous trees prepare for their autumn leaf drop and winter sleep.

The intricate mechanism that powers this annual shedding of leaves from deciduous trees and plants is ingenious. And the chemical gears that snip leaves off their stems can only continue to turn when powered by a live plant.

The first trigger that starts leaves in preparing for their skydive to earth is when the day length begins to shorten as winter approaches. During the growing season, the light-capturing marvel called chlorophyll (which makes leaves look green) is continually being manufactured and broken down during the longer summer days. As autumn approaches and night length increases, chlorophyll production slows down to a standstill until eventually all the chlorophyll disappears. Other pigments that are normally masked by the abundance of chlorophylls begin to show through, and we see a magnificent palette of yellows, oranges, and reds.

At the same time as this color transformation, each leaf is busy sending its accumulated stores of essential minerals, sugars, and other hard-won products out of the leaf, down the branches, and into the roots for safe storage underground all winter. As minerals like iron, phosphorus, and other valuable nutrients are transported off-leaf, other color changes are occurring, a visible testament to the chemical lab work being accomplished by each leaf before it falls.

In preparation for the undocking of leaf from stem, a layer of cork-like material has been forming to reduce and finally cut off the flow between leaf and stem. Any sugars, secondary pigments, or waste products still trapped in the leaves reveal themselves as the various autumn colors we wait for each year, now that the green chlorophyll is no longer king. When the leaf has surrendered all the nutrients it can transport, a magical thing happens: a chemical is sent to the very “seam”, or separation layer, where the leaf is attached to its stem. This veritable scalpel, a chemical called abscissic acid, snips the cell walls that attach leaf to stem, and breaks the bond holding each leaf to its living stem. A seal has already formed at the tear-line, so the branch is protected from fluid loss and bacterial entry. The leaf is released from its stem to perform its next chapter of life under its parent plant: as a cover to insulate the roots from frost, and as a time-release fertilizer for the roots as the leaves gradually break down into the soil.

The following spring, as the sap begins to flow, the roots pump back upward all the stored nutrients into the waiting buds of the branches, and new leaves emerge fed with resources recycled from their predecessors that are now serving a second “life” on the soil below.

The entire process of leaf drop is a response to chilling winter temperatures that would otherwise freeze the water inside each cell of a deciduous plant’s thin leaves, burst the cells, and damage the leaves beyond function, ending up with the complete loss of each leaf’s accumulated storehouse of nutrients, minerals, and sugars. Evergreen plants (think pines, cedars, etc.) have leaves that contain a functional “anti-freeze” to prevent them from freezing. In contrast, the strategy of deciduous plants is to avoid the whole threat of frozen leaves altogether by snipping off each leaf after its valuables are sent off to the roots, accumulating the fallen leaves over the plant’s roots like a warm blanket during their winter sleep, and reabsorbing the disassembled leaf nutrients each year as the blanket of leaves decomposes.

The role that fallen leaves contribute to the health of our trees, shrubs, and landscapes, and whole ecosystems, for that matter, should make us think twice before we rake up and haul away fallen leaves on our own property. Not only do last season’s leaves fertilize next season’s growth, the mulch they create helps to hold in precious soil moisture over drier seasons, and helps to deter weed growth. If we remove leaf “litter”, we are short-circuiting the naturally self-sustaining sequence of nutrient recycling, and nature’s carefully balanced systems.

What we witness as autumn leaves falling may look like a dying process, but it is very much a living cycle of perfectly orchestrated phases of growth, storage, and slumber.

What an ingenious cycle.

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