As daylight decreases and cold weather sets in, deciduous trees begin to shed their leaves en masse, a phenomenon so striking it gives autumn its other name: Fall.

This annual shedding is an evolutionary tactic that helps trees conserve moisture. Broad leaves, as opposed to the needles of evergreen trees, have a relatively large surface area and they give off large amounts of moisture through evaporation. In winter, when the ground freezes and the tree can’t get water through its roots, water conservation becomes necessary for the tree’s survival. If live leaves stayed on the tree all year, the tree would be in danger of fatally drying out.

But what actually happens to break the attachment between a leaf and its twig? The weakening of a leaf’s attachment to the tree is a response to decreasing sunlight and cooling temperatures. The weakening of that attachment also plays a role in the leaf’s autumn color change.

When the combination of sunlight, darkness and cold reaches a critical point, most broad-leaved trees respond by creating a barrier of special cells where the leaf stem meets the twig. This area of special cells is known as the “abscission zone.”

The abscission zone is composed of two parts: a separation layer, or abscission layer, and a protective layer. The location of the separation layer determines where the leaf breaks off. The protective layer seals off the wound left on the twig after the leaf falls. That seal protects the tree against moisture loss and the entry of harmful organisms.

All leaves eventually die. If they remained on the stem indefinitely, they would become a point of infection and decay. Also, if leaves broke off without a separation layer, the result would be extensive tearing, the same as when you tear a live green leaf off a tree. The tear provides an excellent entry site for fungal spores or bacteria.

The separation layer forms either just before the leaf falls, or earlier in the season, depending on the tree species. If the separation layer forms early, it sits there and remains quiet until it is needed. Separation layers that form early are often found in species that grow in more unpredictable environments; if bad conditions arise during the growing season, the tree can shed its leaves and shut down.

The protective layer lies next to the separation layer and consists of a group of cells that undergo cell division to form a sealing layer. After the walls of these cells are formed, they become filled with gum and create an impenetrable corky barrier. All cells to the exterior or leaf side of this layer die.

To some homeowners’ dismay, the leaves of beeches and some oaks tend to stick onto their twigs even after they’re dead. That creates a second round of leaf-raking in the spring, when new buds push dead leaves out of the way.

In short, for our area, decreasing daylight and falling temperatures combine to cue the formation or awakening of an abscission zone, which, in turn, prevents further production of chlorophyll, the pigment that makes leaves green. When chlorophyll production shuts down, the previously hidden red, yellow and gold pigments reveal themselves.

Leaves don’t just “turn” colors. Instead, they lose the green. Leaves show us their “turn colors” just before the attachment to the twig weakens and breaks. To the tree, the entire process of changing leaf color and leaf drop is a strategy for survival.

“Growing Green” is contributed by Diane Dorn, Lehigh County Extension Office Staff, and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613.