Autumn leaves are falling

Just as the leaves approach their peak colors during fall in the Turtle Mountains it seems that we get a windy day, and the whole show turns into a twisted version of “Gone with the Wind.” This year was no exception.

I often wish those trees could hold on to their leaves a bit longer, but that is not the way it works in temperate environments such as North Dakota. After all, this phenomenon is not about color shows for humans, it is about the plants ability to adapt to winter conditions.

Plants do not simply lose their leaves in the fall. There are actually several important processes that are put in motion as the days of autumn shorten. It will culminate, of course, in the abscission, or shedding of leaves as the trees senesce or go dormant during winter. It is a delicate dance, involving enzymes, hormones, and other compounds in and around the base of the leaf.

One of the first noticeable changes is the drop in chlorophyll. The plant will also transfer some important substances from the leaves back into the plant. Magnesium for example, an important atom in the chlorophyll molecule will be returned. So will sugars, amino acids, and other substances. It is a way for plants to cut their losses before shedding their leaves.

Enzymes and plant hormones will help create a weakened layer of cells, the abscission layer, at the base of the leaf stalk where the leaf will eventually break away from the branch. The cementing material between the cells of this layer becomes weak, as will the cell’s walls.

On the branch side of the abscission layer, a protective layer will form. The cells of this layer will produce a thick outer covering of water repellent substances that form a seal and help in the eventual separation of the leaf from the branch. The connection becomes weak. A little breeze and “autumn leaves are falling.”

What remains after the leaf has dropped is the protective layer, what we call the leaf scar. If one looks closely at these leaf scars, the margin of the leaf stalk can be easily identified as are the scars left from where the leaf veins entered the twig. More importantly, however, the area is sealed off from exposure to the elements and potential diseases.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.