Most of us rarely consider that plants have to compete among themselves for space, sunshine, nutrients, and other vital resources. Some methods that may come to mind are different growth rates and variation in leaf arrangement, shape of crown, and overall height. One way that may be surprising is termed allelopathy, or producing their own herbicides. These phytotoxins, or “plant killers”, inhibit the growth of other types of plants. Their production is more common in desert regions since rainfall washes them from the soil and provides a good substrate for microbial degradation. There are, however, a number of plants in the New Orleans are which use allelopathy. The oaks and magnolias are good examples. Many of us have tried in vain to grow grass in their shade. We have raked, planted, fertilized, added soil, and prayed, but with rare success. Since all of a tree’s resources must come from the area under its foliage, it should not be surprising that they have developed a means to keep other plants out of their “pantry.” Another example is golden rod, a beautiful fall flower which grows in open, sunny places. The phytotoxin produced by these p;lants inhibits the growth of woody plants such as trees. When you think about it, it makes sense that a plant requiring an abundance of sunlight would have a method to prevent itself from being shaded out by taller, more permanent plants. These phytotoxins have received little attention to date, but in the future humans will surely make more use of Mother Nature’s safe herbicides.
Why do leaves fall off plants and how does it happen? First, all plants shed their leaves. The most obvious are the so-called deciduous trees like pecans, sycamores, cypress, tupelo, and elm, all of which shed all their leaves at once. Others, such as magnolia, live oak, and wax myrtle, are non-deciduous and shed their leaves as each leaf reaches old age, though many oaks have two times during the year when they drop a lot of leaves during a short period of time.
To explain why leaves fall, imagine a hackberry during summer. Each leaf has a complete connection with the circulatory system of the tree and there is a free exchange of water and food. At the base of the leaf, near the stem, there is an area known as the abscission zone. To the casual observer, it doesn’t appear different, but microscopic examination reveals cells with no fibers which are smaller than those of neighboring areas. With decreasing day length in fall, a number of chemical changes occur within this zone and two distinct layers form. The first, the separation layer, is characterized by short cells with thin walls. These characteristics render the separation layer weak and the weight of the leaf, often helped by a breeze or rain, causes it to fall. The second is the protective layer which is formed by the deposition of fatty material within and between the cells on the stem side of the separation layer. This is the so-called leaf scar which is so obvious on stems during the winter months. If you look closely you may bee tiny dots which represent bundle scars, the ends of veins which previously carried nutrients to the living leaves. Sometimes these bundle scars appear to form animal faces within the leaf scar.
Late next autumn, on a crisp morning when there is a nice new leaf fall, reflect on the physiological phenomena precipitating the shower of leaves.