Purdue University Agricultural Extension Service Forestry Department
P-5
October, 1951 (8C)
Our Fall Festival of Color
Every fall the roads to famous Brown County, and other southern Indiana scenic areas, are jammed with Hoosiers enjoying the fall festival of color. Many and varied are the speculations as to why the leaves change color in the fall providing this great scenic wonder. The legends of the Indians have it that celestial hunters had slain the Great Bear. They explained the red as his blood dripping on the forest leaves and the yellow as fat splattering out of the kettle as the hunters cooked the meat. Many people suppose the change to be due to frost.
Actually the change in coloring from green to the various bright fall shades is a chemical process which takes place in the leaf and tree preparatory to winter.
During the spring and summer growing season the leaves function as manufacturers of food for the tree. The green color during this season is due to small green lumps (grains or granules) in the cells of the leaves which are known as chlorophyll bodies. These chlorophyll bodies combine carbon from the air with oxygen, hydrogen and minerals from the water supplied by the roots to make plant food. In the fall, when growth ceases and finally stops, the chlorophyll breaks down chemically. Whatever food is left over in the leaf is at this time transported to the body of the tree for storage. In the cell cavities there can now be found a watery substance containing drops of oil, crystals of various kinds and a small number of strongly refractive yellow bodies. This accounts for the predominant yellow coloring so familiar in the fall foliage.
Chemical combinations with other substances in the leaf cells produce the various other colors from the brilliant red of sumac and sweet gum to the red-browns of the oaks. Quite often this, for example, is caused by the fact that there is more sugar in the leaf than can readily be transferred back to the tree.
At the same time that color is changing in the leaf, other preparations for winter are progressing. At the base of the leaf stem, where the leaf is attached to the twig, a new layer of cells is formed. These cells as they develop sever the tissues that support the leaf healing the cut in the process. Finally the leaf is blown off, or falls of its own weight, leaving a leaf scar on the twig. This process varies in speed with the species from the black walnut which sheds its leaves early to some of the oaks which retain their dead leaves almost all winter.
The hardwood trees of Indiana, also known as the broad-leaved trees, get protection from winter snow and ice because they shed their leaves. In contrast to this, the evergreens, or conifers, have no definite time to shed their leaves since their leaves are either needle-like or scale-like and are better adapted to bear the burden of winter's ice and snow. The trees in this group are the pines, spruces, cedars, firs and hemlock.
Although the fallen leaves on our lawns are regarded as somewhat of a nuisance, their usefulness in the farm woodland continues. The mineral substances in the cells of the leaves are in this way carried to the forest floor. As the leaves decompose, these valuable elements, such as nitrogen and phosphorus, are returned to the soil and enrich it for further tree growth the next year. At the same time the fibers of the leaves form an accumulation of humus which serves to keep the forest soil well aerated and capable of holding moisture. This process is of great importance to the well-being and growth of our forest trees. If fire is allowed to sweep through the woods, this humus layer is