THE SCIENCE OF FALL LEAVES
GARDEN TIPS – Written by Marianne Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Published October 30, 2016
Fall is my favorite season of the year. Growing up in the northeast part of the country, I have always been enthralled with trees changing color in the fall. The bright red, orange and yellow colors of autumn are marvelous. I can recall collecting the prettiest leaves on my way home from elementary school and bringing them to my mother.
After I became a science nerd, I wondered how this miracle of nature happened each year. It is because fall’s shorter days and cooler weather cues deciduous trees (ones that lose their leaves) that winter is coming. The leaves stop making carbohydrates via photosynthesis. Chlorophyll, the green pigment in leaves essential to photosynthesis, starts to break down. As the chlorophyll dissipates, the yellow and orange pigments already in the leaves become visible. In addition, anthocyanin pigments may develop in the leaves of some trees, resulting in red and purple leaf colors.
A variety of weather conditions influence the development of fall color. Fall colors will be their brightest when fall days are sunny and nights are cool and dry. Cool, dry conditions speeds the breakdown of the chlorophyll. Sunny weather promotes the production of the anthocyanins. If trees are drought stressed going into fall, the development of fall color may be delayed or can also result in early leaf drop.
Natural leaf drop is also part of the fall leaf phenomenon. Leaf drop occurs because the shorter, cooler days also prompt a layer of cells (called the abscission layer) to form where the leaf is attached to the twig. This layer of cells blocks the flow of water and nutrients in and out of the leaf. As the leaf begins to decline or senesce, it starts to produce ethylene, a plant growth regulator. Ethylene stimulates the production of enzymes that break down the cells in the area where the leaf is attached to the tree. This weakens the attachment and the leaf falls from the tree.
Some species of trees, such as oaks and beech, do not shed most of their dead leaves all at once in autumn. Instead, the leaves drop gradually over the winter or when the new buds begin to grow in the spring. This is called marcescence. Occasionally, marcescence occurs on many other types of trees as a result of a very early hard freeze. An early freeze surprises trees before the abscission layer has had time to form.
One thing you might not know is that needled evergreens also shed their “leaves” or needles in the fall, despite the descriptive epithet of “evergreen.” Each year the oldest needles on evergreens, such as pines, turn yellow, orangish-tan, or brown and drop from the tree. In most years, the change in color and drop of needles happens gradually and goes unnoticed except for the piles of needles beneath the trees. Other years, it can happen all at once and cause concern when it is observed.
In our area this phenomena is most often noticed on long-needled pines, such as ponderosa pine and white pine, as well as on arborvitae. If you note it and are concerned, check to see if the yellowing or browning needles are the older growth towards the inside of a branch. If the tips of branches on the outside of the tree or shrub are turning yellow or brown, there may be a problem worthy of concern.
Despite recent rainy weather, my two red maple hybrids, a Marmo maple and an Autumn Blaze maple, are turning delightful shades of red and orange. How about the trees in your yard?