By Frances Owen, Conservation Education Intern, Cerro Gordo County Conservation Board
The first day of autumn has passed. Days are getting shorter, nights are getting colder, and the leaves are beginning to change color and fall. We appreciate the color change for its natural beauty, but do you fully understand why it takes place? Did Jack Frost’s touch cause the leaves to turn yellow and red? Have you ever wondered what is really happening?
It has to do with winter dormancy. During the spring and summer trees are very busy. Through a process called photosynthesis, a green pigment found in leaves called chlorophyll, helps plants capture energy from the sun so it can be stored as carbohydrates for later use. The leaves of deciduous trees, or broad leafed trees, are damaged by cold temperatures and cannot continue to photosynthesize year round; so as the days get shorter and nights get colder, they prepare to go to sleep. Trees gradually remove essential nutrients from the leaves like phosphorus and nitrogen, to store in twigs and branches. Decreased daylight and fewer nutrients cause chlorophyll production to slow and eventually stop. The green pigments begin to fade allowing other colors in the leaves to show through. The trees also release a chemical called phytochrome. Phytochrome causes a barrier to form between where the leaf stalk attaches to the tree. This slows the movement of sugar and water in and out of leaves and helps them break off.
The exact color of the leaf will depend on the acidity of the sap and on the other pigments present. Sumac leaves turn a deep red color because their sap is acidic, while oaks and ashes will sometimes turn a purplish color because their sap is less acidic. Birch and aspen leaves turn yellow because they lack orange pigment while oak and hickory tree leaves turn brown because of a buildup of brown pigments called tannins.
The weather can also significantly impact the color changes we see. Freezing temperatures inhibit chlorophyll production but they also prevent other pigments from forming. Windy and rainy weather will cause leaves to drop before chlorophyll production slows and the green pigments fade. The best weather for producing vibrant leaf colors is warm days followed by cool nights. Red pigments are further enhanced by dry weather which causes sugar to build up in the leaves.
Most people think of broadleaved or deciduous trees and plants turning color, but conifers (trees with needle-like leaves) go through a yearly color change as well.
The same processes are present in trees with needle-like leaves. Some species such as bald cypress develop color and lose their leaves, however most conifers do not drop all of their leaves. Typically they only drop needles that are two or more years old and their color change is muted by the surrounding green needles which remain on the tree. In these cases some people worry about the health of their tree when some of their evergreen leaves turn yellow, brown, or more unusually red. But these processes are the same as in deciduous trees.
Some people travel all the way to the northeastern United States to view the fall color change, but we can view beautiful colors here in Iowa. Check out the fall colors tour as part of Cerro Gordo County’s Recreational Experiences Close to Home! You can pick up a flyer at the Lime Creek Nature Center with the driving route, visit our website at: www.co.cerro-gordo.ia.us (click on outdoors, then “Recreational Experiences Close to Home”) to download a copy, or call 641-423-5309 to request one. Our local colors will likely peak the weekend of October 11th and 12th. To find out more about Iowa’s Fall colors you can check out the Fall Color Report online at www.iowadnr.gov or call 515-233-4110.