By Staff
This Saturday, the 23rd, is the official first day of fall. That doesn't mean much to us here in the upper Midwest. We know it's fall when the woods transform from lush green to brilliant yellows, oranges and reds. However, that's about all many of us know about this event. We've long forgotten the whys and wherefores of this magic act taught way back in our early school years.
Leaf color comes from substances within the leaf cells. Green comes from chlorophyll, a critical substance for photosynthesis where plants use sunlight to convert water and carbon dioxide to sugar, the sustenance of most plants. Carotenoid is yellow, orange or brown and is found in many plants. It's what makes corn yellow and squash orange, to name just a couple, as well as imparting those colors to tree leaves. Anthocyanins add red color to plants, as in strawberries, cranberries, cherries and red maple leaves. Chlorophyll and carotenoid are always present in the leaves when a tree is actively growing. Anthocyanins are produced in the fall and only by some species.
To explain how these colors interact we have to start somewhere in the tree's life cycle so let's pick the summer months. The tree is in the height of the growing period. Chlorophyll abounds and completely covers up the yellow, orange and brown of the carotenoid. As we enter into fall here in the north country the days become shorter and cooler. With less sunlight the photosynthesis slows, hence less chlorophyll. It's a gradual process but eventually all food and chlorophyll production stops as the tree goes dormant for the winter. As the chlorophyll subsides the yellows, oranges and browns of the carotenoid show through. Since this is a factor of day length it happens quite consistently every year regardless of varying temperature, moisture and other climate conditions.
The red and purple colors of anthocyanins that add so much to the fall color scene are another story. Here, both temperature and sunlight are major factors. If we have a spell of warm, sunny fall days and cool but not freezing nights it will be a good year for reds. With all the warm sun leaves continue to make lots of sugar but the cool nights prevent the sap from flowing from the leaves to the branches. Some species such as sumac and maple solve this problem by producing the red anthocyanins which allows the plant to recover these nutrients before the leaves fall off. Conversely, if the combination of sunny days and cool nights does not occur, sugar is not produced and then blocked so anthocyanin production is minimal and the precious red colors are scant. As you can see, the persistent old wives tale of frost being involved in color production is just that. The only role frost plays is to hurry the process along or even shut it down if the freezing is severe.
So that brings us to why do leaves fall off? The woody parts of a tree can withstand freezing but the sap filled leaves cannot. As sunlight decreases the leaf veins that transport the sap to the tree gradually close. Cells form at the base of the leaf stem creating a separation layer. Once its tie with the mother tree is cut off the leaf quickly dies and drops. The exceptions are oak leaves, which don't form a complete separation layer, and evergreen needles, which have a waxy coating and fluids inside that resist freezing.
Maple trees provide the best color show because their carotenoids are brilliant yellow and orange and high anthocyanin production provides lots of red. Forests that are predominately maple occur in the northern reaches of the Midwest, Northeast and southern Canada. The best fall colors I've personally found are in southern Ontario along Highway 17 between Sault St. Marie and Wawa and in Minnesota along the north shore of Lake Superior. These should be gearing up about now. Here in Michigan Lake and Wexford counties will be putting on a great show in early October. Locally, Russ Forest in Cass County is a good place to look in mid October.
Carpe diem.