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Fall Color Season
Article #315 October 2020
By Bill Cook

The annual color change makes autumn the favorite season for many.  While gorgeous, it’s part of a much longer physiological process of tree survival during a harsh season.

            As I was driving along US-2 west of Iron River, a buddy of mine asked me why do the trees change color?  I responded with a goofy comment, then followed-up by saying it’s all about chemistry. 
            The timing is relatively consistent, as it is controlled by a balance between dark and light hours.  This window is roughly ten days to two weeks from the end of September and into the middle of October.
            Tree species comply, more or less, with their biological clocks, but not every species uses the same clock.  The ashes and balm-of-Gileads go first.  Oaks will persist longer.  Sugar maple is a stickler for punctuality.  Trees along the Lake Superior shore have milder microclimates and often change colors at the end of the window.  Forests on the shallow soils of northern Iron County are among the first landscapes to transform.
            Red maples along our highways often cause comments about an early color change.  These trees are responding to unfavorable growing conditions rather than the onset of an early fall. 
     There is a lot of biochemistry related to the annual undressing of the forest.  Those particularly valuable elements get pulled into storage.  The magnesium heart of the chlorophyll molecule is a good example.  As the green chlorophyll molecules break down, other colors appear.   But, that’s not the entire story. 
            Should the first hard frost happen while the leaves retain good stocks of sugars, we should usually see a better color show.  Color intensity is also a function of tree health and September temperatures. 
     While timing is largely controlled by photoperiod, the intensity and visual quality of the fall colors can be impacted by weather.  The compounds with red, yellow, and purple pigments may be brighter, or persist for a little longer, with warm days and frosty nights.  But it’s a risky business to predict color change patterns. 
            The process is part of what trees undergo to make themselves hardy for the cold and dryness of winter.  Northern trees have some astounding and fascinating adaptations.
     The geography and expansive forest of Michigan provides some of the most colorful fall displays in the world.  Annual treks by thousands of leafpeepers provide testimony to this fact.  It is a good time to be in the woods, as bird hunters will quietly attest.
     Much of our hardwood, or broad-leaf, forest consists of maples, aspens, and oaks.  We eagerly anticipate the crimson and gold of maples, which usually outperforms the color changes in other parts of the continent.  Our aspens also turn a lively yellow color, especially with the right weather conditions.  Oaks exhibit deep purple when the frosts come early.  A bright yellow canopy over a snow-white paper birch stand is an experience not soon forgotten. 
     But, let us not ignore the softwoods, or evergreens.  Although they do retain needles year-round, they don’t retain them all.  The older needles, nearer to the trunk, fall off every year.  The only exception is the glorious tamarack.  Not only does it lose all its needles each fall, but it typically departs the season in a flaming blaze of gold!  It is the last tree to change colors as the forest bids farewell to the growing season. 
     Once again, the much anticipated season of color change will soon lie behind us.  This time of year, many of us hold our breath, lest we miss those few days when the forest canopy alights with the fire and brilliance of the last hurrah of the summer. 
     Every season hosts a multitude of changes in the forest and all that lives there, including us humans.  Fall color is arguably the favorite of all seasonal changes and without doubt the outdoors is teeming with interesting events this time of the year. 

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Photo caption.  A bigtooth aspen leaf shows bright red and yellow after the absorption of chlorophyll molecules and sugars trapped in the tissues after an early frost. 



TRAILER- This website was created by a consortium of forestry groups to help streamline information about forestry and coordinate forestry activities designed to benefit the family forest owner and various publics that make up our Michigan citizenry.  This website is maintained by Bill Cook, Retired Michigan State Extension Forester/Biologist.  Direct comments to cookwi@msu.edu or 906-786-1575.