Leaves’ last gasp: When chlorophyll slips, those fall colors are revealed

Our locally prominent sweet gum trees flash the full spectrum of fall colors when the transition comes.

When summer’s greenery fades, the warm hues parade.

That is the poetic essence of the botanical process that leads to our fall colors. The chlorophyll that gives our foliage its pervasive green appearance sags when the leaves on deciduous trees recognize the end of the season and shut down business as usual.

The day job of leaves is photosynthesis, absorbing sunlight and converting it into glucose to nourish the trees they are serving. Chlorophyll, made all the while that the solar cooking is ongoing, creates the green that we see.

Trees know in their herbaceous way, however, that winter is coming. Persistent freezes ahead require that trees brace themselves. They do this by sacrificing their leaves and shutting down food production. They block the conduits between leaves and branches, closing those doors and preparing for dormancy to wait out the frigid season.

A wall of new cells forms at the point of attachment for each leaf. That creates a barrier sealing the veins of the branch to which the leaf is attached. The leaf begins dying and, with chlorophyll production ended, the green that has long colored the leaf fades.

Hereabouts, we are near the beginning of the green slipping away to reveal the bells and whistles of what humans celebrate as fall foliage. The green of leaves is a cover. When it departs, we see a mishmash of reds, oranges and yellows that have been there all along.

That is what botanists tell us. Instead of the leaves just acquiring those gaudy colors, the fizzling chlorophyll no longer hides underlying hues, letting them shine through.

Before and after the green cloak of chlorophyll, leaves contain compounds that yield the autumn palate. Oranges come from beta-carotenes, reds are primarily the handiwork of anthocyanins, while yellows glow from content of flavanols.

Different species of trees have varying mixtures of these color-producing compounds, and other variations are seen from one tree to the next within the same species. Other factors include weather conditions leading up to and during the color change of the leaves — clear or cloudy skies, rainfall amounts and temperatures.

The biggest trigger of the fall leaf color transition is the daily length of sunlight, the photoperiod. When the days grow sufficiently short, it wants to begin.

We can see variations with the mix of secondary conditions. Fall foliage colors are maximized with mild, sunny days and crisp cool nights. On the other hand, cloudy, rainy weather filtering daytime sun and overcast conditions preventing crispy cool nights make foliage colors less spectacular.

There are chemical reactions that create new coloring compounds during the transition period. When changing leaves do get good doses of sunlight, those short-lived leaves are able to produce extra sugars which tend to show up especially as reds and red-oranges. These provide an additional splurge of colors about which the trees might care little, but foliage-watching humans revel in them.

Fall colors typically peak at this latitude and at this elevation about the last week of October, really not long from now. It seems we are running a bit behind this year because of some recent above-normal temperature along with a good bit of cloudy skies.

One might suppose that when the change starts, it might move along rapidly. As of this writing, very little foliage has shown much interest in running off the green. As is typical, already there are some shrubby-sized sumac trees exhibiting some blood red colors. (Keep an eye peeled for them on the sides of highway right-of-way areas.)

Always among the first to show the season is coming on are the catalpa trees. These hereabouts, indeed, are well along fading to a pale yellow and some even already are shedding leaves. Another non-tree that is giving us some early colors is the Virginia creeper vines that in some instances already are glorious orange to red.

The rest is coming in short order. The showiest here typically are the maples, especially sugar maples. These give the full spectrum from red to orange to yellow, but some of the bursts of red-orange are almost electric.

Hickories are pretty early to turn, and they and the yellow or tulip poplars are smashing in creating huge, uniform bursts of bright yellow. On an otherwise dull day, a grove of mixed hickories and poplars is almost like having forest lights flicked on.

Most of our predominant oaks are less than flashy, the green of their leaves fading too quickly toward the brown of winter. Some white oaks, however, do knock out some nice purplish reds that are as pretty as anything in the timber. Scarlet oaks are not as plentiful as many species, but they also flash some rich reds in the tree line.

One of our more common hardwoods is the sweet gum, found both in bigger woods and as one of the eager species found in open fields that are trying to regenerate into forest. Sweet gums in the right weather conditions seem to shine with the best of everything in fall colors. Some of their star-shaped leaves go to a mild yellow, but others take on burgundy to fiery red to garish oranges — and these may be mixed with some lingering greens that set off the warm colors.

Enjoy the artsy colors of fall when they come, for all too soon the cheerful hues will fade to brown and worse. Winter’s humbug tapestry isn’t far away.

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