If you thought (or hoped) I had forgotten my annual treatise on the fantasia of fall – you’d be WRONG! I’m not going to pass up a review of the wonders of my favorite time of year – AUTUMN!
Road in Vilnius Lithuania
Let’s start with the science story behind fall’s phantasmagorical foliage fling.
Once a new leaf is finally fully formed sometime in June, the next thing a tree spends its energy on is preparing the following year’s leaf buds. The job of the leaves currently in place is to create and store food as carbohydrates/sugars necessary for that new leaf bud. And unlike us, trees are unique in that they’re able to manufacture their own food/energy, courtesy of photosynthesis.
Chlorophyll is the green pigmented chemical in leaves that uses the energy provided by the sun during the process of photosynthesis to convert water taken up by the roots and carbon dioxide taken from the air into sugars and starches – nutrients that the tree needs for growth – while giving off oxygen as a by-product.
But the chlorophyll that gives trees their green of summer is not the only color pigment always present in leaves. Working just as hard in the leaves are the carotenoids (carotene and xanthophyll). These pigments are important in capturing light energy needed in the process of photosynthesis. It’s just that the carotenoids are masked by the overpowering green of the chlorophyll during the summer months.
But with the coming of autumn, as daylight hours shorten and temperatures drop, cells near the juncture of the leaf and its stem start to divide very rapidly. This creates what is called an abscission layer. This corky abscission layer of cells is the site where the leaf will eventually break from the tree and flutter to the ground. That corky layer then serves to protect the branch through winter after the leaf leaves.
Leaf Landing in Lake
The rapid growth of that corky layer begins to physically block transport of nutrients needed by the leaf to manufacture the carbohydrates. Consequently the photosynthesis that’s been creating chlorophyll (which is not a very stable chemical and has been rapidly breaking down and then replaced throughout the summer) ceases.
Once the chlorophyll is gone, the carotene and xanthophyll chemical pigments that have been present all summer long now take center stage.
These carotenoids (carotene and xanthophyll) give their characteristic orange and yellow colors to not just leaves, but also to carrots, corn, canaries, and daffodils – as well as egg yolks, rutabagas, buttercups, and bananas. And they account as the predominant color in about 15-30% of our tree species – typically they’re found in the hardwood species of hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.
Look close – there are two girls standing in the Halland Forest, Sweden at the demarcation between the forest and the pines.
The third class of color chemicals that leaves have (after chlorophyll and the carotenoids) are the reds and purples of anthocyanin. But unlike the other two classes, anthocyanin has not been present in the leaves the whole summer. The anthocyanins are created brand new – just for autumn’s fall foliage!
Why the tree would expend energy, when it’s shutting down for the winter, for the creation of a new product is not understood. But we do know the brighter the sunlight during this period, the greater the production of anthocyanins – and the more brilliant the resulting reds and purples. This direct proportion of redness to sunlight exposure explains why the periphery of hardwood trees are bright red, while the foliage lower down and inside are the more typical oranges and yellows.
Anthocyanins also account for the coloring of cranberries, red apples, blueberries, cherries, strawberries, and plums. They are present in only about 10% of hardwood species – mainly maples, sourwood, sweetgums, dogwoods, tupelos, cherry trees and persimmons.
But in a few lucky areas — most famously New England — up to 70% of tree species are the type that produce the anthocyanin pigment. That high concentration of a single color (the reds of anthocyanin) accounts for the intense but relatively brief autumnal color display in New England. Whereas in most other areas that have a higher mix of tree varieties the colors may not be as intense, but the season lasts longer.
Of course, the reds of fall are not only in New England – but all over the world
You can see where your particularly area is in relation to reaching Peak Color with this interactive map:
Fall Foliage Map
Time to enjoy some of my new favorite photos of the fantasia of fall …
Allée of Birch Trees
Farm in Romania
Autumn Road in New England
Park in Paris
And of course to many of us, fall means FOOTBALL! But few places enjoy such a spectacular autumnal setting as Boulder, Colorado …
I’ll see you again next year about this time.
