Yep, my favorite time of year is once again upon us. And by now I suspect you are all close to being experts on the science of the panorama produced by ending photosynthesis. (Like that’s going to stop me.)
After a new leaf is fully formed, the tree will spend its energy on preparing for the next year’s leaf buds. They rely on the leaves already in place to create and store food as carbohydrates/sugars for the energy needed for that new leaf bud. Trees are unique in that they’re able to manufacture their own food/energy, courtesy of photosynthesis.
We all know the green pigmented chemical in leaves as Chlorophyll. It utilizes the energy courtesy of the sun during the process of photosynthesis to convert water (from the roots) and carbon dioxide (from the air) into sugars and starches while giving off oxygen as a by-product.
Of course the green from chlorophyll is not the only color pigment that’s always present in leaves. Equally important are the carotenoids (carotene and xanthophyll). These pigments are used in capturing light energy needed in the process of photosynthesis. It’s just that the carotenoids are masked by the green of the chlorophyll during the summer months.
Once there’s the nip of autumn in the air, as daylight hours shorten and temperatures drop, cells near the juncture of the leaf and its stem start to divide very rapidly. This creates the corky abscission layer where the leaf will eventually break from the tree and flutter to the ground. And then that corky layer serves to protect the branch through winter.
The rapid growth of the abscission layer physically blocks transport of nutrients needed by the leaf to manufacture the carbohydrates. Consequently the photosynthesis that’s been creating chlorophyll ends.
And 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.
When it comes to the forest, they provide the predominant color for about 15-30% of our tree species. They’re most commonly found in the hardwood species of hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.
But there’s a third class of chemicals that leaves have (after chlorophyll and the carotenoids) that produce the reds and purples of anthocyanin. Oddly enough, unlike the other two classes, anthocyanin has not been present in the leaves during the entire summer, but are created brand new – just for autumn’s fall foliage!
It’s a mystery why a tree would expend extra energy when it’s shutting down for the winter to use stored energy to create a new product.
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 anthocyanin accounts for the intense (but relatively brief) autumnal color display in New England.
So while other areas that enjoy a mix of tree varieties the colors may not be as intense, but the season lasts longer.
And this year, rather than individual photos of fall I’ve found a wonderful aerial video to share featuring foliage of New England:
