Leaves

Dying leaves of autumn, like these of a sugar maple, bid us a colorful farewell.

One of the most glorious things about fall is triggered by the end of photosynthesis in deciduous trees.

The annual shutdown also ushers in one of the gloomiest things about fall.

What happens is the changing of the leaves from basic green to artful colors — and then all too soon to dull, crispy remnants that drop away, leaving stark, naked trees.

Photosynthesis is the natural process through which trees use the energy of sunlight to produce nutrients in their leaves. When the leaves are soaking up rays and cooking up dinner for the trees, they create chlorophyll in the leaves. The chlorophyll gives off the green that colors the leaves for their functional lifespan.

When the trees close their doors on photosynthesis for the season, getting ready to go into winter survival mode in weeks ahead, chlorophyll begins to fade. As the green in leaves retreats, we see a painter’s palate of glowing hues left behind.

Those visual flavors unveiled by the departure of greens are what we know as autumn colors, and they put on one of nature’s greatest shows in the hardwood forests and woodlots. It is a signal of dying leaves, but the foliage goes out with a beautiful bang.

Chemistry is mostly what fall foliage colors are all about. It is triggered by shorter days and cooler temperatures that prompt trees to start shutting down the leaves’ photosynthesis workshop operations. They do this by cutting off the flow of nutrients between tree and leaves by forming a layer of cork-like material at the point of attachment.

Once the leaves are cut off, the green pigments of chlorophyll that they have been producing start fading. Carotene pigments already present in leaves begin to show through with their warm yellows and oranges.

As the change is occurring, some species of trees produce a burst of red pigments from anthocyanins that are formed from a reaction between proteins and a load of new sugars produced in what are essentially dying leaves. With these, too, fading green reveals the warmer colors.

So, departure of the green reveals yellows and oranges that have been masked, while reds and purplish tints are created anew. Combinations give us all sorts of in-between hues. At the peak of those colors, our woodlands and just the trees in our yards take on incomparable beauty — right before going dismal.

The color, of course, represents the seasonal death throes of the leaves. The transitional color parade passes in a relative few days, all too quickly fading into dull browns and grays, and then most leaves drop. The trees enter winter slumber as barren wooden skeletons.

Coming into October, you can hardly see the change coming, most of the lush green of summer still abundant. Some of the first clues come in catalpa trees, a species that grows weary early on, the green of the big heart-like leaves weakening and yellowing before others even hint at change.

The transition timing varies somewhat according to weather conditions. Cool nights and warm sunny days bring it on faster and more intensely. Lingering summer conditions and frequent rain and cloudy conditions both delay and mute production of fall colors.

An early frost gives the change a boost of speed, while a hard freeze sends some species rudely toward a quicker brown-up and an earlier shedding of leaves.

Most of the change, however, is a result of the photoperiod, the daily length of sunlight. That is a constant. Leaves, therefore, will go through the transition from photosynthesis ending to dropping at roughly the same time regardless of weather.

Traditionally at this latitude, our leaf colors start changing rather rapidly in several species right about now. Maples and sweet gums are highly evident with reds and oranges, while hickory species and yellow poplar are among some of the brightest with yellows, all these mixed in with still-lush greens of some slower changers.

In most years, this region can be expected to hit maximum color about the third week of October, just a few days from now. That’s kind of an averaging thing, as some species like sumacs and black gum are already on the back side of maximum color by then, while others like many of the oaks are still hanging on to much of their green at that point.

By the end of this month, many trees already will be briskly shedding their faded foliage, some already naked. It happens faster than most of us color fans like. When the time comes, much of a tree’s leaf load can go in a very few days. When foliage is near its end on a tree, a significant rain with a bit of wind can largely strip what remains overnight.

At some point later this month we can expect a larger brown-out of lower vegetation. Most weeds, vines, grasses and all the ground level growth have seen the end of the growing season. A good deal of this stuff has faded from the green of its own photosynthesis.

A frost that comes somewhere around the peak of leaf color typically slams the door on ground-floor greenery, in short order changing the lower landscape over to a basic brown.

The color flush of changing foliage is eye candy for us, but it is just a flicker in time. We wish it would linger, but alas, just ahead looms the gray of winter.