In autumn's light: When seeking the magic and mystery of the season, it's all about the light

Fall is upon us, the season so aptly described by the poet Mary Oliver as “the old gold song of the almost finished year.” Harvest is done, acorns are littering our driveways and trails, and deciduous trees are recoloring and casting off their leaves. Best of all, although the days can still be quite warm, there is that faint flirtatious promise of the long-awaited rain.

I've often heard it said that California “has no seasons.” Instead of four distinct phases divided by the equinoxes, we Californians must make due with only two - hot dry summers and cool wet winters. In this view, fall is simply the calm before the storm. A time when not much is happening out there in nature.

In truth, fall is anything but static, especially in the North Bay. Nature is super busy out there. The next time you take a hike, try making a list of the changes you notice. Here's my short list:

The days are not only getting shorter, but the character of the light is different. It seems softer, less harsh, more golden.

Leaves are changing color but not all at the same time or rate. Some stay yellowish and orange, others become vibrantly red and purple. Still others are just plain old brown, or still green.

Birds are migrating. Recent arrivals include hermit thrushes, varied thrushes, and very vocal golden-crowned sparrows with their descending mournful “oh dear me” call.

It's easy to spot deer trails in the dry grass, peppered with piles of oblong pellets. Bucks have rubbed the velvet off their antlers and are acting strange.

Spiders are everywhere. They've been here all along, but now the females are larger and webs are more visible on dewy mornings. On warm breezy days, I can catch a glimpse of tiny ballooning young spiders, hoisted on shiny strands of gossamer silk by the wind.

None of these observations are unusual. Yet, have you ever wondered why the light changes?

Questioning the light

How do plants sense that winter is coming? Why do leaves change color? How do birds know when to migrate, deer know when to mate, or spiders know when construct their silky egg sacs?

Answers to these questions, like many in nature, are not fully understood by humans. We do know, however, that the mechanisms that trigger seasonal change in the responses of plants and the behavior of animals involve physics, chemistry and biology.

Let's start with the light. The earth is tilted 23.3 degrees from a vertical axis, always towards the North Star. The axis is an imaginary line running from one pole to the other. As it orbits, our planet presents a different aspect to the sun. In summer, the northern hemisphere is tilted toward the sun while in winter it is tilted away. Because we are also spinning on our axis one full turn every 24 hours, this tilting affects day length. The northern hemisphere receives much more total light during a 24-hour period in summer than it does in winter.

But why does the light look different? Imagine you're holding a flashlight exactly perpendicular to a black piece of paper. The light hits the paper directly and forms an exact circle. Bright intense summer sunlight. Now tilt your flashlight so the light hits the paper at an angle. Here the light forms not a tight circle but a diffuse oval or egg shape, spreading the same amount of light over a greater swath of paper. This is sunlight in fall and winter.

Nature's reactions

We humans have scientific instruments, calendars and culture to tell us where we are in the year. Plants and animals, on the other hand, have to figure it out in different ways. And many of those ways involve biological and chemical responses to the amount and quality of the light.

Plants, along with many bacteria and fungi, are able to detect the length of day and night using a specialized pigment called phytochrome. How phytochrome does this involves a complicated chemical conversion of one kind of molecule to another. This conversion happens in response to the ratio of different wavelengths of light at the extreme red end of the visible light spectrum. As the days get shorter and the nights get longer, and as the sun's energy hits the earth at a greater angle, the phytochromatic “lightswitch” is tripped.

Phytochrome is thought to play an important role in many aspects of plant development, from flowering and seed production to the production of chlorphyll and, of course, leaf color change in fall.

We often think of leaves changing color in response to cooler temperatures. And that is certainly part of the story. But weather is fickle and unreliable. The only consistent factor from year to year is the change in day length. Enter phytochrome.

On phytochrome

As days get shorter, the phytochrome “switch” triggers a cascade of biochemical processes in leaves.

First, chlorophyll production slows down. Chlorophyll is the green pigment in plants responsible for converting the sun's energy into food. Green leaves also come equipped with yellow and orange pigments called carotenoids (think “carrot”) as well; we just can't see them. As the green disappears, the yellows and oranges shine through.

The brilliant reds and purples are another story. Unlike the carotenoid pigments, these fall colors are not already in the leaves. They are actually created when leaf sugars are converted to pigments called anthocyanins. This type of coloring does depend on the weather. Strong autumn light during the day and low nighttime temperatures (but not freezing) help this process along.

The most brilliant foliage years, therefore, are those with a long stretch of mild sunny days in fall, when the light can work its magic. An early hard frost will simply kill the leaves. They'll turn brown and the show is over. New England is famous for its fall foliage precisely because it tends to have consistent weather patterns that favor the creation of anthocyanins. In California, you may notice that leaves closer to the tops of trees tend to be more red than those on lower branches. It's all about good fall sun exposure.

The pineal gland

So far we've seen how changes in light trigger seasonal responses in plants. But what about animals? While seasonal behavior in animals is governed by a complex interaction of environmental and physiological cues, one fascinating aspect related in part to day length is the pineal gland.

Almost all vertebrate animals have a pineal gland. So named because it's shaped like a pinecone, the pineal gland produces the hormone melatonin, which plays a central role in the circadian rhythms and seasonal cycles of many animal species. In humans, the pineal gland is about the size of a pea and is tucked near the center of the brain, right between the two hemispheres. The production of melatonin by the pineal gland is triggered by how much light is detected by our retinas. As less light is received, melatonin production goes up. As the lights go up, melatonin goes down. The use of melatonin supplements for sleep disorders and jet lag is thought to reset this rhythm (hey, it works for me!).

What fascinates me is that while our pineal gland is buried deep within our brain, it's direct connection to sunlight can be traced back through the history of vertebrates. Scientists who study evolution of the pineal gland have concluded that it derives from an ancient photoreceptor. Indeed, in some modern fish, amphibians and reptiles (including our beloved “blue-belly” lizard), it is linked to a light sensing organ on top of the head. Referred to as the pineal eye, this light-sensing organ can trigger a change in an animal's behavior and even the pigmentation of its skin. This is why the skin of lizards and snakes turns dark when they're cold (to absorb more rays from the sun) and light when their warm (to reflect light).

Because of its role in perceiving light, and therefore its connection to the sun, the pineal “third eye” also continues to hold spiritual sway for many. Perhaps to physics, chemistry and biology, we should add poetry and spirituality to fully understand that “old gold light of the almost finished year.”

Jeanne Wirka is a naturalist and ecological consultant. She lives in Sonoma.