The Back PorchWhen Everything Changes


When everything changes

This story is featured in Montana Outdoors September–October 2017 issue

At the start of the fourth week of September, days in Montana start to become shorter than nights. Elk and antelope breeding seasons peak. Aspen leaves hang golden before the fall.

During the fourth week of September, moose approach the apex of their rut. Bull trout spawn in coldwater tributaries. Asters still bloom where many other prairie wildflowers have long since faded. Ground squirrels and marmots have already begun their long winter naps.

It’s the autumnal equinox, the start of fall. Some cultures equate the transition with oncoming winter and even death. But for hunters, anglers, hikers, and floaters, the fourth week of September is a time to anticipate and delight in.

And it’s all due to daylight length.

Scientists call it “photoperiod.”

Because Earth tilts slightly on its axis, our planet’s annual dash around the sun means daylight is longer than darkness north of the equator from late March to late September.

The seasonal shift touches everything on Earth, from behavior to temperature, climate to reproduction, birth through death. Every schoolchild has to be taught this. Every plant and animal inherently knows it.

There are short-day plants and long-day plants. No one knows why asters evolved to bloom in September and pasqueflowers in April. It works, and that’s all that has to happen.

In the fourth week of September, at Montana’s latitude, leaf color presents the most visible effect of changing day length. Decreased daylight triggers plants to stop producing chlorophyll, the chemical agent that turns leaves green and masks other colors.

When the green disappears, yellows and reds reappear. Temperature and precipitation come into play, but day length is the driving force. Leaves change color even during unseasonably warm Septembers and Octobers.

In the animal kingdom, birds provide dramatic examples of the effects of photoperiodism. Back in early spring, birds responded to lengthening daylight by putting on a layer of fat to use when migrating and breeding. That happens again in August in response to decreasing daylight, except the energy stores are used only for migration, not reproduction.

When sufficiently plump, birds in the northern hemisphere fly south, turning their spring navigational maps upside down. Young birds that hatched just a few months earlier have to pull this off even though they have never been to their wintering ground or experienced migration.

For Montana’s mammals, the fourth week of September represents a mixed bag. Some reach a peak of behavior, like rutting elk and antelope. Others, like male deer, haven’t even begun their rut.

Bears have their own fall behavior. Triggered by decreasing daylight, they enter a stage known as hyperphagia, where they eat for 20 hours a day or more—gorging themselves on up to 20,000 calories a day—as they beef up for winter hibernation.

Even underwater, daylight length reigns supreme. Photoperiod affects fish behavior and movement, usually in combination with water temperature and flow.

All these animals have a built-in biological clock. So do humans, though most people don’t pay it much heed. Alarm clocks, calendars, and the bright glow of TVs, smartphones, and computer screens mask nature’s timetable.

The exceptions are those of us who spend time outdoors. Like wildlife, we pay close attention to the length of daylight and darkness. When the autumnal equinox arrives, our internal alarm clock starts ringing, because we know that the next few months will be the best time of year to be outside.Bear bullet

Bruce Auchly manages the regional Information and Education Program in Great Falls. Ed Jenne is a Missoula illustrator.