Foliage presents amazing natural transition


As we turn the calendar to October, most of Pennsylvania will soon enjoy the annual transition from the green leaves of summer trees into the flaming foliage of autumn. Entire ridges will be awash in yellow, red, orange, russet or bronze as each species of tree displays its own fall colors. Many folks will enjoy driving throughout the region to experience the glorious show of autumn splendor about the landscape. And photographers like me will be out to capture the endless photo opportunities that nature provides this time of year.

Beech trees turn a light bronze, while hickories will show golden bronze. Poplars will display leaves of golden yellow. Dogwood will turn a lovely purple. Red maples live up to their name with foliage of brilliant scarlet, and sugar maples will turn a beautiful orange. Oaks are the last to show their fall colors of red, brown or russet.

Of course, some years tend to provide a much better show of fall colors than others. Last year seemed to be a good one, while the previous two autumns not so much. Timing is everything, and when most of the trees reach their peaks around the same time, the results can be spectacular. Predicting the “what” and “when” of fall foliage tends to be mostly folklore, so let’s look at some of the basic science behind how and why leaves change color.

Trees in our part of the world grow their new leaves each spring, and those leaves become individual food factories for the tree throughout the spring and summer. Leaves derive their green color from the pigment of an amazing substance called chlorophyll. Through an extraordinary chemical reaction, chlorophyll absorbs the energy from sunlight and uses it to transform carbon dioxide and water into sugars that the tree uses for food via the process known as photosynthesis. Most green plants rely on chlorophyll and photosynthesis for their existence.

As fall approaches, leaves stop their food-making processes, the chlorophyll breaks down, allowing other pigments present in the leaves to show their color. The first of those, known as carotenoids, tend to be mostly yellow to orange in color. Carotenoid pigments are responsible for the distinctive colors of corn, carrots, daffodils and even bananas. Carotenoids tend to be present in the leaves of many trees and shrubs, which is why many of the first leaves to change color tend be yellow hues. Another group of pigments, known as anthocyanins, produce the bright reds and purples in fall foliage as well as apples, grapes, blueberries, cherries, strawberries, and plums. Most anthocyanins are produced in leaves later in autumn in response to bright light and excess plant sugars within leaf cells.

While weather-related factors, such as rainfall, sunlight and temperature, can have some effect on the intensity and duration of fall foliage colors, photoperiod is the primary determining factor of when leaves make their fall transition. Photoperiod is the scientific term used for the relative length of daylight and darkness throughout the year. Photoperiod is also the trigger for many of the remarkable transitions in nature, and fall foliage is certainly one of those. The shorter days of autumn start the biochemical processes in a leaf that ultimately cause the seasonal color changes. Low temperatures can enhance the formation of anthocyanin pigments that produce the bright reds in maples and other species, but an early frost can weaken the red color. Rainy or overcast days sometimes tend to increase the overall intensity of autumn colors.

Photoperiod is also the primary trigger for another seasonal phenomenon that many archery hunters look forward to–the rut for whitetail deer. During the rut, even the wariest, old buck can abandon its natural caution as it rambles about during the daytime in search of potential mates. For that reason, many bowhunters desire to be in the woods during that brief window of opportunity each fall.

Several years ago, I attended a presentation by a renowned deer expert who had developed a system to predict the peak of the rut for specific regions of the country. His system, as I recall was based largely on moon phases. A few weeks later, I was interviewing a deer biologist and asked him about moon phases and weather-related factors for timing the breeding period for white-tailed deer. His answer was short and compelling.

“The gestation period of a white-tailed deer is about 200 days, or 28 1/2 weeks. And most whitetail fawns are born around the third week of May in Pennsylvania,” he said.

He went on to explain the importance of the timing of fawn births. By late May, food sources are plentiful for the doe that needs plenty of nutrition in order to nurse her fawns adequately. Late May also gives ample time for the fawns to grow and accumulate enough body fat to see them through a harsh winter. He then pointed out that if you count back about 200 days from the third week of May, it will put you right about the second week of November, which is generally considered the peak of the rut in our regions, give or take a few days. There is always some wiggle room for anything in nature, but once again, photoperiod, rather than weather or moon phases is the primary factor.