Virginia Climate Advisory

volume 15 number 3

Leafing Through Autumn


As the temperatures begin their steady retreat from mid-season extremes and the oppressive heat and humidity of still afternoons are gradually replaced by crisp, cool breezes, the end of summer is near. For many, thoughts quickly turn to autumn. And, as it seems to us here at the State Climatology Office, these thoughts are just as quickly turned into phone calls inquiring about the fall colors. Words such as when, where, why, and how wonderful, often emanate from our phone receivers around that time of year.

We do our best to answer as many of these requests as we can, but to be quite honest, we just don't know the answer to many others. But, due to our own interests in these subjects (and our consription to best serve the public) we set out to see if we could find someone who does.

In this pursuit, we uncovered many interesting characters, such as the individual who claimed the colors of his pet parakeet often mimicked those of the upcoming fall, and the one who is convinced of the prescient powers of woolly bear caterpillars. Most of these we weeded out, and with the remaining information, we put together the following story of fall.

But before we start with our story, we need to catch up a little on our plant physiology. So we'll pick up with our description of trees in late spring.

By the beginning of May, most deciduous trees in Virginia are leafed out and ready to take advantage of the long days and many hours of sunshine of the coming season. The solar radiation intercepted by the leaves during this time provides the energy to drive the processes of photosynthesis. Photosynthesis uses this solar energy along with carbon dioxide (CO2) and water (H2O) to produce sugar (CH2O) and oxygen(O2). The sugar is then used by the tree as fuel for life.

This chemical process of photosynthesis is made possible by the substance chlorophyll. Chlorophyll is green because it absorbs all of the other colors of light, and thus primarily reflects green light. There is so much chlorophyll present in the leaves during the late-spring and summer, that the whole leaf looks green. In the summer, when water is plentiful and the days are long, these chemical reactions can proceed very rapidly.

As a matter of fact, all the extra CO2 that is being put into the air actually allows these reactions to run a little faster, because in most cases, CO2 is the limiting ingredient in this process. Thus, with higher levels of CO2, trees are actually able to grow faster and produce more sugars. These sugars are not only their primary food source, but that of all living things.

The broad leaves of deciduous trees enable them to catch a lot of incoming solar radiation, as well as providing a larger area over which to intake CO2. However, at the same time, large amounts of water are lost through these leaves through evapotranspiration. This is generally okay in the summertime, because there is usually enough water present in the soil for their extensive root systems to uptake an adequate supply. Only a very bad summer drought will lower soil moisture levels to the point that H2O becomes the limiting factor in the chemical reactions of photosynthesis.

As summer continues, primary production of sugars is high enough not only to fuel all the metabolic processes including growth, but also high enough that extra is produced to store away in the roots, trunk and branches for use in the coming winter. For, in the winter, broad leaves become a liability. The days are short, the sun is low, precipitation is reduced, and the ground becomes intermittently frozen.

If the trees were to keep their leaves, the water loss through them would exceed the benefits obtained through the much reduced photosynthetic reactions and they would eventually die of thirst. If dehydration didn't get them, winter precipitation in the form of freezing rain or snow probably would. The weight of the ice and snow collected by the leaves would be so great that tremendous damage would occur as branches, as well as entire trees would be downed.

Therefore, deciduous trees drop their leaves for the winter, and slow down their metabolic activity in order to live off the fuel supplies that were stored away during the previous summer. When the days once again begin to lengthen, the ground thaws, and the spring rains begin, new leaves are produced in preparation once again for the primetime of summer.

Now, to the issues at hand, how do trees know of the approaching winter, and thus can shed their leaves, and why in the process do they create such a showy fall display?

Well, after June 21, the days begin to shorten, and the leaves begin to intercept less and less solar radiation. This, however, does not start to become a problem at this latitude until mid- or late-September. Then, metabolism starts to catch up with production.

Certain hormones, which control some life processes in the leaves, are produced in supplies which are proportional to the amount of light striking the leaves. As the amount of light starts to diminish, so do the levels of these life sustaining hormones, and the leaves begin the processes leading up to their death. Declining daylength is the major signal to deciduous trees that winter is on the way. Thus, trees on north facing slopes begin their fall display a little sooner than those on the sunnier south facing side. You might wonder however, why the displays of fall in the mountains begin sooner than the display by the shore.

The reason, is that while daylength is the most important harbinger of winter(from a tree's standpoint anyway), it is not the only one. Another clue to them is the arrival of cold air. Remember that snowfall is often fatal to the unfortunate tree that is caught with its canopy still full of leaves. Thus, those cool nights of the late-summer and early autumn, suggest that winter weather could soon be on the way and that trees should soon start the process of bidding their leaves farewell.

Since the mountains of Virginia begin to feel the cold air first, trees there begin to shed their leaves first. The trees of the piedmont soon follow, wit those in the tidewater area shortly thereafter. the combination of shorter days and cooler temperatures give rise to the patterns and dates of the fall season across the state shown on this illustration.

So, now you know why and when leaves begin to drop. But what about those colors?

As mentioned earlier, chlorophyll is green. In the summer, leaves contain an abundance of chlorophyll, and thus, they too are green. However, when the hormone levels in the leaves begin to drop in response to lessening light, the leaves begin to undergo some changes.

A thick-walled layer of cells, which was once inhibited by these hormones, starts to form across the base of the leaves, and chokes off the supply of nutrients. This layer of cells, called the abscission layer, eventually becomes thick enough that it effectively detaches the leaves from their stems and they fall off.

But before that happens, the leaves put on a final display to celebrate a summer season job well done. As the nutrient supply dwindles, so do the chlorophyll amounts. The green coloration slowly fades, and the other colors in the leaves begin to appear. Brilliant flames of yellows, oranges and reds flare up. The substances responsible for these colors have been in the leaves all along, but have been masked by the dominant green of the chlorophyll. The yellows and golds, typically displayed by poplars, hickories and beeches are caused by carotenoids and xanthophylls. These are the same substances which color carrots orange and egg yolks yellow (respectively).

The amounts of carotenoids and xanthophylls present in the leaves are about the same every year, so that the displays of yellow are quite constant from year to year. The substance responsible for the reds, anthocyanin (the pigment which colors apples), however, is produced by sugars remaining in the leaves, and thus can vary depending on the amount of sugar present.

Factors which control the sugar amounts include temperature and precipitation. Cool temperatures slow the uptake of sugars by the branches so that more remains in the leaves, and precipitation provides the moisture needed to produce them in the first place. Therefore, ample late summer rainfall and an abundance of crisp, cool days in the early fall provide for the most spectacular displays of red by such species as the maples, dogwoods and sumacs.

Our story of fall is now complete. So when next year comes around, and the dog days of summer begin once again to give way to cooler conditions of fall, pay particular attention to the weather of the season, for within it lie the clues necessary for predicting the upcoming fall display, not on the wing of some parakeet or on the back of a wooly bear caterpillar.