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A flock of evening grosbeaks has been seen at a feeder in Highland Park. The first snow buntings have arrived from the tundra, juncos have joined the house sparrows feeding in our backyard, and the leaves on the huge old cottonwood across the alley are turning from green to gold. You don't need to be Tom Skilling to notice that fall is here and winter is just behind it.

The evening grosbeaks are seedeaters from the northern forest. They are among the many northern birds called irruptive species. In years when food is plentiful in their home forests we see only small numbers of them here. If food is scarce in the north they come south in large numbers. Some of them get as far as the Gulf Coast, providing birders in Louisiana and Mississippi with rarities to add to their life lists.

Snow buntings are usually seen on open ground. Beaches are good places to look, and windswept plowed fields seem to make the birds feel right at home.

Cottonwoods are with us all the time of course, but in their plantish way they are migrants as well. Unlike animals, which can move back and forth in annual cycles, plants can migrate only once per generation. Seeds make a one-way trip, and the tiny cottonwood seeds, clothed in a web of down, are designed for long journeys. The trees release them in huge numbers in late spring or early summer. They are why we call this tree a cottonwood, and they can pile up several inches thick in fence corners and along other windbreaks. Given the right wind conditions, a cottonwood seed from the tree across the alley could alight in Detroit.

The reproductive strategy of the eastern cottonwood fits its way of life. It has the live-fast, die-young habits of a pioneering species. Borne aloft on cottony parachutes, its tiny seeds carry little stored energy. They have to land in just the right sort of place to have a chance at survival. For a cottonwood, the right sort of place is a spot where the plant can enjoy full sun. Even a little bit of shade is too much for a newly sprouted seedling. Cottonwoods cannot survive in the shade of their parents, and they quickly die in woodlands. In the Illinois of 200 years ago they would have been confined to open wet areas where fires--they are very fire sensitive--could not reach them. With fire removed from the landscape and treeless vacant lots available everywhere the eastern cottonwood has become the most abundant naturally planted tree in the city.

Oaks, hickories, and walnuts follow a very different reproductive strategy. They produce a smaller number of seeds, but each seed carries a large amount of nutrition. Most of the seeds fall to earth under the tree that produced them. Some are transported by squirrels and blue jays, but none of them goes very far. These are plants of more settled communities, not opportunists searching for new openings somewhere.

Eastern cottonwoods are Populus deltoides. We have several other trees of the genus Populus around. They are all fast-growing pioneers intolerant of shade and inclined to die young. Quaking aspens are a Populus species; in northern Wisconsin they are known simply as "popples." Lombardy poplars, the tall slender trees that occupy the backgrounds of many Italian paintings and the borders of many landscaped properties in the Chicago area, are members of the genus, as is balsam poplar, a species rare enough to make the endangered list in Illinois.

Once established, cottonwoods can grow several feet per year, and they ultimately reach heights of 100 feet or more, making them the largest members of their genus. According to my very inexact measurements, the tree across the alley is about 90 feet tall. Its diameter at breast height is just under five feet. The record cottonwood for the state of Illinois is 133 feet tall and eight in diameter.

As big as it is, the tree across the alley is probably less than a century old. Cottonwoods grow fast, but they do not live very long, even in ideal circumstances. This tree's circumstances have to be described as less than ideal. Pavement encloses it on two sides, and the bark is now growing out over the concrete. On a third side the tree is gradually engulfing a fence post. Between garages, sidewalks, and the pavement of the alley, about 70 percent of the tree's roots are cut off from both water and air.

Nonetheless the tree looks healthy. Its crown is broad and leafy. It was a rich green all summer. Cottonwood leaves are glossy and rather stiff, and when the wind blows they clatter instead of rustling. The green is fading now, revealing a bright yellow.

Leaves get their green color from chlorophyll, the magical stuff that enables green plants to convert solar energy into chemical energy in the form of simple sugars. The process of photosynthesis gradually destroys the chlorophyll, but in spring and summer the tree replaces what is lost. As the growing season draws to a close, a layer of weak, gelatinous cells forms at the base of each leafstalk. This is the point where, eventually, the leaf will separate from the tree. This cell layer makes it difficult for water and chemicals to pass into the leaf, so the chlorophyll that is destroyed is not replaced. The green color fades, and other pigments--which were masked by the chlorophyll in summer--reveal themselves.

One of these pigments is carotene, which is yellow orange. Another, xanthophyll, is more purely yellow. Both of these pigments are involved in photosynthesis in some way, but we don't know exactly how. Carotene is the source of the color of carrots, and one form of it becomes vitamin A in animals. Trees that turn red in fall are usually full of pigments called anthocyanins, the same pigments that produce the red in beets and red cabbage.

During the growing season sugars are moved out of the leaves during the night to clear the way for more sugar production the next day. In October the nights get cold, and the tree's physiology slows down. The sugars stay in the leaves. The ideal weather for fall color is warm, sunny days, when the leaves can make sugar, and cold nights, when they can't transport it. The sugars in the leaves produce high levels of carotene, anthocyanins, and the rest.

Broad-leaved deciduous trees dominate forested areas around the world at these latitudes. Moving north from here, evergreen trees with needles instead of broad leaves take over. In warmer lands the broad-leaved trees are evergreen. All the oaks in Illinois lose their leaves every autumn, but in Louisiana the live oaks stay green all year.

Before long the red and yellow pigments in the leaves will break down, and everything will turn a dull brown. The brown leaves will drop to the ground--and into our gutters--and for the next several months photosynthesis will essentially cease in our part of the world.

The needles on evergreen pines, spruces, and firs don't live forever. They are produced and shed periodically. More of them drop in autumn than at other times of the year, but there is never a time when they all drop off.

Crows and starlings will use the tall cottonwood across the alley as a roosting site in winter. Chickadees and downy woodpeckers will search for overwintering insect eggs and larvae in the buds and in cracks in the bark. The sugars created during the growing season will be carried to the roots. Early next spring the sap will begin to flow, carrying energy to the tips of the branches, where the flowers, long, slender structures called catkins, will emerge and begin releasing pollen. The pollen will fertilize the next crop of seeds, and incidentally cause some sneezes among allergy sufferers.

The leaves will open after the flowers have done their work, and the job of creating and storing energy for next year's crop of flowers and seeds will begin. The tree will be slightly thicker, its branches slightly longer. It will be one year closer to the end of its life.

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