When the leaves fall off.

On the ground lie the lungs of the planet. Here are eight things to do with the and lots of other neat autumn stuff.

WHAT BETTER NAME than "fall" for a season when trillions of leaves fall from billions of trees? On the parts of the planet tilted now away from the sun we see less daylight. The weather cools. Leaves that moved and transformed tons of water, air, and nutrients with power from the summer sun now sputter and stall in weak fall sunlight--some "flame out" in beautiful colors.

We go leaf-looking in hardwood regions: admiring the fall colors, supporting th highway fund, and encouraging country inns to stay open just a little longer. But bright foliage soon turns to drifts of dead organic matter. Here lie the lungs of the planet: leaves turning brown and shriveling as if they had smoked pack a day for far too long. Even with evergreens, older leaves (needles) are dying and dropping to the ground, forming layer upon layer of tree debris.

Fall looks like an ending. But it is neither ending nor beginning--just another stage in the constant maintenance and renewal of living things. Now is an excellent time to assess tree conditions and plan your season's projects. You can see things in the fall that are obscured by the summer leaves that cover every live twig and branch. Also, there are mountains of dead leaves to be deal with.

Walk with me out into fall's aromatic old-attic (dry) or clothes-hamper (damp) smell--the smell of a monumental organic recycling project. Dead leaves are decaying and venting a musty brew into the air. It gets in your nose and tweaks your sneeze reflex. There's lots of rustling from crinkled, dry leaves lying on every flat surface. A breeze, a step, a scampering squirrel, even the light touch of a falling twig sets off a cascade of rustling.

We'll probably hear another fall sound--a whining roar--that in recent years ha spread across North America. Combine the bugling of an elk with the sound of a very large vacuum cleaner, and you have it--the sound of the leaf blower. Repeated at intervals from place to place, the noise resembles territorial animal calls. It does involve territorial animals--us. Blasting away with leaf blowers, we move leaves from our territory onto someone else's.

In fall we work very hard to make sure fallen leaves don't stay in the one plac they belong--on the ground. We regard dead leaves as trash. And since we've bee trained that trash must be disposed of out of sight of civilized folks, we wave our wind wands and blow those leaves into piles, into bags, into trucks. We blo them into the streets to be washed down drains and beaten into mush by the constant tread of traffic. We sneak them over to the next-door neighbors; they blow them to their neighbors, who blow them on . . . and on until dunes of leaves are lodged in uncared-for sections.

Dead leaves are worn-out tree engines, but they're still full of fuel. Kick a few worn-out tree engines around with me. A 60-year-old maple, doing its fall thing, has dumped around 200,000 leaves (about 120 pounds). A 30-year-old, mixed-species forest has dropped 10 million leaves (weighing about 3,000 pounds per acre. But there's more going on here than colorful scenes and falling leaves.

SOIL MOISTURE IS INCREASING. On a hot summer day, a large tree like our 60-year-old maple will move 7,500 pounds of water from the ground out through its leaves into the air. That's equivalent to the cooling power of six room-siz air conditioners. No wonder it's cooler around growing trees. No wonder the soi gets damp in the fall.

THE DEAD LEAVES WE ARE WALKING ON BUILT THEIR OWN REPLACEMENTS BEFORE FALLING OFF THE TREE. Working leaves fuel tree maintenance and growth, seed production, and creation of a replacement leaf behind each working leaf. By the end of summer, each small new leaf is rolled and wrapped in an insulating bud. Warmer weather and longer days next spring will quickly pop these new leaves out of their coverings, ready to go to work.

TREES HAVE STOPPED PRODUCING OXYGEN. Summer leaves use sun power to combine carbon dioxide and water, making tree fuel (sugars) and oxygen. The process is called photosynthesis (meaning, appropriately, "putting together with light"). Working leaf exhaust is oxygen, exhaled into the air (lucky for us since we nee it to breathe). Now that trees are dormant, oxygen is used and carbon dioxide i released from tree respiration and decaying or burning leaves and other organic matter.

But we are not going to gasp for oxygen or feel greenhouse warming from this process, because most of the carbon from carbon dioxide taken in through the summer is still held in tree branches and stems. Oceans and growing plants in those parts of the planet now coming into summer are taking in carbon dioxide and giving off oxygen.

DEAD LEAVES ARE BECOMING FERTILIZER AND HUMUS. A one-acre, 30-year-old forest will pull about 350 pounds of nutrients from the soil annually--240 pounds of this falls back to the soil in leaves and twigs. Key elements in leaves in orde of quantity are: calcium, nitrogen, and potassium, followed by small amounts of magnesium, sulfur, and phosphorous. Layers of leaves also slow rainwater/snowmelt runoff, intercepting and absorbing water, and improve soil aeration and water retention by adding humus. If you let the trash truck haul your leaves away, you are throwing away mulch and nutrients.

EIGHT THINGS TO DO ABOUT DEAD LEAVES

* If they're not interfering with you, leave them alone.

* If they're in your roof gutters, get them out.

* If they're on your lawn, mulch them with a mower or leaf mulcher.

* If they're in your street, help yourself to some free mulch and fertilizer.

* If your neighbors blow them into your yard, thank them.

* If your community hauls them to landfills, organize efforts to compost and use them instead.

* Stabilize steep slopes by piling leaves against the upslope side of logs an branches laid across the slope.

* Improve soil by using a compost maker or "leafalator," my name for a large wooden box for blending dead leaves with not-so-good soil.

HOW TO EVALUATE A NAKED TREE

Look for:

* Broken branch stubs that slant up from the stem, catching and holding water and potentially leading to decay.

* Cavities and woodpecker holes, indicating internal decay and insects.

* Clashing crowns: areas where tree branches rub against each other or lean o roof, gutters, chimneys, or vents.

WHAT TO DO ABOUT WHAT YOU SEE

If you haven't a clue, don't do anything yet. Make some notes with your questions and concerns; then consult these areas in your phone book:

* City: Check in city government listings to see if your town or city has a tree department.

* County: Find your county Extension office, in county government listings under Extension Service.

* State: Find State Foresters, generally under Forestry in state government listings. For small urban plots, see if they have an urban forestry program. Fo larger, rural lots, check with the service forestry program.

* Private: Ask the city, county, and state folks about private tree and forestry services in the area appropriate for your situation. Also, check in your phone book under tree services and foresters/consulting. As titles imply, tree services tend to deal with individual tree problems, foresters with trees at the forest level. Once you have built a base of knowledge by chatting with people in the categories above, you may want to hire tree services or foresters to do some of the work.

If you're comfortable wielding saws, axes, and clippers, you can do some of the needed work yourself. Use a pole saw to prune off broken branch stubs and limbs that clash with your house or with each other. Before you cut, look at where th branch comes from a main stem--you'll see a slight swelling, like a collar. Cut the branch just beyond this collar; don't cut flush against the stem or so far out that you leave a stub.

Cavities and decay in trees create great conditions for woodpeckers and cavity-nesting animals. Leave these trees unless they are located such that the may fall on something important, like your house.

Trees may clash with each other and with structures, necessitating pruning branches in some cases and cutting trees in others. Warning: Unless you know about undercuts and wedges, don't try to fell large trees yourself. They can fall in unpredictable directions and do major damage. If you need tree cutting and pruning projects on your property, there are three reasons fall is a good time to do them:

1) Shrubs and plants beneath your trees have died back or become dormant, so there's less chance they'll be damaged by falling trees and branches.

2) Trees have "hardened off" for winter, making them less likely to be seriousl damaged if they're hit with falling limbs or equipment. (One of Murphy's laws says you always hit trees that you're trying to leave untouched.)

3) It's cooler in fall; more comfortable outside.

Now, from what you've seen, consider the future. Was your backyard drab this fall? Consider adding a red maple, sugar maple, or other species that turns bright colors in fall. Choose an area with room--remember, little trees become big trees.

Is everything gray and bare now that the leaves are off? Add shade-tolerant evergreens like hollies or hemlocks to provide splashes of green through the winter. Rhododendrons near the base of hardwoods will add bright blossoms in spring and stay green through the winter. Their leaves will do temperature tricks, puckering up when it's cold, then spreading to their usual generous siz as winter days warm.

WHAT'S NEXT

We've covered the basic tree-happenings of fall. There's much more, but to cove them we'll have to turn this column into a book. (That's our plan.) The next time we meet, the subject will be tools; I'll be carrying catalogs. There are wonderful tools available to help you measure, prune, chip, and plant. Most aren't available in hardware stores (at least in the quality I'll find for you) but they're only a catalog and phone call away. I'll include special instructions on how to wrap a dibble for Christmas.

IF YOU LOVE IT--LEAF IT!

If you love your back yard--leaf it. In summer, maintain layers of growing gree leaves--in live trees, of course. In autumn, mulch fallen leaves into the soil to add nutrients and humus.

When the DeCoster family moved from a 200-acre tree farm in Maine to a three-quarter-acre lot in Virginia, we needed leaves, lots of leaves, to feel a home. We bought a house with a well-wooded lot, but the soil was red clay, hard as cement when dry, slick and sticky as Vaseline when wet.

So I built a "leafalator": nothing more, really, than a large wooden box. But i will produce rich-as-chocolate-mousse soil when you blend together dead leaves and not-so-good soil and let them stew for a season.

To build: Use preservative-treated decking planks. Nail or screw the planks together in the shape of a box with the top open. Butt each plank against the next so there is a small crack between them, as with decking, letting air and water move in and out. Hinge one side so that you can drop it down and take a wheelbarrow inside if you wish. To make your box sit evenly on the ground, use level as you're building.

Build to the size you prefer. My leafalator is six feet wide, 10 feet long, and three feet deep. It's not a lovely-looking thing, so I placed it in the woods and piled firewood in front of it.

Children can be helpful with leafalators. Small ones think it's neat to hold planks and move dirt and leaves. Dangling car keys can heighten older ones' interest in helping. Spouses can be bought with dinner out or exchanges of chores.

Each fall I choose a new area in my backyard, cajole whoever's around into helping, dig up some of the hard red clay that rules around here, and wheel it back to the leafalator. Then we mow awhile, running a bagging mower over leaves accumulated on the lawn. The fluffy mulch goes back to the box in the woods. Improved soil that has been stewing in the leafalator since last year gets hauled out to the area just excavated. Now, into the nearly empty box I throw a three-inch-deep layer of clay, then six inches of leaf mulch, repeating layers up to two or three feet; ending with a loose mix of leaves and clay on top so that rain can penetrate.

I leave the mix to settle and stew through the winter. Next spring and summer spading forks will be used to mix the mass of leaves and soil occasionally; you'll want to toss it like a salad. After each mixing, helpers and "helpee" have a cool soda or beer, sitting in the shade and admiring the flowers, trees, and shrubs that seem to be whispering, "Thank you!"

QUESTIONS FOR YOUNG PEOPLE

(See if you can find the answers in "When the Leaves Fall Off")

When green plants are growing, they convert carbon dioxide into the oxygen we need to breathe. Why don't we sense a lack of oxygen when plants stop growing for the winter? (Hints: When it's winter north of the equator, what season is i south of the equator? Also, look in encyclopedia to see what important life processes and chemical reactions occur in the oceans.)

If your family car worked like an oak tree, would any of these things be true?

* The engine would be green leaves, running on water, carbon dioxide, and sun

* Car exhaust would be refreshingly clean, moist, oxygenated air.

* The engine would use some energy to run the car, some to make it bigger, some to make seeds for new cars, and some to build a replacement engine.

* When an engine was worn out, it would fall off the car and biodegrade into humus and useful nutrients.

* Tires would move water and nutrients from the ground to the engine and use some energy from the engine to build replacement tires.

* Worn-out tires would biodegrade into the soil as humus and nutrients.

* The car would take 60 years to go from 0 to 100 feet straight up.

* You couldn't get a stereo tape player and radio as an option.

* Color choices would be shades of green.

Series author Lester DeCoster specializes in working with nature (including human nature). He is president of The DeCoster Group, a Reston, Virginia, consulting firm offering expertise in writing, public relations, forestry, and environmental science.