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Mother Nature recycles fallen leaves

| Sunday, Nov. 11, 2007

Now that the leaves are off the trees and on the ground, a question comes to mind.

Where do all those leaves go over the winter• In the spring, there isn't the heavy layer of discarded leaves that is in the woods now.

If you think about it over a longer period of time, if all the leaves from all the past autumns for thousands of years piled up, shouldn't the wood be covered to the tree tops• Well, it isn't, so what happens to leaves after they fall to the forest floor?

At first, the leaves, often still bright with color, blanket the ground with a multicolored quilt that is as spectacular as when those same leaves were still at the top of the trees.

One of my favorite things to do at this time is to shuffle along trails, kicking up piles of newly fallen foliage ahead of me, and become immersed in the crunching, crackling sound of boots against dried leaves.

But that loose, fluffy layer doesn't last long. Rain pounds the now-brown blades and presses them down against the soil. By the end of November, the spent leaves are compacted into a continuous brown layer hugging the woodland hillsides.

This is a time of the year when we feel that nature's processes have stopped for the winter. However, this is the season that the recycling of leaves is just beginning. From the soil, a variety of bacteria, fungi and tiny animals start to disassemble the complex chemicals that the trees, through photosynthesis, took a season to assemble.

First, let's look at how the leaves were constructed in the first place. From the first leaf formation last May, when leaves unfolded from buds swelled by abundant water and warm days, chlorophyll took two simple substances, carbon dioxide and water, and used them -- carbon, oxygen and hydrogen -- to build more complex substances, especially sugar.

To build something complex from something simple takes energy. Lots of energy. Think of building a house from wood, bricks, wall board and shingles. It takes time and energy.

It is the same for trees. To build roots, trunks, leaves and food from carbon dioxide, water and many small traces of elemental chemicals take time and energy. The primary product made by plants is sugar, a complex chain of carbon, hydrogen and oxygen molecules.

The energy used to make sugar from simple and abundant carbon dioxide and water is light -- light from the sun. Photosynthesis is a process exclusive to green plants that makes something complex from something simple.

This process isn't as simple as I have outlined, but I've provided the basic idea.

The blanket of fallen leaves covering the forest floor at this time of the year is the product of a season's worth of the composition of complex sugar from simple carbon dioxide and water. Now that the leaf litter is flattened again on the forest floor, a new, and reverse, process begins.

Leaves that were built (composed) by the trees, are dissembled (decomposed) by a vast army of nearly invisible organisms. Living in the soil are bacteria, fungi, mites, nematodes, earthworms, insects and even small mammals that view the layer of leaf litter as food rather than a pile of detritus from the autumn.

Probably the most important are the bacteria. These are one-celled microscopic life forms. They are not animals, but a group of living things all their own. Soil bacteria feed -- or, in other words get their energy -- by taking the complex sugars and other parts of leaves and chemically combining them with oxygen. The process breaks the energy bonds built by photosynthesis, and the sugars are converted back to simple carbon dioxide and water.

The root-like stands of fungi are doing the same thing as they grow into the once-living tissue. Little by little, all the complex parts of the once-green leaves are dissembled. The bacteria and fungi use the energy to live, and the leftover carbon dioxide and water are released into the soil. Thus, over the winter, all those leaves that fell in the past couple of weeks are being slowly decomposed by micro-organisms in the soil.

This is part of one of the fundamental cycles in nature called the carbon cycle.

Just like the circular wheel of a bicycle, carbon is being rolled through the environment -- first as a gas free in the atmosphere, then captured by plants and used to build sugars and other materials. Plant leaves fall to the ground. There the leaves are broken down by bacteria and put back into the atmosphere.

The carbon cycle rotates over and over and over, so that it is possible that a carbon molecule that was once part of a leaf hanging on a tree in a nearby forest was released to the air by bacteria last winter during decomposition, came in contact with a leaf in your garden and was built into the pumpkin you will have in the pie at Thanksgiving. That's recycling at its very best.

I've simplified the story greatly. Along with cycling carbon, the organisms of decomposition break down other, less common substances that are part of the plants, and return these to the soil. Nitrogen, magnesium, potassium and others are combined in the various complex compounds and made into parts of plants and animals. When those plants and animals die and decay or decompose, the chemicals are released in their elemental form. They remain in the soil and become the nutrients used by plants to grow a healthy forest, meadow or aquatic stream vegetation. Nothing is wasted, and what might be the discards of one living thing are the nutrients of another.

So, when you think of rotting leaves, don't think of a negative process. Rotting, or the more accurate term, decomposition, is preparing the soil and providing the building blocks for a flush of new greenery next spring. The leaves that fell this fall already are on their way to being the new leaves of next May.

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