Now that the leaves have changed beautiful colors and are falling to the cold ground – what’s next? I bet you can smell it. Those leaves play an active role in replenishing the soil with what it needs for when spring rolls around. I really feel like summer has passed when I can smell the crisp air and leaves decomposing.
In nature, a forest produces about a pound of dead leaves and wood per square yard every year. This may not sound like much material, but consider the amount that would fall on an area the size of a football field. An American high school football field, including end zones, covers 6400 square yards. A football field-sized area in a forest can be covered by 6400 pounds, or 3.2 tons, of fallen organic matter every year. To give an idea of how much material this is, remember that an average-sized car weighs only 1.5 tons. How can so much material fall to the ground in so little time and disappear by the next spring? Let’s zoom in and learn about the processes that are happening around us every time the leaves fall.
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When it comes to decomposing organic matter like leaves, fungi and bacteria are the major players breaking down the leaves on the ground. Here, we do not use the word “organic” in the same way it is used by grocery stores and farmers at the markets. In the scientific community “organic” describes any material made up of molecules containing carbon and hydrogen atoms. All living things, for the most part, are considered organic. Organic matter is broken down into carbon dioxide and the mineral forms of nutrients like nitrogen through the complicated process of decomposition. This matter is also converted into fungi and bacteria as these organisms feed on the organic material and reproduce. What were once colorful leaves on the ground, breaks down into new organisms like fungi and soil. Scientists call the organisms that break down organic matter decomposers.
When considering the decomposition of leaves, decomposers like earthworms and other soil animals break the leaves into smaller pieces in a process called fragmentation. This step is important because smaller pieces have more surface area to support the growth of bacteria and fungi. Bacterial growth is especially affected by fragment size since fungi can penetrate substances more easily than bacteria. If the pieces are too large, the bacteria’s growth can be stunted. The fungi can spread itself through the entire forest floor, living on the dead leaves and twigs that have fallen from the trees above. The fungi and bacteria help the plant material decompose while extracting many useful substances of their own for their personal growth.
Rainwater percolates through the leaves, fungi, and bacteria dissolving and carrying away some of the chemicals in the leaves through a process called “leaching”. The movements of earthworms and other soil animals stir the leaf fragments and mineral soil particles together in a process called “mixing”. The result of leaching and mixing can be seen in the changes among the leaves. The fallen leaves started out whole, and were green, yellow or red. As they decomposed, they were reduced to small dark brown shreds as their fragments became heavily colonized by fungi. Finally, at the end of the processes of decomposition, they have become fine black particles of organic soil and matter, and their original shape no longer exists. Many of the chemicals which remain after decomposition become nutrients for living plants including newly germinated seedlings. These nutrients can be taken up by the plant’s roots in the soil and are used to help make new leaves, twigs, branches, roots, flowers and seeds in the coming spring.
Brooke Friesen is a Naturalist for Walking Mountains Science Center. Originally from Michigan, she enjoys everything about the fall – the apple cider, the colors, and the leaves crunching underfoot.