It's one of the pleasures of fall: walking in the woods on a warm day, scuffing my feet through a deep layer of newly fallen leaves. Looking down, I notice the gold coins of aspen leaves against the bread-knife serrations of brown beech leaves. My feet make that “swoosh, swoosh” sound that takes me back to when I was a kid.
It's November, and where I live the color blast has faded. The woods are gray and brown. The much admired “fall foliage” has drifted earthward to become the more prosaic “leaf litter.” I understand the term, but the word litter grates a little. It connotes trash, yet leaves are just the opposite of trash. Their contribution to forest health, to the ecosystem, is incalculable. They help make the forest what it is.
Fallen leaves provide insulation from cold, benefiting myriad creatures. They act as mulch, preventing soil drying. They absorb raindrops, which helps prevent erosion during hard rains. And, they eventually become soil themselves, that light, airy, fragrant, moist humus that is key to a healthy forest.
Almost the minute a fallen leaf hits the ground, it starts down the path of decay and decomposition. But it's a slow process. Leaves are composed of cellulose and lignin and are a chewy meal.
“They are pretty tough, like everything else in nature. But there are whole groups of organisms that are able to break them down,” Kevin Smith told me. Smith is a researcher with the US Forest Service's Northern Research Station in Durham, New Hampshire, and an expert on decay and decomposition in the forest. When he takes a walk in the woods, “a lot of it is on my hands and knees,” he said.
Larger animals work to crack leaves into more manageable pieces; every time a wild turkey or ruffed grouse scratches through the leaves, it's doing just that. Millipedes and the tiny terrestrial crustaceans known as pillbugs and sowbugs munch on them. Bacteria and fungi – particularly the ascomycetes, or cup fungi –work diligently to extract what little nutrients there are, the tree having retrieved most of the good stuff –potassium, phosphorous, and nitrogen – before letting the leaves go.
There are a lot of things that determine how fast leaves decay: how much moisture there is, the shape of the leaf, whether it is hairy (hairy is harder for munchers to get a grip on), how much nitrogen is left in it (fungi and bacteria crave nitrogen), if there are tannins (oak leaves have a lot of tannins and are harder to break down), and the proportion of lignin to cellulose (lignins have a chemical structure that's more of a challenge for the decomposers). Some parts of the leaf, especially the stalk or petiole, are much tougher than others.
A recent study by Massachusetts Institute of Technology researchers found that temperature also plays an important role. Though there are innumerable leaf types and lots of variables when it comes to leaf decay, as temperatures rise the rate of decay goes up the same across the board. Their findings, which they've translated into a mathematical model, could help improve climate change models and better predict carbon dioxide levels, because as leaves decay, much of the carbon they free up ends up in the atmosphere as the greenhouse gas carbon dioxide.
In the end, months, perhaps years from now, what will be left of the newest layer of dead leaves in the forest?
Well, not much, Smith said. But what is left is important. It's known as humus.
“Certainly for our northeastern forests, the life of the forest depends on humus. I couldn't say it more strongly,” Smith explained.
Humus holds moisture and helps with soil aeration, buffering trees from two of their major stressors: getting their roots waterlogged and having them dry out. It also stores calcium and magnesium, two elements essential for forest health.
Fall, then, is “the time of year when there's this great loading of leaves and organic matter onto the forest floor that will fuel these processes. What we see as the shedding process is a big part of what helps maintains soil fertility and the growth of trees and the next generation of trees,” Smith said.
That's why it always pains me when I see people bagging leaves to throw away. I've been known to stop and offer to take them off their hands. I know of a better home for them.