(Image: CHS)

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Fall leaves might evoke warm memories of cider pressing and pumpkin carving. Or they might remind you of a chore you had growing up, or still haven’t managed to escape. Leaves on the ground might symbolize the impending gray we live through in the Pacific Northwest. No matter how you feel, with certainty, leaves fall, and that should happen starting right about now.

The colors of Capitol Hill’s street trees in fall definitely have curb appeal, but I would argue the bigger, more compelling story happens after they hit the ground. We all know inherently that leaves decay. It’s why letting them form a slippery, mucky mess on the sidewalk isn’t great. We rake them up because eventually they turn from red to brown to mush. But what if I told you that decaying leaves contribute to a food web far more diverse than they do while they are green and on a tree?

Indeed, a green leaf might be important food for a native butterfly’s larvae that goes on to pollinate a wildflower, or be eaten by a bird. A fallen leaf is munched by a host of invertebrates, fungi, and bacteria. Fallen leaves make a home for wintering insects, and in turn, become a place for birds to look for important winter foods. The decayed material builds soil and fertilizes plants wherever they fall. A fallen leaf is the base of a pyramid of biological diversity.

Pikes/Pines has talked about the value of leaving leaves where they fall. Sometimes that isn’t practical or even safe. If you are a gardener, you might want to harvest this bounty to mulch your beds. Or you might not be able to endure neighborly judgment for leaving them be. I just want to brighten your world by demonstrating their worth, while they still might be an impediment or a nuisance.

Even before a leaf drifts to the ground, fungi can already take hold. A good example of this is a fungus that colonizes leaves of our local bigleaf maples is the Speckled Tar Spot (Rhytisma punctatum). This well named species forms dark spots on leaves that are more noticeable as fall color arrives. This fungus follows the leaf to the ground, overwintering until the following spring when they release spores to colonize new leaves. (All plants are in a constant dance of self preservation, so seeing a few of these on a maple leaf isn’t a death knell – but a few too many and could be a sign of other underlying maladies.)

Speckled Tar Spot (Rhytisma punctatum) on Bigleaf Maple. (Image: Brendan McGarry)

While there is no simple explanation of leaf decay, some generalizations hold true, so let’s walk through how it happens. A leaf falls to the ground and for our imaginary story, we’ll say it’s pristine and free of infection. If our leaf falls in water, the very first thing to happen might be that some of the carbohydrates and sugars leach out. That doesn’t deter our first wave of detritivores: fungal species start digesting material through the excretion of enzymes, which begin to break the leaves into usable nutrients. This also makes the leaf more accessible to bacteria and other microbes, which happily feast on the newly dissolved organic compounds.

If you grow houseplants (or have a sloppy compost system), you might be familiar with the next visitor — likely the first insects to arrive are fungus gnats. Their larvae like to eat fungus, so they show up to continue their short life cycle. Along with them will be a host of species that munch on the larger portions of the leaf, feasting on whatever remaining carbohydrates are left or even enjoying the cellulose. On the tails of these digesters and munchers are yet more invertebrates and bacteria who eat their waste and leavings.

From there, it’s not hard to leap upward in the food chain — larger species of insects, amphibians, fish, and birds have parts of their nutrition connected to the decay of a simple leaf and the endless cycling of carbon and nitrogen. Larger insects eat those eating the leaves and in turn are eaten by a frog, who is then swallowed by a snake, who then feeds an owl. Of course drawing a straight line between them often isn’t realistic — again, we’re generalizing.

However, not all plant species are alike, let alone their leaves, and even the leaves of different individuals of the same species can have marked differences. A 2019 study in the American Southwest found that two individuals of a certain cottonwood species contained 3%, and 30% tannins in their leaves, a compound that seriously inhibits decay. Conifer needles (a reminder that needles are just modified leaves) contain more lignin, which also resists decay.

Leaves look different and are made up of different components because they have different needs and face different challenges. The result is that the creatures waiting to consume fallen leaves are diverse and have specific adaptations. It’s a riotous, exciting world in that leaf pile in the gutter on Broadway. Of course, another aspect of this is that diverse plant assemblages beget diverse systems, including what results from their leaf litter. Greater diversity of leaves falling can equal a greater diversity of species showing up to break them down. So maybe the leaf litter curbside at Dick’s isn’t quite as excitingly diverse as leaf litter in a showy corner of the Arboretum, but it’s still cool to think about.

(Image: CHS)

We know about all these processes not merely because there are diligent biologists, but because the carbon cycle is an important process across all aspects of life on earth. The natural decay of organic compounds contributes nearly 90% of the yearly carbon release into our oceans and atmosphere. There are multiple stories tied to climate change and biodiversity as we follow our leaf into the dirt.

In some cases, it appears that ecosystems with less leaf litter diversity (be it species or genetic) capture less carbon because less species show up to take up the available carbon. This is made worse if the leaves decay quickly, increasing carbon dioxide emissions from microbes who get first run at the food. And because organic material decays faster at higher temperatures (regardless of the makeup of the leaf), rising temperatures also mean decaying leaves release carbon into the atmosphere quicker, creating a positive feedback loop and skipping some of the species that rely on their turn at the buffet as a leaf slowly falls apart.

These are alarming ideas even if my explanation is a touch simplistic. Unfortunately, systems being more connected and complex doesn’t bode well for a planet faceing less and less biodiversity paired with interrupted, rapidly altered processes. That’s your Halloween scare for the month.

But not to leave on a dour note, the next time you rake aside some leaves and find a bunch of creepy crawlies under there just think about what I’ve told you. Every year we turn around and find that the leaves of last year are mostly gone and a new crop is ready to fall, because of these unassuming, but massively important processes and organisms that are right in your yard or along the sidewalk. I think that’s just a treat.

HELP KEEP CHS PAYWALL-FREE
Subscribe to CHS to help us hire writers and photographers to cover the neighborhood. CHS is a pay what you can community news site with no required sign-in or paywall. To stay that way, we need you.

Become a subscriber to help us cover the neighborhood for as little as $5 a month.