The Other Side of Fall Leaves

By Steve Midway

Around this time of year throughout much of the northern hemisphere, we take pause to notice the beautiful spectrum of colors presented in fall foliage.  Have you ever wondered what happens to all of those leaves, where they go, and what types of animals may benefit from this fall bounty?

Leaves change from green to yellow to red before being deposited
in rivers and fueling a long-term food chain. (Source)

As a quick refresher, throughout the fall the green chlorophyll that photosynthesizes energy for trees all spring and summer is being replaced by other pigments in the leaves in a predictable sequence.  The yellow pigments appear first, as the yellow compounds are actually present in the leaves along with the green chlorophyll all year long—just not enough to overcome the green pigments.  Next come the orange, then the red pigments, which, unlike the yellow, are actually created in the fall.  Rainfall, temperature, sunlight, and a host of other factors influence the timing of fall leaf color change, but ultimately photoperiod (hours of sunlight in the day) is the main driver causing leaves to change and fall regardless of other conditions.

Not all leaves change at once, but rather, they
change over a period of weeks. (Source)

Okay, now that we are all up to speed on leaf colors, what does all this have to do with fish?  Actually, a lot.  Following the dark red colors, the trees no longer need the leaves, so they are released.  As useless as these leaves may seem to the tree, they are still chock-full of organic matter and other potential foodstuffs for others to enjoy!  When they fall in our yards, we rake them and collect them, but when they fall in or near a stream, they represent a huge addition of nutrients to the system.

Fallen leaves entering flowing waters represent
the start of a stream-wide feast. (Source)

For tree-lined waterways, the annual leaf-dump initiates a chain reaction of feeding that helps sustain all levels of biological diversity and abundance through the tough winter months.  When leaves first enter the water, they leach out dissolved organic matter, DOM.  This is the brown stuff that stains puddles the color of tea, or that brown leaf-stamp that imprints on your car (for those of us who forget to wash our cars in the fall). The DOM is mainly available to bacteria and fungi, but this is only the beginning of the process; the ecosystem-wide meal is still to come.

One of the best ways to see the fall color is along a stream or river. (Levan)

Both the bacteria and fungi and the now-decomposing leaves serve as a major food source for the aquatic invertebrate community.  This can include a wide range of species—for example many anglers will be familiar with stonefiles and caddisflies.  As the leaves continue to break down, both shredder and collector insects further process the leaves for food and shelter.  In turn, these aquatic insects are the main food source for a healthy fish population.

Stream invertebrates like these caddisfly larvae are happy to shred up
decomposing leaves and make them into homes. (Source)

You don't have to look far to realize that invertebrate diversity and abundance is a near-critical requirement for stream fish.  Of course some fish eat other fish, but at the lowest level, fish need a healthy supply of aquatic insects.  And this food supply is largely dictated by the quantity and quality of the annual leaf fall.  So as you take a moment to enjoy this year's autumn foliage, consider the importance of the leaves not only while they are on the tree, but also their importance after they fall, and all the hungry little fish that are eagerly staring up into the orange and red sky along with you.

After months of leaf processing, the improved health of the
aquatic insect population directly benefits fish. (Source)

Note: Numerous studies have confirmed the importance of leaf matter to stream ecosystems.  For those interested in more information, the links below represent a sample of some of these studies.

Wallace et al.—declines in abundance and biomass of invertebrates.

Graca—a review of invertebrates and leaf litter.

Woodward et al.—using leaf litter to evaluate stream health.

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Check out the video below, where some fish, like grass carp that are native to Asia, actually eat the leaves as they fall onto the water: