Fallen Leaves: A Tree’s Gift to the Stream

There were still some green leaves hanging onto the trees when this photo was taken in Lancaster County, PA in late October. Newly fallen leaves rested on the tops of rocks, waiting for a gust of wind or higher waters to pull them into the stream (Photo Credit: Rebecca Lauver).

After a gorgeous season of fall foliage, the leaves of our deciduous trees turned brown and have fallen. Once they are on the ground, it can be easy to forget about the ecological importance of these leaves. However, the second portion of the leaves’ lives have just begun- now is their turn to give back to the ecosystem that formed them!

Once tree leaves fall to the ground, they turn into leaf litter. Leaf litter provides food and shelter for the organisms below and improves soil health. Whenever it is possible, it is always good to leave leaves on the ground where they are! If these leaves happen to be near a stream, there’s a good chance that some of them will make their way into the water. Suddenly, the leaves find themselves in a wet environment filled with new microbes, aquatic insects, and a wild ride of riffles and runs.

Once in the stream, there are 3 processes that leaves may undergo: leaching, conditioning, and fragmentation. Leaching is when the soluble organic compounds in a leaf are pulled into the water. Stroud Water Research Center has coined the combination of leached organic compounds and stream water as “watershed tea.” Just like when you stick a tea bag into a mug of water and see the tea disperse throughout the hot water, so do the nutrients from the leaves disperse throughout the stream. The dissolved organic compounds provide important nutrition for the microbes and other organisms within the stream.

You can see a variety of oak leaves, american beech leaves, and tulip poplar leaves and seeds on the streambed. Pools of stiller water can also result in pile ups of leaves (Photo Credit: Rebecca Lauver).

Another process that the leaves might undergo is called conditioning. When a leaf is conditioned, fungi and bacteria in the water start to break down the leaf tissue and absorb nutrients for themselves. A conditioned leaf is easier for some other organisms to feed on. In a way, conditioning ‘cooks’ the leaves, making it an easier meal for other organisms, like aquatic macroinvertebrates, that may need some help with their digestion process.

The final route a leaf can take is fragmentation. This can happen when fungus takes in parts of the leaves, by physical abrasion from the stream, or by being shredded by aquatic macroinvertebrates (Abelho, 2001). You could imagine that during a flood, a leaf would get tossed around and caught on branches and rocks in the water, ripping it into smaller pieces. There is also a feeding group of macroinvertebrates called shredders (in addition to scrapers, predators, and collector/gathers) that rip leaves into smaller pieces as well. Shredders feed on leaves that fall into the stream, breaking apart this coarse particulate organic material (sometimes denoted as CPOM) into fine particulate organic matter (FPOM) that another feeding group, called collector/gatherers, can then feed on. Pretty much they take big pieces of leaves and turn them into smaller ones that other organisms can eat as well. Shredders include some caddisflies, some stoneflies, amphipoda, some beetles, and a range of other macroinvertebrates.

This debris dam is formed by larger branches intersecting across the stream. Water trickles through the leaves and through a few open gaps to the sides and under the leaf build up. (left) This leaf pack hides a plethora of tiny macroinvertebrates and microbes between the folded leaves (Photo Credit: Rebecca Lauver).

In order for leaves to undergo any of those 3 processes that were just mentioned, they have to be held in the water for an extended period of time. The structure of a stream, from the way it curves to the distribution of riffles and pools, influences how many leaves it can capture and how long it can hold them for. Most leaves travel between 0-70 meters downstream before they are trapped (Abelho, 2001). One way leaves can be caught within a stream is by getting snagged by another one of trees’ gifts to the streams: woody debris. Woody debris can include anything from a fallen tree trunk to smaller branches. You’ve probably seen instances where a large branch fell across a stream, resulting in a backup of other branches, twigs, and leaves. This buildup is called a debris dam and it is another important feature of a healthy stream.

Within a debris dam, you may have also noticed that there were a lot of leaves piled up on each other. These pile ups are called leaf packs and they can also occur on a smaller scale behind rocks or smaller branches in a stream. Leaf packs are hot spots of macroinvertebrate and microbial activity and are locations where leaching, conditioning, and fragmentation can occur. If you are curious to learn more about leaf packs and discover some educational tools around leaf packs, check out Stroud’s webpage on the Leaf Pack Network!

Just as there are different mixtures of tea for human consumption, there are also different mixtures of “watershed tea.” The varying species of leaves falling into streams creates unique mixtures for the organisms awaiting their own tea. This is another reason why encouraging native species, rather than nonnatives, along streams is crucial; the organisms within the stream rely on a specific tea combination that nonnative leaves can’t create.

A cranefly was hiding out under a submerged leaf pack. (left) If you look closely, you can see a small stonefly within the red circle on this oak leaf (right) (Photo Credit: Rebecca Lauver).

Different species of tree leaves also decompose in their own way. For instance, alder leaves generally decompose quickly while some species like oaks take much longer due to the chemical and physical properties of their leaves. Having a diversity of native leaves within a stream ensures that a diversity of organisms will be supported.

There are many potential paths for the deciduous leaf detached from a tree after a spring and summer of growth. Perhaps it fell to the ground in your backyard where it provides cover for small critters through the winter, or in an upland forest where it will gradually be turned into soil by terrestrial processes that mirror aquatic ones. Or perhaps it fell into your neighbors stream where it will now be leached, conditioned, and fragmented by the awaiting aquatic organisms. Whatever its path is, it now gives its nutrients back to the local ecosystem- another present during this holiday season.