By David Wartinbee, for the Redoubt Reporter
Photo courtesy of David Wartinbee. Fall leaves bring nutrients to the forest, from fungi to a variety of insects.
Leaves are falling. Who could miss it? This time of year, some roads can be completely covered, and clouds of billowing leaves get lofted into the air by every passing car.
All too soon our birch, alder, aspen and cottonwood trees stand naked as snow covers the ground. Considering that the average tree can drop over 200,000 leaves, it’s no wonder they cover the ground with their fall yellow, tan and brown colors.
Have you ever wondered what happens to all those leaves? I am sure you have noticed that while there are always a few leaves to be found on the forest floor, there is no evidence of years and years of leaves piling up under the forest canopy. Why aren’t we wading hip-deep through many years of leaves piled on the ground?
The answer to where they went is fairly straightforward. Many of the leaves were partially consumed by a variety of insects before they ever left the tree. Fallen leaves often have insect chew holes, gnaw marks or evidence of leaf-miner caverns. Once the leaves pile up on the forest floor, they get rained on and packed together. Now the fungi, bacteria and a variety of invertebrates start breaking down the newly fallen leaves.
Threadlike mycelia from a myriad of different fungi grow into the moist leaves. These mycelia secrete cellulose digesting enzymes that break down the leaves. Then the invertebrates, like beetle larvae, earthworms and amphipods, do their jobs. They chew up the leaves and also feed on the fungi. Within a year’s time, most of the leaves have been transformed into an unidentifiable mass of organic humus in the forest soil. Those leaves on the very top of the pile may dry out often enough that fungi and invertebrates won’t break them down this year.
However, next year when they are on the bottom of the leaf pile, they’ll get broken down first. Eventually, all the leaves become part of the soil’s humus and increase the moisture-holding ability, as well as the overall quality of the forest soil.
Because moving leaves tend to work their way downhill, many enter streams and lakes in our area. Leaves that enter stream or lake waters will also be transformed within a year or so, just like those in the terrestrial areas. The aquatic processing requires fungi and bacteria to break down the cellulose, and there are several aquatic insects that specialize in chopping up the leaves into smaller fragments.
When leaves first fall into a stream, they are unpalatable to aquatic insects. However, once soaked in the water for a few days, the fungi and bacteria start to infest the leaves. With a new layer of fungi and bacteria on a leaf, the total leaf protein content actually rises significantly, and various insects will now begin to eat them. The first aquatic insects on the scene will begin the breakdown process by chewing up the leaves into fine fragments.
The aquatic insects that specialize in grinding up whole leaves are called shredders. This guild of insects can often be found within leaf packs that pile upin front of rocks or logs. Their work is to skeletonize the leaves, leaving only the thick stems and a lacelike outline of the initial leaf. Some of these are fairly large and distinctive aquatic insects, and include the inch-long cranefly larvae, or some even larger stonefly nymphs.
The leaf fragments from shredders pass through their guts with very little of the nutrients extracted. This shouldn’t be considered remarkable, since most aquatic insect guts will extract less than 5 percent of the energy contained in and on the leaf fragments.
Another guild of aquatic insects called collectors start gathering the excreted fragments. They use a variety of techniques to filter the fine leaf particles out of the water column. Some mayflies use special hairs on their legs to collect the particles while others form fine mesh nets; like a miniature set-net site. The larvae of pesky blackflies have a specialized set of antennal fans that are able to filter out ultratiny leaf fragments. Once collected, the leaf fragments are passed through the gut and released into the water column for someone downstream to collect and use again.
Since each insect is able to assimilate so little of the energy in each leaf particle, these fragments often pass through dozens of insects before they are completely degraded. This process of passing through sequential feeders is called nutrient spiraling. Some streams have shorter distances for this spiraling due to an increased number of inhabitants. Due to the cold temperatures of our streams and the relatively low insect diversity, we would expect the spiraling to require a long distance downstream and a large number of consumers to be involved before particles are consumed entirely.
The attacking of these leaves by fungi and bacteria can be impacted by the chemical nature of the receiving stream. Those that are slightly acidic will process leaves more slowly than those with more neutral waters. Areas with low oxygen levels, such as found on the very bottom of deeper lakes, can also slow down the processing speed. Whether leaves fall onto the soil or into the water, they will be broken down into their component molecules, and elements that in turn can be used by plants when forming leaves in subsequent seasons.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.
