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What happens to all the leaves?
December 2012 www.kenaiwatershed.org
By Dr. David Wartinbee, Kenai Peninsula College – Professor of Biology, KWF Board of Directors
There are estimates that large trees can produce more than 200,000 leaves each year. As Fall approaches, valuable nutrients are extracted from the leaf and drawn into the trunk and roots. During that removal of nutrients, changes in biochemistry within the leaf, and changes in temperatures, we get the beautiful “fall colors”. Then the millions of leaves are dropped and they cover the ground some distance from the parent trees.
The leaves pile-up along the ground and many get blown into our rivers and streams. The smaller streams receive a larger proportion of leaves than larger rivers since smaller streams may be completely shaded by overhanging limbs & trees. Leaves falling into moving waters get swept into piles above rocks, roots, or in-stream obstructions. Other leaves settle-out into deeper, slow-moving pools.
Initially these leaves contain mostly cellulose and are of little nutritional
value for most organisms. ( While cellulose is a polymer of glucose, only a few bacteria or fungi can break down cellulose. ) However, after soaking in the stream for only a few days, the leaves become covered by aquatic fungi and bacteria that are able to break-down cellulose. As the fungi & bacteria layer grows, the overall nutritive value of the leaves increases significantly. Now the leaves become a desired food source for a variety of in-stream insects. The fungi and bacteria are the source of the nutritive value…and were once described by Stream Ecologist Ken Cummins as the “peanut butter on a tasteless cracker”.
The guild of aquatic insects that work on those leaves are called “shredders”. Basically, they feed on the leaves and convert the leaf into a skeleton of its former self. The fine particles that were chewed from the leaves provide nutrition for the growing insects. A couple of the common shredders in our area are the large, dark stoneflies called Pteronarcella. We don’t often see them unless we dig into a pack of leaves and specifically look for them. Another group of shredders are called craneflies. As fat, worm-like larvae, they borrow through the leaves and chop them into fine particles. ( Craneflies are often see as adults during the summer since they look like giant mosquitoes buzzing around. Note that they don’t feed as adults. )
The particles that the shredders create from leaves will pass through their gut mostly intact. Insect guts are able to extract only about 5% of the leaf particles that passes through. So, there are lots of small particles released into the water by these shredders. The fine leaf fragments are then a food source for another guild of aquatic insects called collectors. Collectors trap the fine particles drifting with nets, leg hairs, or special antennal fans. They then consume the particles and pass them through their gut for a second round of nutrient extraction. Some of the common insects that use these fine leaf particles are the mayflies, many chironomids, many caddisflies, and the black flies. After the particles have passed through a number of guts, they are then so small that it only takes bacterial action to completely transform the leaf particles. Eventually all the sugars and nutrients that were in the leaf have been transformed into a water soluble or animal form. The leaves no longer exist.
This spring when the ice melts from our rivers and streams, there will be only a few fall leaves left from September. They will be gone. Where have they gone? During the coldest time of the year, the aquatic insect community, the collectors and the shredders, has been busy feeding on
the leaves that arrived a few months earlier. These stream insects that fed on leaf particles all winter will become the food for young salmon fry as they emerge from stream gravels in the spring. Leaves truly provide some of the fuel needed for a healthy river system. Without one piece of the puzzle, the aquatic insects or the leaves, the system ceases to function as a healthy river system.