Leaf Fall

Last weekend in Northwest Arkansas we had our first real taste of fall weather. Temperatures dropped from the low 80’s into the 40’s and even 30’s. Then a misty rain and a breeze came along and provided a perfectly miserable couple of days. This cold snap is a reminder that our trees will soon start turning and then shedding their leaves for the winter. We are all set for three to four weeks of glorious color followed by a blizzard of falling leaves.

At the Morgan house, we can let leaves lay where they fall in the back of our lot. That practice provides cover for insects through the winter and as a result food for over-wintering birds and other small animals. The leaves also return valuable carbon and nutrients into our soil for the next season. However, neighborhood convention is that lawns be cleaned of dead leaves to keep up a neat appearance. So, we go along with the neighbors. The leaves from our lawn are collected and chopped up. The first group is used to fill the compost bin that has become depleted over the summer. The rest are put into wire cages and allowed to age for a couple of seasons. Eventually, Sharon spreads that leaf mulch in her native plant beds. No fertility is allowed to leave the property. If we are lucky, we even get some of our neighbor’s leaves that blow up against our fence.

Beyond providing for a few weeks of color and sightseeing, leaf fall is an important event for both the terrestrial and aquatic ecosystems. Leaf fall is a part of cycling of minerals and nutrients through our ecosystem. An idea of the importance of the event is simply the mass of material that is involved. Take the Ozark National Forest as an example. The forest covers roughly 1.2 million acres (https://www.stateparks.com/ozark.html). To simplify this calculation, I am assuming that the forest can be characterized by Red Oak. That is not a good assumption, but it will provide an order of magnitude estimate. Next assume that 75% of the area in the forest is actually forest. Leaf area index is the ratio of the surface area of the leaves on a tree (one side only) to the surface area of the ground covered by the tree’s spread. Breda (Journal of Experimental Botany, Volume 54, Issue 392, 1 November 2003, Pages 2403–2417,) estimated the leaf area index for a Red Oak in a dense forest as 4 to 4.5. So for every acre of trees, there are 4 to 4 ½ acres of leaves. The weight of Oak leaves according to Jurik (Amer. J. Bot. 73(8): 1083-1092. 1986.) is around 40 grams per square meter. Doing a little math to convert everything to metric and multiplying, it turns out that just shy of 600 million kilograms (1.3 billion pounds) of leaves fall in the Ozark National Forest each autumn. Those leaves may be up to 13% protein (https://www.fs.fed.us/database/feis/plants/tree/querub/all.html). Rounding to 10% protein and multiplying, it is found that 60 million kilograms (130 million pounds) of protein fall within the forest each year.

Given the mass of material falling as leaves, it is not surprising that leaf fall is a significant source of energy in forested ecosystems. For headwater streams (those near the streams source) in forested watersheds, leaf fall may be critical to the energy balance for the entire year with respect to the aquatic community. In those streams the forest canopy covers nearly the entire stream blocking sunlight. With little direct sunlight reaching the stream, there is little photosynthesis taking place within the stream. Leaf fall into the stream makes up the shortage. The leaves provide a source of carbon, nutrients and minerals to the stream system. Vannote et al. (Canadian Journal of Fisheries and Aquatic Sciences, 1980, 37(1): 130-137) noted this relationship in their 1980 paper, the River Continuum Concept). It was pointed out in the paper that stream ecosystems could not be studied simply on a square meter basis like lakes. Streams flow. So it is necessary to consider where along the continuum of the stream the square foot being studied existed. Headwater streams, without photosynthesis were dependent upon material falling into the stream from the watershed, primarily leaves and woody debris, for energy.

When leaves fall into a stream or other body of water, they are quickly colonized by fungi and micro-organisms which start the decomposition process. As the leaves decompose, they become more palatable and soon macroinvertebrates (bugs) start munching. Those bugs are then eaten by larger organisms in the familiar food pyramid. Left over parts from the leaves and waste products from the bugs and fishes drift on downstream helping to support those communities as well. Leaves then are a critical part of the nutrient and carbon cycling in streams.

Good streamside management maintains a healthy forest in the riparian zone. In headwater streams especially, that forest helps to maintain a healthy stream including reinforcing the streambanks and providing for the aquatic community. Healthy streams are better able to process nutrients and provide us all with clean, dependable water.

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