Each autumn’s colourful mantle of newly fallen leaves adds to a mass of organic decay commonly exceeding the bulk of all living things in the forest. Together with fallen deadwood, this immense bed of compost harbours unparalleled biological diversity and nutrient riches for new growth on land and life in water.
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Rain and moisture, aided by frosts and thaws, leach up to a third of a fallen leaf’s contents within two or three weeks. Fungi colonize the new litter immediately after it falls, together with bacteria consuming the lion’s share of it, even beneath the snow in winter.
Vast multitudes of mites, springtails, worms, and other invertebrates accelerate decomposition, riddling litterfall with tiny holes, which are soon invaded by microbes. Leaves can take one to several years to break down, leaving ever tinier, tougher, slow-moldering particles of humus that swarm with grazing and predatory microorganisms. Most of the forest’s greatest, if unseen, masses of creatures—beetles, ants, centipedes, spiders, salamanders, and shrews—all line up in the long detritus food chain.
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Leaves provide up to 90 per cent of the energy for life in the headwater streams in which they fall. Litter nutrients pass from aquatic fungi and bacteria to insect larvae, crayfish and their predators, or collect in sediments, sustaining aquatic plants in shallows. Runoff from the forest floor and snow melt-fed ground water seepage carry another jolt of nutrients during the spring freshet.
Later in autumn, a lake’s surface is just as cold as its depths, and winds can churn the entire water body, circulating leaf sediments normally stuck at the bot- tom. The flushes of nutrients create sudden blooms of invisible planktonic algae in the middle of lakes, in turn feeding an explosion of minute crustaceans and other zooplankton eaten by insect larvae, minnows, and the fry of larger fish.
This article was originally published in the October 2020 issue of Cottage Life.
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