The hidden fungus.

Step 1 of 6

Fruit of a hidden body

The red mushroom with white spots is a fly agaric, and beside it, on the litter, are small bonnets. But what you see above the ground is not the whole creature, any more than an apple is the whole apple tree. Choose Underground under Look underground, and the soil washes out of the front half of the block.

Step 2 of 6

Eating from outside

The white threads are hyphae, each a few thousandths of a millimetre across, and together they make a mycelium. A fungus has no chlorophyll, and its cell walls are made of chitin, the stuff of insects’ shells. In the tree of life fungi are closer to animals than to plants. But they cannot swallow anything.

Step 3 of 6

Warmer and faster

The bonnets are decomposers: their threads grow into fallen leaves and digest them. The lower chart follows a leaf that falls today and loses mass over the years, and its half-life is the time it takes to lose half. Like most chemistry, the fungi’s work speeds up about $2.2$⁠ times for every 10 °C, up to about 30 °C. Now the soil is moist, at 10 °C, and a leaf’s half-life is about a year and a half.

Step 4 of 6

The best soil

Fungi need water: their enzymes and the food they absorb move in thin films of water round the soil grains. In dry soil those films break, and the threads stop working. Soil moisture is the share of the spaces between the grains filled with water, and it is at 25%, dry, for now. The upper chart shows how fast the fungi work at each moisture.

Step 5 of 6

Too wet

If water is good for fungi, is more water better still? The threads, like us, breathe oxygen, and the soil’s air sits in the same spaces the water fills.

The litter on the floor is a matter of balance: leaves fall at a steady rate, and each year the fungi break down a share $k$⁠ of what lies there. Litter builds up until the two rates match, $L = F/k$⁠, so when rotting slows, litter piles up. That is how peat bogs form.

Step 6 of 6

Trading with the tree

The fly agaric is not a decomposer. It lives in a mycorrhiza, a “fungus-root”: its threads sheathe the tips of the birch’s fine roots and run out from there into the soil, far beyond anywhere a root reaches. Watch the dots moving along the threads: amber one way and sky blue the other.