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Summer 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

July 10, 2012

The upside of downed trees


The violent derecho that ripped through Washington may have created human misery, with smashed roofs, power outages and spoiled food, but it generated a windfall for the organisms that live in and feed on fallen limbs and logs.

The rate at which woody debris is consumed depends on the species of tree (pines break down faster than oaks) and where and how the tree falls.

fallen log suspended above ground

Logs suspended above the forest floor by side branches decay more slowly than logs lying in contact with the ground, where they absorb more moisture.

fallen log lying on ground

Compared with logs falling upslope or downslope, logs falling parallel to the contour of a slope will rot faster because they are more effective at catching and holding debris and water, which also helps slow erosion.

fallen logs on a slope

After debris falls, the wood initially dries out and cracks, creating thoroughfares for insects to enter. As beetles, ants and termites traipse into the log, they introduce microbes and fungal spores that contribute to the breakdown of the wood.

A log's nutrient-rich inner bark and sapwood are the first to be exploited, causing the naturally resistant outer bark to slough off. The log's hard, nutrient-poor heartwood is the last to disintegrate.

log anatomy
Life in a rotten log

Persistent gnawing and decay eventually creates spaces in the log large enough for vertebrates to use as shelter. During hot, dry summers, rotting logs provide cool, damp havens for myriad animals and serve as a moisture reservoir for plant roots.

The log is finally rendered into a fertile mound of dark, spongy, mineral-rich, carbon-sequestering humus. Much of its fertility is due to the biologically diverse activity that deposited nitrogen-rich waste in and around the collapsed log, turning it into an ideal nursery for tree seedlings.

Source: Quinney Natural Resources Research Library

Flow of water

Parallel with slope contour

Downslope

Outer bark

Inner bark

Sapwood

Heartwood

Pileated woodpecker feeds on carpenter ants.

Ambrosia beetles cultivate fungi in chambers.

Bark beetle larvae chew galleries along surface of inner bark.

Shrew

Shelf
fungi

Moss

Termites

Earthworm

Oak seedling

Centipede