The Main Structures of
Old-Growth Forests

Structures are the physical arrangement of various pieces of an ecosystem, such as the spacing of trees in the forest. Old-growth forests have four main structures.

• Big trees--The huge trees are the main "factories" of the old-growth forest, because the trees bring energy into the forest through photosynthesis. The trees also are storehouses. Each tree stores many tons of organic material and nutrients, which are eventually recycled back into the ecosystem. The big trees are also the source of the next two structures.

• Large snags (standing dead trees)--The big trees die from tree diseases such as heart rot and root rot, or are killed by lightning strikes or insect damage, or their tops are broken off in a windstorm or snowstorm. Snags are used by many different kinds of wildlife, including pileated woodpeckers and spotted owls.

• Large fallen trees on the forest floor--Fallen trees decay on the forest floor. It takes many decades for a fallen tree to decay completely. As the fallen trees decay, they become homes for many living creatures, including carpenter ants, folding-door spiders, centipedes, salamanders, and shrews. Mushrooms and other fungi grow on the rotting trees, and eventually the rotten trees turn into nurse logs, as young trees grow on top of them.

• Multilayered or continuous canopy--The big old trees have large branches and deep crowns. Younger, smaller trees spread their branches through spaces between big trees. Shrubs such as rhododendrons create another layer. An old-growth forest has so many layers of branches that the canopy is essentially continuous. Lichens and mosses that live on these branches survive on rainwater and moisture from the air.