The Main Structures of Old-Growth Forests
Structures are the physical pieces of an ecosystem, such as the trees in the
forest, and their arrangement.
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
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.
Deep, complex, and 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.