What is an Old Growth Forest?
Old growth forests are ecosystems dominated by large trees at least 250 years old, and have all of the following characteristics present at the same time:
- Large living trees and a multi-layered canopy. The canopy is the leaf and branch layer of tall trees that form a roof over the forest community. In an old growth forest, the larger trees, two hundred feet tall or more, tower over the younger trees. Both grow together in a mixture of species. The uneven canopy is efficient at trapping moisture during the drier seasons. The huge trunks often survive fires, for they are reservoirs holding thousands of gallons of water and are protected by thick bark.
- Large standing snags. Snags are standing dead trees from which the leaves and most of the branches have fallen. Snags may stay erect for over two hundred years. As their branches fall off, sunlight is able to reach the forest floor, allowing species that require light to take root. Insects and woodpeckers open up the dead wood, providing habitat for many other species, which in turn become food for larger predators.
- Large down trees. Fallen trees help to hold soil in place, and as they decay over a period of two to five hundred years, many species of insects, birds and mammals use them for food and shelter. This activity helps raise the concentrations of nutrients like phosphorus and nitrogen in the rotting wood, which the rootlets of other plants can tap for food. Like live trees, fallen trees can hold large amounts of water.
- Large fallen trees in streams. Fallen trees crisscross small head-water streams. The run off is not strong enough to move them and they become temporary “stairsteps” that hold woody debris long enough for 70 percent of it to be processed by insects and bacteria. Fish consume these insects and rely on the pool-forming ability of the forest for shelter from run-off and for temperature control.
Any one of these characteristics may occur in younger forests, but only in old growth forests do all four occur. The old growth forest the students will be visiting is a western red cedar, western hemlock forest.
The Forest Community
A forest is organized in vertical layers. The top layer is the canopy, or roof of the forest community. Here leaves catch the sunlight necessary for trees to create food, release oxygen, and provide shelter and shade in the forest below. Below the canopy are the understory trees; young trees of the canopy species, and smaller, shade tolerant trees that will never become part of the canopy. In the old growth forest the students will be visiting in W-GIPP, western red cedar forms the canopy, while western hemlock and Rocky Mountain maples form the understory.
Beneath the understory is the shrub layer, where knee-high to head-high woody plants reside. Beneath the understory is the herb layer where ferns, grasses, wildflowers, and smaller woody plants grow. Here bunchberry dogwood, bracken fern and queencup are found, among others.
The forest floor is the bottom layer of mosses, mushrooms, creeping plants and forest litter (leaves, needles, twigs, feathers, bark bits, animal droppings, etc.). This is where decomposition occurs and where 95% of all insects live at some point in their life cycle. The final layer is the forest’s basement, laced with plant roots, mycelia of fungi (the threads which nourish the mushroom), and tunnels of animals such as the ground squirrel and shrew.
Each layer of the forest has its characteristic animal species, although most feed in more than one level and some nest in one story and feed in another. Every animal and plant consumes a portion of the available nutrients and has its place in the forest community food chain, directly or indirectly affecting all the other organisms.