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Forest Stratification

Forest Stratificationsimply refers to the different layers of plants in a forest. In a

mature forest, one can typically see several distinct layers of vegetation rising from the

forest floor to the tree canopy.

Young forests may not show clear separations between layers. It is only as forests ageand trees grow to create a tall canopy that layering becomes most visible. To learn more

about forest layers you can investigate the interactive figure and text below.

Canopy LayerPrimary Vegetation:Mature Trees

The canopy, sometimes called the "overstory", is the highest vegetative layer in the

forest. The canopy is filled by leaves deployed from large mature trees. During the

growing season, canopy leaves intercept much of the sunlight available to a forest.

Typically less than 50% of the total amount of sunlight can pass through the canopy to

plants in the forest layers below.

In a deciduous forest, the canopy is typically the last layer to show green in the spring.

Since the canopy trees receive sunlight throughout the growing season, they can wait

longer to deploy their leaves. This reduces the risk of the young tender leaves being

destroyed by a late freeze.

Some trees grow especially tall. Sometimes these tall growing trees can actually grow

through the canopy. Giant trees that poke through the canopy are called "emergents."

Emergents are able to harness immense amounts of unfiltered sunlight above the canopy.

However, the leafy crown of emergent trees are openly exposed to fierce winds that howlabove the canopy. Therefore, emergents are susceptible to be being blown down during

gusty storms.

Understory LayerPrimary Vegetation:Tree Saplings, Small Shade-Tolerant Trees (ex. dogwood, redbud,

musclewood) & Tall Shrubs

Just beneath the Canopy and above the Shrub layer lies the Understory. The

Understory can be thought of as a tree sapling staging ground. In a mature forest, many

saplings can claim enough nutrients and sunlight to reach the Understory. However,further growth is typically impractical as the saplings can not steal enough additional

nutrients from established canopy trees to grow any higher. So many saplings slow their

growth and wait in the Understory until a mature canopy tree dies. How well a sapling can

grow in full shade and how long a sapling can survive in the understory are two principle

measures of a tree's shade tolerance.

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When a mature tree dies and opens a gap in the canopy, all of the saplings waiting in

the understory rush upward. The saplings quick growth is fueled by the sudden increase

in sunlight and nutrients no longer claimed by the deceased tree. The race to reach the

canopy is very much a race for survival. There is typically only room for one new tree in

the canopy. The tree that reaches the canopy continues to grow and expand, gradually

reducing the flow of sunlight and nutrients to the trees below. All saplings thatcommitted to the growth race but failed to reach the canopy gradually weaken and

eventually die.

Shrub LayerPrimary Vegetation:Young Tree Saplings, Mature Shrubs and Bushes

Between the Understory and Litter Layers is the Shrub Layer. This low lying layer of

vegetation is typically between 3' and 7' from the ground surface. Mostly bushy shrubs

occupy this position in the forest. An abundance of food for animals such as deer and

bears is found on shrub layer vegetation. In fact, many of these shrubs depend onwildlife to distribute their seeds. The animals ingest the plants' fleshy fruits and

distribute the seeds in their feces.

Herb/Fern LayerPrimary Vegetation:Tree Seedlings, Herbs, Ferns, Grasses and Weeds

The Herb/Fern layer ranges from the ground surface to about 3' and is considered the

lowest forest layer with leafy living vegetation. This layer is typically the first forest

layer to turn green in the spring. Plants on the forest floor have to deploy their leaves

early in the growing season to capture direct sunlight to kick-start their growth cycle.Once the understory and canopy trees have deployed their leaves, very little sunlight

remains for plants in the Herb/Fern Layer. Most of the plants in the Herb/Fern layer

have short life cycles.

Litter LayerPrimary Vegetation:Decaying plant and animal matter, Fungi, Mosses & Lichens

Alas, lying directly on the forest floor is the final forest layer, the Litter Layer. The

litter layer is the repository of all of the dead matter in the forest. As leaves, trees and

other plants die, they fall from upper layers and land on the forest floor. Here a host ofbacteria, fungi, worms, insects and other waste consumers chow down on the rubbish to

create new nutrient-rich soil.

The newly released nutrients are then extracted from the soil by the plants in the

forest. The plants use the nutrients to create new leafs and organic matter that will once

again return to the Litter Layer. This recycling of matter creates an on-going balanced

cycle that ensures the long term sustainability of the forest.