Forest ecosystems have three layers with specific roles in providing complexity and habitat for different organisms: the forest floor layer, the sapling layer, and the canopy layer.

Let’s start at the top: the canopy layer.

Looking up in a forest tells us much about the diversity of tree species present (composition) and how the trees are arranged in terms of size and form (structure).

The canopy also tells us much about the past, as tree form is often the result of growing conditions. Trees with many large lower branches, for instance, suggest a site was once unforested. Trees grew under high sunlight and were not forced to grow tall. The result was short, limby trees. In Northeast Michigan, many of those sites were once pastureland.

Individual tree health can also be evaluated by looking up.

Tree vigor is illustrated by crown size and foliage density. Stress, for instance, can cause crown dieback. Conversely, when we thin a forest to increase diameter growth for retained trees, we expect crowns to expand under the increased sunlight. More leaves correspond to more photosynthesis, thus more food for tree growth.

Looking up, we can also find evidence of the presence of different wildlife species in a forest.

One needs to look up to find stick nests used by crows, ravens, or raptors, or cavities developed by woodpeckers, which may be used by other birds, bats, other small mammals, and herpetofauna (reptiles and amphibians).

Looking down in a forest is also important.

The future of the forest, the seedlings, are in the forest floor layer. By looking down, we can evaluate whether habitat elements for specific organisms exist. For instance, large, downed logs (called coarse woody debris) are important as cover for herpetofauna, small rodents, and even small carnivores, such as our many species of weasels. Downed logs also provide drumming spots for ruffed grouse and substrates for many species of flowering plants and fungi.

In fact, dead material may drive forest biodiversity (2020, “Frontiers in Ecology and the Environment,” v.18, pages 505-plus).

The third layer of the forest is the sapling layer.

That layer consists of individual trees that are waiting for a disturbance to the canopy that will allow more sunlight through. Saplings take advantage of increased sunlight and grow taller before they grow larger in diameter. For many species of neotropical migratory birds, the sapling layer is important for nest placement.

How those different layers look and function differs by forest ecosystem type.

In northern hardwood forests, some tree species can handle more shade. Those “shade tolerant” tree species (such as American beech or sugar maple) can exist in the low light conditions of the forest floor or sapling layers. Thus, northern hardwood forests usually have many different species of trees of varying ages.

Conversely, natural pine ecosystems with an intact fire regime have only two to three age classes of a smaller suite of tree species and may have sparser forest floor and sapling layers.

To meet many landowner goals and facilitate biodiversity maintenance and climate adaptation, complexity in composition and structure are important across all forest layers.

Threats to one layer of the forest can impact other layers and their associated wildlife and plant communities. Common threats include exotic and invasive plants and animals. Autumn olive, for instance, can dominate the forest floor and sapling layers and decrease the likelihood of recruiting trees to the canopy.

Native wildlife can also impact forest layers.

For instance, intensive and extensive browsing by white-tailed deer and elk impact the forest floor and sapling layers, and thus negatively impact habitat elements for many other wildlife species. Ungulate browse is one of the reasons preferred browse species such as eastern hemlock, yellow birch, and sugar maple are so difficult to regenerate in some area forests and why tree species that are not eaten, such as ironwood, proliferate.

Next time you are in a forest, perhaps look up and down to appreciate all that is there.

Greg Corace is the forest and wildlife ecologist for the Alpena-Montmorency Conservation District. For more information, including assistance with the Qualified Forest Program and related forest planning and management, email greg.corace@macd.org.