Well-structured "layers" of a forest (canopy, understory trees, shrubs, and field layer) provide maximum habitat for animals.
Photographer: Sam Sheline, courtesy of NatureServeIn this field guide, we categorize plants as trees, shrubs, saplings, vines, or low plants. Trees no more than a few years old are referred to as saplings, and baby trees as seedlings. These roughly correspond to height categories.

It is not unusual for ecologists to use the term "layer" to refer to a height category. The canopy (or canopy layer) refers to the tallest trees in a forest (which also get the most sunlight). The understory is everything that grows in their shade, including shorter trees. The shrub, sapling and vine layer is the medium-height category. We use the term field layer to mean any low plants on the forest floor—including not only non-woody (herbaceous) plants such as ferns and wildflowers, but also short tree seedlings or very small shrubs.

Ovenbird (Seiurus aurocapillus) spends a lot of time on the forest floor.
Photographer: S. Maslowski, courtesy of US Fish and Wildlife ServiceCategories like these are significant, because plants of different height categories play specific ecological roles—as wildlife habitat, for instance. Birds that nest or forage on the forest floor (eastern towhees or wood thrush, for example), rely on a moderately dense understory to protect them from predator’s eyes. Bird watchers in the eastern U.S. could expect to find these birds in a natural community or successional community that has a reasonable cover of shrubs, such as the Basic Mesic Hardwood Forest (Coastal Plain / Piedmont Type), Oak - Beech / Heath Forest, certain examples of the Mesic Mixed Hardwood Forest, or the Successional Tuliptree Forest (Rich Type).