Forest Layers: The Understory

Part I: Definitions

Forest Ecosystem: All the living and non-living things that reside within a specific woodland area characterized by the processes and interactions between them

Biological diversity: The variety of plants and animals, the communities they form, and the ecological functions they perform at the genetic, stand, landscape, and regional levels.

Understory: The plants growing at the lowest layer of the forest, typically growing only a few feet high; this layer includes mosses, herbs, small shrubs, and ground-level vines. Other names for this layer include the herb layer, herbaceous layer, and ground layer.

Midstory: The layer of plants growing between the understory and overstory, including tall shrubs and lower canopy trees up to 20 feet in height.

Overstory: The plants growing at the tallest heights and making up the topmost layer of the forest, including trees and shrubs over 20 feet in height. The overstory is also referred to as the upper canopy.

Herb: Small, vascular plants that are found in the understory. Herbs do not have woody stems. They may be annuals, biennials, or perennials, and they do not necessarily fulfill culinary roles for humans like cultivated cooking herbs.

Native: Plants (or other species) that occurred within this region before settlement by Europeans. Native plants include the ferns, grasses, perennial and annual wildflowers, woody trees, shrubs, and vines that covered Penn's Woods when the first settlers came.

Invasive: According to the Pennsylvania DCNR, an invasive plant is one that is not native to an area, spreads quickly, and can cause environmental, economic, or harm to human health. Invasive plants have been brought into areas, and this can happen accidentally or on purpose. They are often referred to as "exotic," "alien," "introduced," or "non-native" species. In their natural range, these plants are limited by factors that keep them in balance, including pests, herbivores, or diseases. However, when introduced into an area where these limitations are absent, some species can become invasive.

Part II: What is the Understory Layer?

The understory refers to the vascular plants, called herbs, that make up a component of the forest at the ground level. Herbs in the forest are different from culinary herbs; although you can eat some of them, "herb" refers to the form and role of the plants in this part of the forest. Herbs are vascular plants that have tissues distributing nutrients throughout the plant, and typically have roots, leaves, and stems. Herbaceous plants do not have woody stems, unlike shrubs and trees.

The understory can vary greatly between different forest types and even within the same forest. While some understories may be dense and crowded with plants, others may be more sparsely populated. Regardless of the density of the layer, herbaceous plants and the layer often serve similar roles in the forest. Although these plants may seem small and inconsequential, they have a measurable impact on the productivity of the forest, and many other species rely on their presence.

Variation in the understory is part of what creates differences among Pennsylvania forests; broadly speaking, there are four distinct major types of forests in the state, but if we look at the ground-level and the different herbaceous species, we can type out more than 50 forest types. Uniqueness among the herbs of the forest can help us distinguish between ecological conditions in the forest, like between dry sites and wetter, bog-like sites.

Part III: Roles of the Understory

Biodiversity

Forests, at the broadest of scales, represent some of Pennsylvania's richest wealth of biodiversity. These are the hotspots for many important plants, animals, and other organisms in our state. A diverse understory contributes to a forest's overall biodiversity.

Biodiversity impacts human beings, often in subtle ways. First, plant diversity helps cycle nutrients throughout the entire ecosystem. Different plants utilize different kinds and quantities of nutrients that are taken from the soil and passed into their tissues. Having a variety of nutrients available in plants to herbivores, like deer, can help improve health for wildlife populations, which we can all benefit from. Second, biodiversity is also linked with carbon storage. Plants of all types, not just trees, can store carbon in their tissue. It is particularly good for humans when trees do it because the carbon is stored for the long term, but even herbaceous plants can store carbon and help cycle it through the ecosystem. Thirdly, genetic diversity refers to the differences among individuals within a single species. Variation in traits means a population of plants has a better chance of some individuals surviving environmental and ecological challenges, like drought or pest pressure.  Lastly, biodiversity in our plants can support diversity among other groups in the forest, like birds, mammals, and more. The more diverse the plant community, the more wildlife, fungi, and insects can use those plants for food, shelter, nesting sites, and overwintering.

Cultural

Aside from the many roles and functions that the understory serves in the forest ecosystem, humans have relied on this part of the forest for resources as well. One way that humans have used forest herbs for centuries is foraging. Mayapple, ramps, and wild yams are all herbaceous plants found in forests across the state that have been used by humans in Pennsylvania for hundreds of years. Other forest herbs are important to people for health-related reasons. Indigenous peoples in Pennsylvania used many herbs in the forest for medicines, like American ginseng, bloodroot, and goldenseal. Some of these herbs cannot be found everywhere in the state, so the cultural association with the understory changes just as much as the forest does across the landscape.

Aesthetic

The forest understory, even without all the good roles that it serves, can be very beautiful! For a landowner with wooded areas, or for the Pennsylvanian who simply enjoys retreating to the forest to take in the sights and sounds, this layer of the ecosystem might be very important for enjoyment, appreciation, and recreation. The understory contains beautiful flowering plants that are often used in our landscaping. Solomon’s Seal, trilliums, violets, spring beauty, and Canada mayflowers are delicate flowering herbs that are appreciated in many garden beds and can be found in some Pennsylvania forests. Other flowering herbs like pink lady’s slipper and Jack-in-the-pulpit are not only beautiful but may also be useful to indicate that you have a healthy forest.

Purple trillium (Trillium erectum) (left) and pink lady's slipper (Cypripedium acaule) (right) in flower. Photo credit: Cathryn Pugh

Non-flowering herbaceous plants also increase the aesthetic appeal of a woodland. Many ferns, like maidenhair fern and Christmas fern, are planted as features in gardens to add interesting textures and various shades of green to a garden. Ferns may grow in clumps or carpet a forest understory, providing insight into the type and health of a particular forest.

The understory of the forest changes frequently throughout the growing season. Many of the plants that live in this layer have short above-ground lifespans. Virginia bluebells and bloodroot are two commonly enjoyed spring ephemeral flowers, meaning they complete their lifecycle before the deciduous trees leaf out in the spring. Over the summer, the composition of the understory changes. Plants like white baneberry, wood geranium, and violets come into bloom during this season, and ferns are out in full force. By the fall, wood asters and goldenrods take over as the dominant flowering herbs in the woods. Throughout the seasons, there are many types of flowering and non-flowering plants to enjoy in the forest for woodland enthusiasts.

Carolina spring beauty (Claytonia caroliniana) (left) and Virginia bluebells (Mertensia virginicaI) (right) in flower. Photo credit: Cathryn Pugh

All types of herbaceous plants have different appeals to the people who enjoy the forests of Pennsylvania, and understanding the understory can help increase the appreciation for the plants found here.

Part IV: Wildlife

Many woodland owners and enthusiasts have questions about how wildlife utilizes forest layers. Here, we address examples of how wildlife use the understory of the forest for food, hunting, and breeding. We also have links to additional resources if there are one or more types of wildlife you are interested in investigating further with other extension resources.

Part IV: Wildlife and the Understory

Wildlife

Amphibians

Amphibians of all types (salamanders, frogs, toads) rely on the understory layer of the forest for survival, habitat, food resources, and reproduction. The most abundant (non-insect) animal in Pennsylvania, the red-spotted newt, is among this group. With increased plant diversity in the understory, more amphibians can take advantage of this space. Small, herbaceous plants can change the temperature and available water on the forest floor. For animals like amphibians that breathe through their skin, this allows them to maintain body temperatures and skin moisture levels that help them survive. Many insects also utilize this part of the forest, and they are often consumed by different amphibians.

Old forests with shade at the ground level can provide some of the best habitat for terrestrial amphibians. In addition to herbs, coarse woody debris and mosses may also provide suitable habitat for frogs, toads, and salamanders.

Birds

Many forest interior birds rely on the understory of the forest. Some species of birds, valued by bird watchers and wildlife appreciators, utilize the understory of the forest for nesting, foraging, and more. Pennsylvania is an important pit stop for many migrating birds in the spring and autumn, including those that forage for insects and other food resources in the forest. Some birds spend the majority of their lives in the lowermost stratum of the forest. Ovenbirds, warblers, hermit thrushes, sparrows, juncos, wrens, and wild turkeys all use the understory for ground nesting. Herbaceous plants provide cover and help these birds hide their ground nests from predators.

Insects

Insects in forests often get a bad reputation; most of the insects we talk about are forest pests and can damage some of our preferred plants and trees. However, every forest needs insects to maintain its health. Insects function as pollinators, are important parts of the forest food web, and increase our state’s biodiversity. In the understory, pollinators rely on flowering plants. Moths, butterflies, and bees use this layer of the forest to collect nectar. One group, called mining bees, lives in forests and is often the earliest bees to emerge in the spring. This is important for plants like blueberries. Other insects, like gnats and flies, pollinate some of Pennsylvania’s native forest herbs, like Jack-in-the-pulpit. In addition, many of the insects that reside in the understory become important sources of food for other wildlife. Birds, mammals, reptiles, and amphibians consume the many beetles, flies, and other insects that use the understory as habitat.

Mammals

Pennsylvania is home to many mammals that rely on the forest for habitat, and nearly all of them spend some time among the herbs in the ecosystem. Some species of mammals directly consume the fruits from herbaceous forest plants, like in the cases of red foxes eating the berries from pokeweed, or chipmunks utilizing the fruits from Canada mayflower. Famously, white-tailed deer are frequent grazers of forest herbs, and often have preferences for the lily-like species that grow in this layer. Insect-eating wildlife will also forage in this layer and seek insects to consume. Some mammals, such as red and gray foxes, will also use this layer as hunting grounds for other species living among the herbs, like mice, voles, and shrews.

Reptiles

Forest and woodland-dwelling reptiles are often found utilizing the lowest layer of the forest. Like amphibians, reptiles are exothermic and rely on the conditions of the environment to regulate their body temperatures. The herbs of the forest can help to minimize water loss from the soil during hotter periods, allowing reptiles to stay cool. The understory can also be used as cover or in camouflage by reptiles as they hunt and move around on the forest floor. The Eastern wormsnake, a helpful insect-eater and species of concern in Pennsylvania, uses the cover provided by the understory to avoid detection from avian predators when it emerges from its burrows.

Part V: The Understory and You

The understory has value to many woodland enthusiasts: landowners, hikers, birders, and anyone who lives in Pennsylvania and utilizes these spaces. For those interested in improving the conditions of their forest understory, the understory can offer the opportunity to learn more about the woods, starting from the bottom up.

Caring for the forest starts with knowing what is in it. Our forests in Pennsylvania often face challenges from invasive species, deer, and development. Invasive species can crowd out our native herbs and change the plants we see in our woodlands. Deer herds often browse and eat large volumes of herbaceous plants, making it hard for groups of these plants to reproduce or persist.

Learning about the forest herbs around you is one of the best places to start. Understanding what is currently in the woodlands around you offers the chance to recognize what type of forest you are in, which types of wildlife your woodlands may support, and how you can improve the quality of the woodlands for plants, animals, and yourself. Forest inventory, or the documentation of the types of plants in a forest, can be a difficult task, but it is highly recommended for anyone looking to better understand what goes on in the woods around them. For more resources on how to conduct an inventory of your forest herbs, check out our courses on Woodland Stewardship: Management Practices for Landowners or Plant Identification and Usage: Native Plants in the resources section below.

After learning about what plants you have in the understory of your woodlands, you can decide what your goals are. Do you want to manage for more wildlife? For more foraging herbs? Or another goal? The management goals you choose will help you steward your forest lands responsibly. Oftentimes, this may look like removing invasive plants, then moving to areas with the strongest or healthiest populations of native plants and encouraging their spread and reproduction. In any case, the resources provided by your local extension office can help you make the right choices for the type of forest you own or manage.

Resources

Articles

Forest Stewardship: Conserving Biological Wealth in Forests

Spring Ephemerals for Residential Gardens

FISH: Pennsylvania Frog and Toad Breeding Phenology

Pennsylvania Wetlands: Vernal Pools

From the Woods: Amphibians

Restoring Pennsylvania's Game Birds

Conserving Wild Bees in Pennsylvania

Bees in Pennsylvania: Diversity, Ecology, and Importance

Forest Stewardship: Wildlife

Tree Squirrels

White-Tailed Deer

Videos

Woodland Amphibians: Frogs and Salamanders

Forest Birds

Do Insects Matter to People?

Herbaceous Forest Openings for Wildlife

No Ugly Forest Moths

Woodland Reptiles: Snakes and Turtles

Online Courses

Woodland Stewardship: Management Practices for Landowners

Plant Identification and Usage: Native Plants