Earth Partnership for Schools



WOODLAND RESTORATION INTERACTIVE WORKSHOP > CHARACTERISTICS OF FOREST ECOSYSTEMS

Cant' see the forest because of the trees? Often when we think of a woodland, we focus on the trees. Yes, they are large, they dominate visually and by sheer biomass. But a healthy forest ecosystem is more than just trees. A forest also includes a diverse combination of herbaceous plants, shrubs, seedlings, and an abundance of birds, mammals, insects, reptiles, amphibians, and microscopic creatures.

The species composition of forests varies from one place to another, even within the same woodland type. Yet, we can learn general characteristics of a forest to better understand a forest community.

Learning about the structure, environmental influences, and succession in a forest provides you with a general model to plan and restore your native woodland.

Vertical Structure-Vegetation Layers
Horizontal Structure
Environmental Influence
Succession
Succession Model

 

Vertical Structure - Vegetation Layers

The vertical structure of the forest is divided into distinct layers, each adapted to increasingly filtered sunlight if going top down. The layers are: canopy, understory, groundlayer, and the forest floor. Not all forests have each layer. Generally; extreme environments such as deeply shaded woodlands, boreal forests, and frequently flooded woodlands have fewer layers.

The Canopy is the upper-most layer of a woodland. It is composed of dominant and associate tree species. These largest trees shield the layers below, shading them and influencing the microclimate. Under a dense, continuous canopy of sugar maple and basswood, the forest community below is generally cool and moist. Under an open canopy of oak or pine woodlands, the microclimate is warmer and dryer. Where light is allowed into the understory, shade intolerant canopy species may regenerate.

The understory is the next layer. It represents the middle layer and casts additional shade on the layers below. The understory may be subdivided into a subcanopy and shrub-seedling layers.

The subcanopy consists of a variety of small trees, some of which may be ironwood, chokecherry, serviceberry, mountain maple, hop-hornbeam, and younger individuals of canopy species.

The shrub and seedling layer consists of woody plants that usually have multiple stems such as dogwoods, viburnums, elderberry, blueberries, and raspberries. This layer also includes immature offspring of shade tolerant canopy species. These saplings and seedlings may take their place in the canopy with the death of the mature trees overhead. The understory is home to many woodland nesting birds and provides abundant food resources for wildlife.

The groundlayer is on the forest floor. Wildflowers, sedges, ferns, grasses, creeping shrubs, horsetails, liverworts, and mosses comprise this layer. Many woodland herbs are adapted to low light, sheltered conditions, and cannot tolerate direct light environments. Other woodland plants -- ephemerals -- adapted by grow rapidly and blooming in the early spring before the canopy leafs out and casts shade. Another adaptation is to develop large leaf surfaces to capture sunlight, and/or grow in colonies. Example of these plants include ferns, mayapple and wild ginger. When restoring a woodland, it may be years before the soil is favorable and light intensity is reduced enough for many of these plants to prosper.

Underfoot lies the forest floor. This lowest layer is composed of litter, humus and topsoil. On the forest floor lie fallen trees, leaves, and branches in various stages of decomposition. Here a diverse community of creatures live. Microscopic soil bacteria, fungi, and nematodes along with worms, insects, millipedes, and other small creatures break down the organic debris into humus and mix it with soil. Their nearly invisible action is critical for forest nutrient recycling and continued growth.

 

Horizontal Structure

The composition and distribution of species can vary widely from one spot to the next. Environmental gradients, such as moisture, drainage, slope, slope aspect, soil type, and light intensity, influence these horizontal patterns within a forest. Ecologists call these species composition changes along an environmental gradient a vegetational continuum.

Other factors that influence the distribution and composition of species include: gaps in the canopy where individual trees die from old age; disease; lightening strikes; storms; seed availability; large clearings created from widespread insect and disease damage; or destruction from fire. New generations of trees and shrubs fill in these gaps and clearings. As a result the forest develops patches of different-aged vegetation, with trees varying in size and composition. Naturalists are able to read the history of the land through observing the mosaic of plants growing in a forest.

 

Environmental Influences: How the Environment Influences Where Species Grow

A natural community is built on relationships. These relationships influence the structure and appearance of any particular plant community. You, as a member of the human community, are influenced by your family, home environment, friends, and community. Similarly, plants and animals are influenced by each other and by the environment in a natural community. Some interrelationships between the native vegetation and the environment are easily recognizable, such as soil, moisture, drainage, and topography. Understanding these environmental aspects will help you determine which communities will grow best on your site.

Soil

The key soil-related factors affecting the type of community that will grow on a particular site are as follows: 1) the soil type; 2) the pH which ranges from acid to alkaline; and

3) the amount of organic content.

Moisture and Drainage

The amount of moisture available is another key determinant for the mix of species that will grow at a site. It is so important that many botanists classify communities along a moisture gradient. Soils and, therefore, woodland communities are classified as dry (xeric), dry-mesic, mesic, wet-mesic, and wet (hydric).

Topography

The lay of the land, whether it is flat, hilly, upland or low-lying, further refines the composition of a woodland. Low areas, such as those along rivers or in depressions, support species adapted to wet conditions. High, dry uplands support species tolerant of drought. The direction a slope faces, known as slope aspect, also plays a role. Plants growing on north-facing slopes encounter cooler temperatures and deeper shade. Maple forests will grow on a north-facing slope. Species on south- or west-facing slopes encounter dry, exposed conditions. Dry to dry-mesic oak woodlands will grow on south-facing slopes.

 

Succession

Forest communities experience disturbance that may create a cycle of change. Fire, windthrow, flooding, and human disturbance are some major change factors. Many textbooks describe the process of succession as a series of progressive changes in vegetation and animal life which may culminate in a climax community -- a stage where a community is stable and able to perpetuate itself. This is a simplified and idealized view of a complex process. Site specific factors exert varying degrees of control over succession. Some of these factors include previous land use, seed availability and dispersal, topography, soil, moisture availability, slope aspect, the growth habits and strategies of colonizing species, and the prevalence of invasive exotic species.

An understanding of the processes influencing succession can direct restoration strategies. The following example of a hypothetical succession in southern Wisconsin will provide you with an idea of how succession can influence your planned woodland restoration. Learn how to use the mechanics of succession in your restoration.

(A note about native and non-native plants: Plant composition of ecosystems in Wisconsin has been greatly altered since European settlement and the introduction of non-native plants. Non-native species are plants introduced from other parts of the world and are often aggressive. Species are considered native for restoration purposes if they were here before European settlement. Exotic, non-native plants often out-compete native vegetation. For restorationists, these non-native invasive species create a frustrating challenge.)

 

A Successional Model in Southern Wisconsin

Succession is often described as a series of stages. Each stage can dominate an area for a differing length of time, depending on site specific factors such as soils, slope, etc.

First Stage
The first stage is a site with bare soil due to some recent disturbance. Natural disturbances would include animal digging or wallows, and siltation from flooding rivers. When we till gardens or plow agricultural fields we are continually reintroducing disturbance to take a site back to the beginning. On the other hand, lawns can be considered a successional stage that is not allowed to change but is maintained in one stage by mowing and other cultural practices.

The bare soil created by disturbance is invaded by plants that can tolerate open conditions. These early successional invaders are often weedy annuals that are prolific seed producers and have seed that is either dispersed widely, and easily, or seed that is able to remain in the soil for a long time until a disturbance provides the conditions for their growth. These plants usually cannot perpetuate themselves on the site after they have modified the soil and micorclimatic conditions by their very presence. These modified conditions allow for the invasion of the site by another group of plants. Some native species that may come in are: daisy fleabane, Canadian and weak St. John's wort, wild lettuce, evening primrose, black-eyed Susan, and native ragweed. Since the introduction of non-native species the first successional stage might be characterized by a predominantly non-native group of weedy plants such as lamb's ears, crab grass, foxtail, pigweed, purslane, and dandelion.

Second Stage
In the second year, perennials start to be conspicuous. Typical non-native perennials include quack grass, brome, timothy, burdock, and some thistles. Some native species that may come in are beebalm, wild ryes, yellow coneflower, vervain, milkweed, common goldenrod, asters, and yarrow. These plants can dominate an area for many years in what has been called an old field. The presence of any particular species is dependent upon a seed source being available. In many cases therefore, there are few native species present in this stage since their natural range has been reduced to small areas.

Third Stage
After a time, woody colonizers may begin to appear. Native shrubs and trees, such as staghorn sumac, gray dogwood, aspen, and hawthorn, gradually shade out sun-loving, herbaceous perennials modifying the site as the pioneers of the forest. Non-native species such as honeysuckle and buckthorn commonly invade at this stage, too. Many of the woody pioneer species are typically found growing along a woodland edge. These species are usually fast growing, sun loving plants and are adapted to exposed conditions. Once woody plant cover reaches 50%, this stage is called the woody pioneer stage. This stage can last 25 to 50 years, assuming no additional disturbance, during which time the organic debris accumulates, decomposes and is incorporated into the soil. However, these plants cannot reproduce in the modified shade conditions that they create, so barring further disturbance more shade tolerant plants follow them.

Fourth Stage
In the mature forest stage long-lived trees and shrubs begin growing that are adapted to the light conditions created by the woody pioneer community. Some common tree species associated with the mature forest in the southern Wisconsin include white ash, oaks, elm, walnut, red maple, and ironwood. Shrubs may include of alternate-leaf dogwood and witchhazel. During the next 25 to 50 years, the pioneer tree species are replaced by these long-lived trees and shrubs.

Last Stage
Over the next 50 to 150 years, the forest becomes dominated by sugar maple, beech, or basswood. The resulting, old-growth stage, depicts a climax or self-sustaining, stable forest. The final species composition varies due to local environmental conditions proximity of seed sources. This stage is self-sustaining without disturbance because the plants are able to reproduce in the conditions they create, such as lower light conditions. These plants do not do well in earlier successional stages and are not good choices for open conditions.

For a more complete discussion of succession with student activities, see Forest, Land and Water: Understanding our Natural Resources listed in curriculum resources.