Home » Gardening Information » Ecology for Gardeners
The Physical Structure of Plant Communities
Every native plant community has a recognizable, and somewhat predictable, structure based on the dominant and subordinant vegetation. A forest has a towering canopy of trees that influence what can and cannot grow beneath them. A grassland has a similar structure, but the plants are herbaceous instead of woody, and this vertical structure is re-created above ground each growing season, not over decades and centuries as in a forest.
Plants within a given community also create patterns of distribution on the landscape as a result of environmental factors such as soil, moisture, and light. Different species thrive with different amounts of moisture, for example. In a prairie, shrubs will grow in the wettest areas, grasses and forbs on the slightly higher and drier land. As a result of these factors, plant communities have recognizable vertical and horizontal structure.
Ecological gardeners use the structure of the native plant community as the basis for structuring their gardens. In the eastern forest region, for example, canopy trees lend a grand vertical scale to the garden, creating a cathedral-like enclosure, while understory trees add a more intimate, human-scale "ceiling." Shrubs can become "walls" that divide spaces horizontally and create privacy. The ground layer is where gardeners can create a tapestry of wildflowers, ferns, and grasses in beds and borders. Ecological gardeners know that structuring a garden like the native plant community also helps support birds and butterflies and other wildlife by offering them an array of spaces for feeding, breeding, resting, and nesting.
Vertical Structure
All plant communities have a vertical structure based on the size and growth pattern of the dominant species. This pattern is called vertical stratification, or vertical layering. Vertical structure is most obvious in a forest. The tallest layer is called the canopy. It is composed of mature trees that cover the entire forest. The tallest canopy trees may be one hundred feet in height or more. The lowest canopy trees grow to about 30 feet. The canopy is interwoven, forming a fairly continuous ceiling over the entire forest. The canopy sets the stage for everything that happens in the layers below it.
The next layer down is called the understory. This layer is composed of saplings of canopy tree species as well as smaller flowering trees such as dogwoods, redbud, shadblow, ironwood, and hop hornbeam in the eastern United States. The understory extends from 30 to 12 feet.
The shrub layer is the lowest layer of woody vegetation. It occupies the area between 12 and 3 feet above the ground. Shrubs grow in patches where light and space are sufficient. A variety of forest birds, including vireos and some warblers, use the shrub layer for foraging and nesting. Many shrubs produce edible fruits that enable birds to lay on fat reserves in preparation for migration.
The lowest above-ground layer of the forest, below 3 feet, is called the ground layer. Here, wildflowers, ferns, grasses, and sedges grow in often-spectacular assemblages. Plants in the ground layer also partition their environment vertically. The spring ephemerals bloom first, typically raising their foliage only a few inches above the leaf litter. As they are going dormant the taller ferns, merrybells, and other herbs overtop them.
The age of a plant community affects its structure. Young forests have a well-defined shrub layer and understory. The canopy becomes more distinct as the forest ages. Different forests have different structures. Deciduous forests have the most elaborate structure, as described above. By contrast, coniferous forests, with their dense stands of tall and narrow trees, typically have very little understory, but can have a dense shrub layer and ground layer of herbs and mosses. Pine forests have the most open canopies of the coniferous forests, with scattered understory trees and a well-defined shrub layer. Ground-layer species are scattered in the sunny openings. Oak woodlands have a structure similar to that of the pine woods, except the trees are smaller in stature and the canopies are wider. The ground layer of grasses and annual wildflowers is exceptional during the rainy season.
Shrub communities have mixed layers of different-sized shrubs, with a ground layer of herbs, grasses, and sedges. In communities dominated by herbaceous plants, the vertical structure is no less distinct. The plants resprout from their roots each year. The earliest plants to emerge in the spring are low to the ground. Each successive emerging plant overtops the next, culminating with the tallest grasses and late-blooming composites that end the growing season.
Horizontal Structure
Light, moisture, slope, and soil have a direct effect on where a plant or group of plants grow in a forest, grassland, desert, or other plant community. Plants form horizontal patterns of distribution in response to these environmental and edaphic, or soil-related, factors, better known to gardeners as microclimates. The resulting diversity of associations and species across the landscape is called horizontal heterogeneity.
In deciduous forests, oaks are found on the drier sites such as sunny slopes, while maple and basswood grow in the moister soil on east and north slopes. Individual trees are spaced according to their canopy size and shape. In marshes, sedges dominate the wettest areas, shrubs the intermediate regions, and wet meadow or woodland plants the upland zones.
Ecotones
Ecotones are the transitions between two plant communities. Because they include species from both communities and often their own unique species as well, ecotones are usually the most diverse ecosystems. In the Midwest, the eastern deciduous forest grades into the prairie in an ecotone called oak savannah. In the West, conifer forests grade into pinyon, juniper, or oak woodlands or into shrubby grassland ecotones. Ecotones also occur at a smaller scale, where wind throw has created an opening in the forest, for example, or at the edges of small tracts of woods. Ecotones support not only a variety of plants but also generalist animals such as skunks, deer, robins, and jays.
Today, the most common ecotones occur on abandoned agricultural land. These fields are first overtaken by pioneer species, generally a mixture of native and naturalized annuals that colonize open or disturbed ground. As the soil stabilizes and grows richer, an increasing variety of plants parades across the landscape, depending upon soil, moisture, and exposure. In forested regions, canopy trees ultimately become dominant. This new edge of grasses, shrubs, and young trees becomes an ecotone between open field (meadow) and forest.