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Roadside Use of Native Plants

Explaining Plant Communities Why Different Plants Exist in Different Places

Evelyn A. Howell, University of Wisconsin — Madison

Native plants evolved within communities — groups of plants and animals living together at the same place at the same time. Communities provide the context within which plants interact with their environment, and supply the resources needed for survival and reproduction. For many, the aesthetic appreciation of native plants lies not only in a celebration of the beauty of their individual forms, colors, and textures, but also in an understanding of the complex interrelationships among species in their natural settings.

The structure and composition of plant communities varies across the landscape, largely in response to changes in the physical environment. North America is remarkable in its natural diversity. Patterns of precipitation, temperature, and day length combine with varied land forms and soil types to divide the continent into distinct regions. Native plants tend to group themselves according to these environmental differences, forming units that are easy to recognize and distinguish.

The cactus — shrublands of the southwest, the marshes of the Everglades, the alpine meadows of the western mountains, and the hemlock forests of the northern midwest all have a distinct appearance. They are dominated by different life forms — succulents, shrubs, sedges, grasses, or evergreen trees. The architectural framework created by the growth patterns of the plants provides different spatial experiences; for example, the exhilaration of the open, wind- swept meadows with the ground carpeted by low- growing flowers and grasses; or the calm of the dark and shady high- ceilinged rooms formed by the hemlocks, with nothing on the ground except a carpet of needles and one or two clumps of ferns and wildflowers where the canopy lets in shafts of light and rain.

Botanists use these community attributes — life forms and structure — together with geographic location and the dominant species present to classify vegetation on a regional basis. The Natural Regions Maps, included in this guide, show the distribution patterns of the major plant associations in the United States.

On a more local level, individual communities become more difficult to distinguish. The species groupings often form a continuum, with no obvious boundaries between community types. Landscapes with large variations in microclimate - different slope aspects, soil types, elevations, often have a mosaic of communities present, blending into a seamless whole. In the Driftless Area of the midwest, for example, it is often difficult to tell on a large tract of land with continuous forest cover where a maple forest ends and an oak forest begins.

No two communities are ever exactly the same, and change is constant. Species enter and leave. Some are able to both establish a presence in the community and reproduce to maintain a population over time; others are less permanent residents. As the microclimate changes, either through the actions of the plants themselves — shade increases in an area as trees mature — or through climatic fluctuations, a drought cycle - or the actions of wind and fire, the vegetation shifts and communities change.

Although communities are variable and dynamic in both space and time, the interactions between species lead to patterns of community structure that are characteristic of particular community types. For example, North American forests have vertical layers — canopy, midstory, understory — that are composed of different life forms — trees, shrubs, herbs. Midwestern prairies have species that are active in the spring and fall — cool season species — and others that are active in mid-summer — warm season species. In the northeast, many communities have some species that bloom in spring, some in summer, and some in fall. These other structures are thought to be a result of "resource partitioning", an evolutionary strategy that allows species that are potential competitors for a resource (space, light, pollinators) to co- exist by specializing on different aspects of the resource.

Other mutual survival strategies that shape the structure of communities have developed in reaction to different kinds of species interactions — for example predator/prey relationships. The fact that grasses have their growing points at the base of the leaf, is thought to be due in large part to a reaction to grazing.

These complex interrelationships are the origins of the diversity of our native species.

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Questions and feedback should be directed to Deirdre Remley (deirdre.remley@dot.gov, 202-366-0524).

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