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Aquatic and Wetland Vascular Plants of the Northern Great Plains

Defining the Wetland Flora

Authors inevitably differ in their concept of which species should be included in a taxonomic treatment of aquatic plants. Each has his own definition for the term "aquatic." Virtually all authors recognize those plants with a submersed or floating growth habit as aquatic. Most also apply the term to the common emergent species. The definition is harder to apply consistently for (1) plants growing in marginal zones of wetlands, e.g., shores, wet meadows, streambanks, etc., and (2) plants displaying a wide-ranging ecological amplitude which enables them to grow in either wet or dry situations. Whether a plant is to be designated as aquatic or not is thus based upon the plant's growth habit and the types of habitats in which it is found. How much weight is assigned to either of these criteria is a subjective decision which largely accounts for differences in the taxa treated by various aquatic plant manuals.

Much confusion can be avoided at the outset by defining the terms used to delimit the group of plants included herein. The term "aquatic" is used to describe those plants growing in water or in soils that are saturated during most of the growing season. Four categories of aquatic plants may be recognized on the basis of growth form and zone of habitation:

1. Free-floating is the term used for plants which float at or beneath the water surface without attachment to the substrate. Free-floating aquatics are transported freely by wind and currents, so they are normally found in abundance only in calm, sheltered waters. Duckweed (Lemna spp.), bladderwort (Utricularia vulgaris), and coontail (Ceratophyllum demersum) are common examples of free-floating aquatics.

2. Submergent describes plants anchored to the bottom by roots or rhizomes. Their foliage is either entirely submersed or some floating leaves may also be present. Reproductive structures may be submersed, floating, or borne above the water surface. Submergent plants occur in very shallow to deep water, depending upon water clarity, substrate, and growth form. Some common examples include pondweed (Potamogeton spp.), water milfoil (Myriophyllum spp.), waterweed (Elodea spp.), and widgeongrass (Ruppia maritima).

3. Emergent refers to those species which occur on saturated soils or on soils covered with water for most of the growing season. The foliage of emergent aquatics is partly or entirely borne above the water surface. Examples of emergent aquatics are many, including arrowhead (Sagittaria spp.), cattail (Typha spp.), common reed (Phragmites australis), and bulrush (Scirpus spp.).

4. Amphibious applies to aquatic species which are capable of growing as either submergent or emergent aquatics. These species commonly assume a semi-terrestrial growth form when stranded by a receding water level. The semi-terrestrial growth form usually differs markedly in appearance from the submersed growth form. Amphibious aquatics include yellow water-crowfoot (Ranunculus flabellaris, R. gmelinii), pepperwort (Marsilea vestita), and water smartweed (Polygonum amphibium).

Many of the species included by this treatment fit none of the categories given above. These are plants that ordinarily inhabit wet meadows, shores, streambanks, exposed mud flats, and other marginal habitats where the soil is saturated for only part of the growing season. Since these habitats are inherent to prairie wetland ecosystems, the plants which inhabit them are logically part of the wetland flora. They are, therefore, admitted to this treatment as wetland plants.

Not all wetland plants are restricted to wetland habitats. A considerable number of them are also found in upland situations. Many are opportunistic weedy species that rapidly invade soil left bare by receding water. Plants like barnyardgrass (Echinochloa muricata), foxtail barley (Hordeum jubatum), and common plantain (Plantago major) are as apt to be found on disturbed upland sites as on mud flats or shorelines. Some of the agronomic weeds, e.g., sowthistle (Sonchus arvensis), Canada thistle (Cirsium arvense), and cocklebur (Xanthium strumarium), are well suited to the disturbed conditions provided by shorelines and stream banks. Still other plants found in both wetland and upland habitats are not weedy at all, but are simply capable of growing in a variety of moisture regimes. The wild lily (Lilium philadelphicum) and blue-eyed grass (Sisyrinchium montanum) are nonweedy species encountered in wet meadows and boggy places, as well as in prairie habitats. Even though these plants are by no means restricted to wetlands, they are encountered in wetland habitats with considerable frequency and are included in this treatment to ensure adequate coverage of the wetland flora.

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