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Northern Prairie Wildlife Research Center

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Classification of Natural Ponds and Lakes
in the Glaciated Prairie Region

Phases of Vegetational Zones

In ponds and lakes with undisturbed bottom soils, most of the vegetational zones are each represented by two or three distinct phases that frequently alternate when there are appropriate fluctuations in water level or changes in intensity or frequency of certain land-use practices. Within each vegetational zone of a pond or lake, two or three phases may occur at the same time. These phases are as follows:

Normal emergent phase.

In deep-marsh and fen zones, and in normal untilled wetland-low-prairie, wet-meadow, and shallow-marsh zones, emergent vegetation composed chiefly of biennial or perennial species is usually of regular occurrence. Plant growth extends above the water surface or dry bottom soil an often forms a canopy or overstory. Subdominant species occur under the emergent cover.

Open-water phase.

Open water without vegetation or without emergent plants extending above the water surface may occur in all zones. In the open-water phase of shallow-marsh, deep-marsh, permanent-open-water, intermittent-alkali, and fen zones, submerged or floating aquatic plants are often present.

Drawdown bare-soil phase.

As surface water in the open-water phase gradually recedes and disappears, expanses of bare mud flats, which often become dry, are exposed. Ordinarily, this phase is of short duration, but in intermittent-alkali zones and occasionally in the more saline deep-marsh zones it may persist for considerable periods.

Natural drawdown emergent phase.

Undisturbed areas with emergent drawdown vegetation are considered to be in this phase. This growth is composed mostly of annual plants, including many forbs, that germinate on the exposed mud or bare soil of the drawdown bare-soil phase. After the drawdown emergents become established, surface water is occasionally restored by heavy summer rains.

A typical sequence of wetland phases as they occur under variable water conditions in undisturbed ponds is shown in figure 1.

GIF -- Graph of Normal Water Conditions

GIF -- Graph of Low Water Conditions

GIF -- Graph of Reflooding of Pond

Figure 1. A typical sequence of wetland phases as related to different water conditions.

In ponds and lakes in cropland areas, the wetland-low-prairie, wet-meadow, and shallow marsh zones are frequently cultivated when they become dry. This results in the development of early successional plant communities that differ markedly from those of undisturbed soils. Following cultivation, these early seral communities often retain their characteristics for a year or two. The open-water phase and drawdown bare-soil phase are of common occurrence in vegetational zones that have been disturbed in this manner. In addition, a cropland drawdown phase and a cropland tillage phase are often present. These are as follows:

Cropland drawdown phase.

Tilled pond bottoms with drawdown vegetation characterize this phase. The plants include many coarse introduced annual weeds and grasses that normally develop on exposed mud flats during the growing season. These species appear as overwater emergents whenever surface water is restored by summer rains.

Cropland tillage phase.

In this phase are tilled bottom soils dominated by annual field weeds, characteristic of fallow or neglected low cropland. Tilled dry pond bottoms devoid of vegetation are also considered to be in this phase. Planted small grain or row crops are often present.
Previous Section -- Vegetation Zones in Prairie Ponds and Lakes
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