Northern Prairie Wildlife Research Center
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.
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.
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U.S. Geological Survey