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Wetland-low-prairie zone.

In natural untilled low-prairie zones, a normal emergent phase, with low-prairie plants, occurs regularly. Occasionally in the early spring, when water levels rise above the tops of low-prairie plants, an open-water phase without submerged aquatic plants develops. Under agricultural use, the cropland tillage phase nearly always persists as dry tilled soil, with or without weedy plant growth or crops. Tilled low-prairie zones may also appear briefly in the open-water phase during extremely high water conditions. Typical plant species found in normal emergent and cropland tillage phases of wetland- low-prairie zones are listed under Class I in appendix A.

Wet-meadow zone.

Wet-meadow zones in the central areas of shallow pond basins are restricted to fresh or slightly brackish wetlands, while peripheral bands of wet-meadow zone frequently occur in deeper, more permanent ponds or lakes with salinity ranging from fresh to subsaline. Characteristic species of plant associations in the normal emergent phase and cropland drawdown phase differ markedly, and major differences in species composition within the normal emergent phase may be correlated with variations in salinity (species listed in appropriate divisions under Classes II, III, and IV in appendix A).

Shallow-marsh zone.

Under natural untilled conditions, this zone is represented by four wetland phases: a normal emergent phase of regular occurrence; an open-water phase, often with submerged aquatic plants, occurring during high water; and a natural drawdown emergent phase, occasionally preceded by a drawdown bare-soil phase that develops during periods of low precipitation. Typical dominant species in the normal emergent phase are grasses or grasslike plants that are intermediate in height in comparison with emergent plants in the normal emergent phase of wet-meadow and deep-marsh zones.

Wetland phases occurring when this zone is tilled include the following: an open-water phase, with or without submerged aquatic plants, which is generally present during the spring and occasionally present after heavy rainstorms in summer and fall; a drawdown bare-soil phase, developing as open surface water disappears; a cropland drawdown phase that becomes established on exposed mud flats, particularly during late summer and fall; and a cropland tillage phase immediately following cultivation. Whenever surface water is maintained for a considerable period in late spring and summer, a distinctive normal emergent phase characteristic of the tilled shallow-marsh zone occurs. This phase is composed of pioneering shallow-marsh species that also appear, although less commonly, in the normal emergent phase of natural untilled shallow-marsh zones.

Shallow-marsh zones occurring in central areas of pond basins are largely restricted to fresh, slightly brackish, or moderately brackish ponds or lakes. In the deeper, more permanent ponds and lakes, the concentric bands of shallow marsh adjoining the more centrally located deep-marsh zones are of regular occurrence throughout the range of salinity, from fresh to subsaline. Tillage of shallow-marsh zones ordinarily occurs only in fresh, slightly brackish, and moderately brackish ponds. Outer bands of shallow marsh in strongly saline alkali lakes are subsaline, in contrast to the greater salinity of the central open areas. Surface water in brackish and subsaline shallow marsh tends to be shallower and less permanent than surface water in shallow-marsh zones of the fresher ponds and lakes. Nevertheless, the spatial relation of shallow-marsh to wet-meadow and deep-marsh remains the same, regardless of salinity.

Differences in species composition are quite pronounced between shallow-marsh plant associations characteristic of untilled and tilled conditions, and among emergent, open-water, natural drawdown, and cropland drawdown phases of this zone. More subtle differences within each phase may be represented as a continuum of overlapping species that is correlated with differences in salinity. The characteristic plant associations occurring under these variable conditions are listed in appropriate divisions under Classes III and IV in appendix A.

Deep-marsh zone.

Four wetland phases are represented in this zone: a normal emergent and an open-water phase, both of regular occurrence, and a drawdown bare-soil (nonvegetated) phase and a natural drawdown emergent phase, both of which develop only during drought. In the deeper ponds, an alternation of the normal emergent phase and the open-water phase is common because of annual and seasonal changes in water depth. The normal emergent phase is generally present in the shallower areas of this zone, while the open-water phase occupies the deeper areas. In permanent lakes, marginal bands of deep marsh are usually represented by the normal emergent phase in the outer, shallower portions, while the open-water phase is typical of the deeper portions that adjoin the permanent-open-water zone. Submerged or floating plants are often found throughout this zone; certain species of these plants occur as subdominants in the normal emergent phase, while many other species are characteristic of the open-water phase. Dominant plant species in the normal emergent phase are in general coarser and taller than corresponding species in shallow-marsh zones.

Deep-marsh zones are nearly always present in the deeper ponds and lakes in which salinity ranges from slightly brackish to subsaline. During high water this zone may also be found locally in some of the deep fresh-water ponds. Species composition of plant associations differs noticeably in the three vegetational phases of deep marsh and under different ranges of salinity within each phase. Characteristic species of these associations are listed in appropriate divisions under Class IV in appendix A.

Permanent-open-water zone.

Intermittent-alkali zone.

Emergent plants do not develop in this zone, apparently because of the high salt content, but one submerged aquatic species, saltwater widgeongrass (Ruppia maritima), is frequently abundant whenever surface water is maintained for a few weeks during the summer (see under Class VI in appendix A).

Fen (alkaline bog) zone.

Pockets of fen zones adjoining the more typical basin wetlands are most frequent along the margins of brackish, subsaline, and saline ponds and lakes. In these situations fen zones are often located on gently sloping terrain with a perceptible flow of ground water on or near the surface, extending from seepage inflow or spring sites to the ponded surface water below. Ordinarily, salinity increases as water moves down the slope, and this is reflected in changes in species composition of wetland plants. Typical fen species gradually merge with and are replaced by species characteristic of salinity ranges in other zones. Vegetation of fens is represented by a normal emergent phase and an open-water phase. Typical plant species for each of these are listed under Class VII in appendix A.