2.2 Swamps

Swamps are common in boreal, temperate, subtropical, and tropical landscapes but may or may not accumulate peat. Swamps commonly occur in valley bottom or riparian settings; they are dominated by woody plants, either large shrubs or trees (e.g., > 10 m tall), and their hydrological regime is highly episodic (Figure 3).

Figure 3 – Hypothetical seasonal hydrographs from temperate and boreal peatlands. Swamps display more extreme variability due to the strength of episodic water inputs; strong surface water inputs exaggerate water table response, while water table fluctuations below the surface may be exaggerated because of low drainable porosity, and strong tree canopy interception and transpiration. Fens have shallower water tables and less variability, due to sustained groundwater and/or surface water inputs. With their water tables in the upper layer of peat, the higher drainable porosity there reduces water table response to water exchanges. Bog water tables respond to episodic water inputs only (rainfall, snowmelt) and water loss is mostly due to evapotranspiration but also to small groundwater seepage losses; they are not modulated by groundwater inflow, and water tables are typically lower than in nearby fens. Values on the vertical axis scale were intentionally omitted but would be on the order of tens of centimeters.

Swamps may or may not receive substantial groundwater input but commonly flood in response to snowmelt or heavy rain or, in the case of riparian settings, overbank flows. Because of surface and groundwater outflows and/or high evapotranspiration, their water table declines steeply during dry periods. This decline is an essential feature since most large woody species cannot withstand persistent flooding. Peat swamps, therefore, must have a hydrological regime wherein drainage is sufficiently impaired to delay decomposition of plant materials, and/or the annual litter deposition is very high to accumulate sufficient organic matter to form peat.

In either case (peat or mineral swamp), water table drawdown must be sufficient to accommodate woody plant growth (Locky et al., 2005). We note, however, that some swamp trees such as bald cypress have root adaptations that permit them to tolerate extended periods of flooding (Harms et al., 1980), although these swamps are not typically peatlands.

In peat swamps, seasonally low water tables often result in highly decomposed peat. This may be enhanced by the labile (easily decomposed) nature of the litter and external mineral/nutrient-rich water inputs. The water in swamps is called minerogenous, referring to the input of mineral rich water from adjacent mineral soil uplands, and hence has minerotrophic ecologic composition and trophic status. The degree of mineral input depends on the rate and chemical composition of groundwater (and sometimes surface water) inflow that can buffer the organic acids released by decomposing plant matter; thus, swamps can be alkaline, intermediate in pH, or acidic.