2.6 Regional Processes
Peatland occurrence is related to latitudinal gradients of temperature and precipitation and zonal gradients of continentality (Rochefort et al., 2012), which control vegetation productivity and decay (thus peat formation) through energy and moisture availability. Bedrock geology and lithology also affect the regional distribution and character of peatlands and their groundwater relations. Regions like the Hudson Bay Lowlands have extensive peatlands (Figure 1) and the peatland complex (Figure 7) due to low permeability sediments in a favorable climate. Precambrian bedrock such as the Canadian Shield in eastern North America are also of low permeability, but irregular topography and thin soils result in isolated peatlands, more commonly bogs and poor fens, since they are poorly connected to groundwater.
Given that precipitation generally decreases poleward and with continentality, and potential evapotranspiration increases toward lower latitudes, bogs have a northern and southern limit in the Precambrian region of eastern North America (Damman, 1979). Regions with a higher seasonal water deficit, such as the more continental Western Boreal Plain, tend to have more fens than bogs (Vitt and Chee, 1990), given that groundwater exchanges in the deep sediments can augment the water budget. Oceanic climates are favorable for peatland formation, especially bogs, since they are cooler and wetter, whereas temperate climates are amenable to peatland formation, often swamps. However, peatlands in temperate climates are commonly more reliant on geomorphic controls that facilitate groundwater inputs, given their higher evapotranspiration losses.
Boreal and subarctic wetlands experience seasonal differences in their groundwater relations owing to the development of seasonal frost. Since the water table is always close to the ground surface, it freezes early in the fall season as air and shallow ground temperatures fall below 0o C. As the freezing front moves downward through the saturated peat profile, a relatively impermeable ice layer forms across the peatland. Such an ice layer can persist for over half the year during which time the peatland surface is hydrologically decoupled from the underlying groundwater. At higher latitudes, seasonal ice occurs over permafrost, ground that remains below 0 oC for at least two consecutive years. Permafrost presents a permanent (i.e., year-round) barrier separating peatlands from sub-permafrost groundwater systems and strongly influences peatland landscape development. Box 2 provides more detailed discussion of permafrost peatlands.
Tropical peatlands share many of the fundamental hydrological processes with peatlands of other regions, albeit without the cold-climate features described above. Given the high potential evapotranspiration in tropical latitudes, tropical peat swamps develop where rainfall is high. In subtropical areas, where rainfall is often much less but potential evapotranspiration is still high, swamps are less common, but can occur where groundwater input offsets the shortfall of precipitation. Box 3 provides additional information about tropical peatland hydrology.